Do you have the desire to be involved in a boatbuilding project?
Why build a boat, instead of buying one ?
If you want to build a boat, you will know the answer.
Anyway, three good reasons for it :
the minimum serious cruiser
|Choise of Construction Materials||Choise of Construction Methods|
GLASS REINFORCED PLASTICS is the most common material used in modern boatbuilding. In more general terms, a COMPOSITE MATERIAL contains high performance fibers such as glass, carbon, and aremid embedded in a polymer matrix like polyester, vinylester, or epoxy. Fibers and resins alone are basically useless as building materials. Unimpregnated fibers resist tensile forces, but fail quickly under compression, torsion, or bending forces. Matrix resins are even considerably lower in strength and stiffness than reinforcing fibers. They are, however, excellent adhesives and are well able to transfer forces to any reinforcing fiber by shear at the interface. Thus, when a bundle of fibers are dipped into a bath of resin, drained and allowed to harden, a composite material is formed with properties similar to steel. It is able to resist enormous forces in tension, compression and bending.
Glass fiber is the most widely used reinforcing material as it combines good mechanical behavior with relatively low cost. It is either cloth (a smooth woven fabric), roving (a course, basket-like woven fabric), or mat (a random combination of many short fiber strands of glass). Fiberglass composites have a significantly lower tensile modulus than steel or aluminum. Moreover, the shear stiffness is also relatively low. Hence, deflection limits are of prime consideration, strength requirements are usually a subsequent issue which are generally found to be satisfactory. Carbon fiber composites are, on the other hand, have a high modulus, on the order as steel, and thus deflection is a less significant problem.
Examination of the stress/strain curve of a composite shows that they tend to have low strain to failure. Meaning, composites are generally completely elastic up to failure.
It is important to realize that, unlike conventional materials, composites are not homogenous. Their properties vary and are dependent on position and the angle under consideration. The Properties of composites are, of course, dependent on the properties of both the fiber and the matrix, the relative quantity of each in the composite and the geometry of the fibers.
Steel is an alloy of iron and carbon, in which carbon can be up to about 1.7 percent of the whole as an essential ingredient. However, it is not just one simple material but a whole family of materials, all within the designated limitation of carbon content. Some with relatively simple characteristics, some capable of almost bewildering variations. To provide this large assortment of properties and attributes besides the main ingredient carbon many elements may be added in a wide assortment of proportions. Most common are manganese, silicon, sulfur, nickel, chromium, tungsten, vanadium, copper, cobalt, columbium, and molybdenum.
For the boatbuilder, the plain low-carbon steels containing from about 0.10 to 0.20 percent carbon is adequate and posses without complication a sum of desirable properties. These are the most common and generally the most useful of the whole family of steels and they are readily available all over the world in a diversity of shapes and sizes.
What is needed by the builder of boat hulls is a steel that is strong, that is stable in properties, that does not loose its toughness with extreme changes in temperature, that is resistant to fatigue, that is ductile enough to be readily shaped but still stiff enough to withstand the forces of the sea without failure, that is amenable to easy welding, that can be readily drilled, tapped, ground, or machined, that does not harden or become brittle locally when cut with a torch, or that does not alter its properties when subjected to red heat for shaping. The plain carbon steels fulfill these requirements very nicely.
However, as versatile as these ordinary steels are, designers in recent years have been specifying other steel alloys for their extra stiffness and strength in an effort to use lighter metal for weight advantages, particularly in sailing vessels and boats of relatively small size. These steels are marketed under a variety of trade names such as "Cor-Ten", "Jal-Ten", "Mayari", etc., depending on the producer, but in essence are relatively low carbon steels (about 0.12 percent maximum) containing small but specific amounts of manganese, copper, chromium, and nickel. This combination provides a material of greater strength than mild steel, coupled with resistance to abrasion and impact and relatively better resistance to atmospheric corrosion. It should be born in mind, however, that while there is a gain in strength, there is also a loss in ease of forming.
As a general rule, although there are numerous exceptions, as the carbon content of steel increases, the hardness and tensile values increase also. But at the same time, ductility decreases until a point may be reached when the steel will have little or no give and may become brittle and fragile. The various alloying elements can suppress or enhance strength or ductility, or provide other attributes.
Steel is elastic, that is strains are recoverable upon unloading up to a point called elastic limit , which is about three-quarters of the yield-point stress . The yield-point strain is a little more than 0.1 percent. It is linearly elastic up to a proportional limit of about two-thirds of the yield-point stress. When the steel is unloaded after being loaded beyond the elastic limit, there will be some permanent set or residual strain apparent. Such behavior is not marked, however, until the yield-point stress level is reached. For all practical purposes, when the yield-point stress is reached, steel will deform plastically at no increase in stress up to a strain of 1 or 1.5 percent. At this point, so-called "strain hardening" begins, and again an increase in stress is required to produce an increase of strain. Ductility in steel and other metals is measured by the amount of plastic stretch expressed by the percent of elongation and by the percent in reduction of area of the cross section as stretching occurs.
Ductility in boat steel is of prime importance. Chief among the factors which effect ductility, besides chemical content, are what are known as stress-raisers, generally highly local, sometimes quite small, sharp-contoured notches where loads or stresses of some sort are highly concentrated. While the general area of the stress-raiser may be carrying well within the ability of the metal, the effect of the notch is to focus many times the actual load on a small area. The ductile limit of the metal is then locally exceeded, and a crack develops which then further concentrates the stress. Such a crack, once formed, is usually self propagating, and will progress until the load causing it is released or complete failure of the metal occurs.
Steel is readily available in a wide variety of shapes and sizes. Bar stock comes in rounds, squares, hexagons, flats, ovals, tees, and rectangles, as well as a number of special configurations. There are a host of angle sizes and wide selection of structural beams, channels, and zee shapes. For hull plating, decking, tanks, and the like, there is a considerable range of sheets and plates, of almost any thickness, width, and length that can be transported or handled. In addition, pipes and tubing can be obtained in rounds, squares, and rectangles in a number of weights and wall thickness. Also, treads, gratings, and perforated items are stocked in an assortment of patterns and designs.
Steel is a material which is forgiving of construction errors because they can be easily cut out and replaced. It is also forgiving in that workmanship has to be really bad before it makes the boat really unsafe, not that that is a valid reason for using it as a boat construction material. Steel does not mind living out of doors during construction. If raw steel is used, the weathering will help to remove the millscale before the blasting and priming phase is reached. If Pre-primed steel is used, regular wire brushing at weld and grinding areas followed by the application of a touchup primer will keep the steel in good shape until the balance of the paint system ca be applied. On the negative side, for home building, steel is not a practical material to work with in a residential area. From start to finish of the steelwork phase it is by far the noisiest material. Sandblasting will turn the whole area around the boat into a desert and the same applies to gardens and houses nearby if there is a wind blowing. The use of pre-primed steel will remove this problem.
Obviously, no material, including steel, is the answer to all boatbuilding problems. But steel seems to have an undeserved reputation for having limitations as structural material for small boats. A few of these objections are specific and have a reasonable basis; most are ill-defined; others are prejudices without any basis whatsoever. Hence, an objective summary of the advantages and disadvantages of steel versus other materials is appropriate:Strength
Without doubt, steel is the strongest of all boatbuilding materials, and in a stregth/weigth basis is superior to wood, grp, or aluminum.
Ordinary mild steels have typical tensile strengths of 3,500 to 4,200 kg/cm2 (50,000 to 60,000 psi) and typical yield strengths of 2,100 to 2,800 kg/cm2 (30,000 to 40,000 psi),which is to say they will sustain loads in that order before beginning to stretch or yield without returning to their original position. At the same time, they are very ductile and will elongate 30 to 40 percent before failure. Thus, a steel boat in a collision or cast ashore on rocks may sustain repeated loads or blows that would irretrievably shatter wood, fiberglass, or ferro-cement boats, and yet still be in floating condition when hauled off, though possibly with some bad dents. The high tensile steels are even stronger with typical yield points of 3,500 kg/cm2 (50,000 psi) or more, and ultimate tensile strengths of 5,000 kg/cm2 (70,000 psi) or better. Ductility, of course is less, but still greater than other materials.
There are other measures of strength besides tensile strength. They are: (1) resistance to impact, (2) stiffness, (3) resistance to abrasion, and (4) fatigue. In all of these categories, steel is in the first rank. As already noted, a steel hull will survive grinding and collisions that would be fatal to other materials. Any vessel that is subject to the repetitive flexions of the sea, is subject to fatigue. Fatigue failure generally starts as a small crack, which can then progress rapidly. The fatigue strength of most materials is usually in direct proportion to their ultimate tensile strength. Wood is not particularly prone to fatigue but its fastenings are. Some of the disastrous cracks and splits that occur in fiberglass hulls are in fact fatigue failures.Weight
On the basis of weight alone, steel would be at a considerable disadvantage; as it is the heaviest per unit volume. But when considered as the quantity necessary to attain the desired strength, steel is usually ahead. However, as it is difficult to weld very thin sheet without distortion there is a limit to the minimum hull plate thickness which is 3 mm (1/8 in.) for all practical purposes. For welding reasons, then, the advantage of steel boats under about 8 meters (27 feet) becomes questionable.Life
It has to be admitted that steel has had a bad reputation in some areas; corrosion being the main one. It is, of course, true that some steel boats have rusted through in very short order due to one of two causes - gross neglect or improper techniques in finishing, building, or design. Today, however, with proper finishing and with reasonable care, such as a once-a-year painting and inspection, the life of a steel boat will be indefinite. Corrosive decay is inexcusable in most cases, because it gives plenty of warning and correction is simple and easy.Maintenance
The exterior of steel hulls, properly finished to start, is a matter of once-a-year paint job, usually less tedious than for wood. Above the waterline, only a light smoothing with sandpaper followed by a finish coat is all that is required. Below the waterline, replacement of any loose barrier coat and an application of anti-fouling is needed. The interior, except in the bilges, which may require local touchup with barrier coat once a year, may go a number of years without repainting. Compared to other materials, maintenance is less than that of a wood boat and more than that of a fiberglass.Noise
There are sounds peculiar to steel hulls which are different from those of wood or fiberglass but they are minor; water sounds may at times be more distinct and sometimes the "talking" of marine creatures is transmitted. Motor noises are no more annoying than those in a wood or glass boat and, in some instances, due to the more solid character of the hull, less.Condensation
Any material that may be temporarily or permanently colder than the surrounding air will condense some moisture or "sweat" to some degree. While wood appears to be less subject to this, it has the alternate fault of retaining condensed moisture longer. Good ventilation, a desirable feature in any boat, readily controls condensation.Fire Resistance
Obviously steel, together with aluminum, is more fire resistant than either wood or the plastics. Fiberglass laminates can burn quite readily once enough heat is built up to ignite them. A steel hull can survive a fire that would be catastrophic to hulls of other materials and probably would be still in floating condition after fire has been subdued.Space
Because of the relatively small frames and other structural members, and the wider spacing possible, coupled with the feasibility of building water and fuel tanks integral within a steel hull, a considerable gain in living or working space is available in steel hulls.Limitation of Shape and Design
Hulls with severely rounded or compounded sections, wineglass shapes, and the like are relatively costly. Most metal-hulled boats are deliberately designed to facilitate fabrication and reduce expenses. This is why most metal vessels are of hard-chine design with "developable" surfaces. A developable surface is one that is either flat, a segment of a cylinder, or part of the surface of a cone.Ease of Fabrication
A great many boatbuilders, both professional and amateur, shy away from steel because they are afraid it is difficult or beyond their talents. This is a false premise. Assuming a reasonable set of lines, a steel boat is no more difficult to fabricate than a comparable one of wood or fiberglass. Once the basic principles of steel handling are grasped, the work is fairly straightforward, though perhaps different. Cutting a piece of steel is easier than sawing a piece of oak, welding steel is far less trying than boring thousands of holes in wood, then driving an equal number of screws, nails, or bolts, and finally plugging the holes. Grinding steel is unpleasant and noisy, but it is no worse than the itching that is companion to working fiberglass, or the discomfort of the sticky, messy, smelly resin that must be used. The uninitiated regard welding as something of a mystery, but actually anyone with a sense of touch and distance can do it ably within a few days of practice. Steel is heavy, some of the plates may be well over half a ton. But, a lever or two, a pair of rollers, a simple jack, and block and tackle or chain fall will place the heaviest steel plate exactly where one wants it easily.
On the opposite side, however must be mentioned the dirty, unpleasant, gritty necessity to sandblast any steel hull.
Steel boats are undoubtedly repaired more easily and quickly (and therefore more cheaply) than any other form of construction. The main reason for this is the high local strength of steel, which means that only the area of damage need be removed and new plate welded in. In a wooden boat many planks may have to be replaced and in GRP it may often be impossible to repair the result of serious collision damage satisfactorily because of the monocoque (stressed skin) nature of GRP construction - The strength of any part depends on the strength of the whole. Another factor that influences repairs and costs is availability of materials. Steel can be found virtually anywhere in the world and in the quantities necessary for the average damage repair is very cheap, such that materials will probably form only a minute part of the repair bill. Another factor that may influence yard repair bills is that steel can be worked on immediately and in virtually any weather. No waiting for the area to dry out, as is necessary with wood or GRP, this being especially significant if the repair is below the waterline. This fact will also benefit the amateur who hires a slip on a daily basis to do repairs himself.Ease of Modification
Another distinct advantage of steel is the ease with which existing steel boats can be modified or converted. The dream-boat seeker, therefore, no longer has to find the perfect boat on the secondhand market; he can find one that is nearly right and modify it to suit his own particular needs.Relative Cost
Steel per ton, or by strength/ton ratio, is by far the least costly basic boatbuilding material readily available. In comparison, the cost of wood, aluminum, or fiberglass is relatively high, often five or six times as much. It is true that the unit material cost cannot serve as a proper basis on which to judge the relative cost of any boat. There is no substitute for the meticulous dollar calculation of all the factors involved. Nevertheless, the following general observations are possible. Over the years, good boatbuilding woods have become more scarce, and the labor of skilled workmen more costly. Ultimately, wood will be priced out of the market. The aluminum alloys suitable for boatbuilding are quite expensive; the cost of fabrication, in general is equal to or greater than steel. The cost of fiberglass boats, is competitive only when the boats are manufactured in a given minimum quantity to amortize the cost of the molds. Custom-built fiberglass boats are not economic, particularly as boat size increases.
Steel, on balance, within the limitations defined, is likely to be the least expensive, most durable material available today for boats 9 m. (30 ft) and up, particularly boats produced on a single or custom made basis, or boats that, because of their intended use must be exceedingly strong and which must also be manufactured with a minimum effort and expense.
Aluminum is light, its weight is about one-third of steel. It is corrosion resistant; the oxide that forms on the outer surface of aluminum, unlike rust in iron, protects the basic metal, whereas, in iron the basic metal will rust continuously. Pure aluminum is rather weak and has a very low yield strength of approximately 350 kg/cm2 (5,000 psi). Alloyed some of the aluminums have a yield strength of 6,300 kg/cm2 (88,000 psi). The common alloying material used to increase the strength of aluminum are such elements as manganese, magnesium, copper, and zinc. Basically, the alloys suitable for the fabrication of hulls are:
According to ISO Standards: Al Mg 4.5 Mn or Al Mg 2.5 in H3 condition
According to British Standards: N8 or N4 in H3 condition
According to American Alum. Association: 5083 or 5052 in H3 condition
The above alloys have a yield strength in the order of 2,250 kg/cm2 (31,000 psi) and an ultimate tensile strength of 3,150 kg/cm2 (44,000 psi). Comparing with mild steel, one notices, that yield strengths are comparable and ultimate tensile strength of aluminum is about 80 percent that of mild steel.
Aluminum alloys, aside from being chemically categorized, are further broken down into heat-treatable and non-heat-treatable alloys. Basically, this means that those alloys that are non-heat-treatable do not depend on any heat treatment to achieve their mechanical properties and can be reheated without any appreciable drop in strength. Furthermore, they can be cold-worked with greater ease than any of the heat-treatable alloys. Where heating is necessary, it must be a controlled heat and 400 C is about maximum.
Advantages: The primary advantage of aluminum is its light weight. It extremely easy to work with, requires very little maintenance, is clean, and can be formed, welded, riveted, or bolted. It is superior for the lining of ice chests, food containers, and tanks. Aluminum has a much better scrap value than steel. In 10 tons of purchased aluminum, a scrap loss of five percent would be high. Welding of aluminum is extraordinarily fast. The welding speed is approximately three times that of steel. The preparation of an aluminum hull for finishing is done by sanding rather than grinding, which is also very rapid. It can be cut with hand tools, and any tool that is suitable for cutting wood is also suitable for cutting aluminum. In fact, an ordinary portable power saw with a carbide blade is the easiest way to cut the heavier material, such as plates and shapes. Lighter material can be cut and intricate cuts can be done with a saber saw or a bandsaw fitted with a metal-cutting blade.
Disadvantages of aluminum are sometimes overlooked in the enthusiasm for total application of this material. Throughout the world, the number of yards capable of repairing aluminum hulls are few. Standard marine shapes in aluminum are not available in the varieties that they are available in steel and those that are available are not usually suited for small craft construction. The price of aluminum may be extraordinarily high when bought in quantities ordered on a one-up basis. Aluminum can corrode extremely rapidly if suitable precautions against electrolysis are not taken and maintained. The cost of welding equipment and the more professionalism necessary for its welding have to be mentioned. Aluminum is one of the best heat conductors; hence, condensation is more of a problem. An additional cost in construction should be expected to compensate for this. Fire protection must be considered in some areas, as aluminum has a low melting point. Consideration must be given at all times during construction to eliminate concentration of stress due to improper welding. Aluminum tears and is notch sensitive, so this is a design consideration. Even the hardest alloys can be easily scarred. Aluminum can be water-stains easily, if the protective finish or the bright finish as it is delivered from the mill is to be retained, proper storage of the material is a requirement.
Wood is relatively easy to cut and shape and almost everyone has some basic knowledge and experience of working with it. It is aesthetically satisfying to work with because of its inherent beauty. Its stiffness, light weight, and resistance to fatigue give wood advantages over other materials for boatbuilding.
Besides its advantages, there are also some well-known disadvantages. Specifically, wood is subject to rot. It also shrinks and swells with moisture and temperature changes. And it looses some of its strength and stiffness after absorbing moisture. The root cause of these problems is the passage of moisture in and out of the wood cells.
To a very great extent, the use of wood/epoxy system overcomes the above problems. Boats built in this system have all of their joints bonded with, and all of their surfaces encapsulated in epoxy resin. Thus every piece of wood, both inside and the outside of the hull, is covered with a barrier coating of epoxy resin through which no significant amount of air or water can pass. The result is that the moisture content of the wood is stabilized. This stabilization in and of itself means that there will be little shrinking or swelling of the wood. The level at which the stabilization occurs, and at which it remains, ensures a continuation of design strength and stiffness. In addition, it prevents rot not only by stabilizing the moisture content, but also by restricting the oxygen supply to the wood surface.
Wood Strength: Wood, of course, is not a single material a fixed set of mechanical properties. Rather, there are many species, possessing a wide range of properties. Unlike metals which are isotropic, wood is anisotropic. A select grade fir displays 1,000 kg/cm2 (14,000 psi) tensile strength along its grain direction while 20 kg/cm2 (300 psi) tensile strength across its grain.
Resistance to Fatigue: Fatigue is an accumulation of damage caused by repeated loading of a structure. For boats which are subject to the repetitive flexions of the sea the fatigue characteristics of the material is as important a design consideration as its ultimate strength. Wood displays excellent fatigue characteristics. Given an equivalent number of load cycles; a wood structure stressed at 60% of ultimate strength would function as long as a fiberglass structure stressed to 20% of its ultimate strength or mild steel structure stressed to 40% of its ultimate strength.
Wood Stiffness: Panel stiffness is an important consideration in boatbuilding. The stiffer a structure, the less deformation it undergoes with a given cyclic load and, therefore, the less cumulative fatigue damage it occurs. Wood, by its very nature, has an excellent stiffness potential.
Moisture content of wood is expressed as a percentage of the weight of the wood perfectly dry. The moisture content of wood varies according to the relative humidity and the temperature of the atmosphere that surrounds it. For every combination of temperature and humidity, there is a moisture level that wood will seek, and it will either absorb or dispel moisture until it reaches this equilibrium with the air conditions. At room temperature of 21 C (70 F) for relative humidity values of 40%, 50% and 60% the equilibrium moisture contents of wood are respectively 8%, 9.5%, and 11.5%. As temperature decreases the equilibrium moisture content also decreases. When using wood in composite with epoxy resin, it is important to only use wood that has a moisture content of 12% or less, and 7% to 10% seems ideal.
Dry Rot is a misnomer for fungal growth. Although there are many types of rot fungi, there are primarily two species in the brown rot family that attack wooden boats. In order for these two species to exist, there must be four conditions present in the wood:
1) the moisture content must be at or near the fiber saturation point (about 25%), rot is unknown in wood with a moisture content below 20%;
2) there must be an adequate supply of oxygen;
3) the temperature must be warm, 24-30 C (76-86 F) is ideal; and
4) there must be proper kind of food.
Most of the commercial wood preservatives use the approach of poisoning the food supply, however, in boat hulls poison seems to dilute rather rapidly in presence of moisture. As already mentioned, encapsulating the wood in epoxy resins, stabilizes the moisture content at a point much lower than the saturation point and eliminates the oxygen supply.
Plywood: As already mentioned, wood is anisotrpic; it has very little tensile strength across its grain. To overcome this shortcoming, and to increase its strength and stiffness in relation to its weight and volume, the practice of cutting wood into thin sheets and then bonding the sheets together with the grain going in perpendicular directions in alternating layers is used. The individual sheets, before they are bonded to each other in layers, are known as veneers. Veneers come in a variety of thicknesses, in a variety of quality grades and from many species of wood. A bonded lay-up of at least three plies of veneers is known as plywood.
Only marine grade plywood should be used for boatbuilding. (Conforming to BS1088 or BS6566). Exterior grade plywood uses the same glue as marine ones but in marine grade only a minimum of voids are allowed in the core and in general better quality veneers are used. In most cases, it has more plies for the same thickness that makes it stronger, but also more difficult to bend.
Douglas fir is extensively used in manufacturing plywood in the US using rotary cut veneers. Douglas fir has annual growth rings that are part early and part late wood, and the early wood is softer than the late wood. As the veneers are rotary cut, the surface presented is an irregular maze of hard and soft areas that are difficult finish. Sanding tends to "dish out" the softer early wood areas, leaving the late wood areas high, irregular ridges. The more you sand, the worse the problem becomes. Douglas fir is a very strong wood. The reason so much of it ends up in plywood panels is that it has low grain strength for its weight and splits under nailing. Crisscrossing the grain of veneers into plywood panels overcomes this problem. However, Douglas fir also has a high expansion-contraction rate (7.6% tangentially, the direction of rotary cutting) that tends to set up stresses within the finished panel. Complicating this problem is the fact that heartwood and sapwood may be mixed in the same panel, causing warping and cracking to occur.
Plywood panels of the mahogany type built of veneers okoume, luan, meranti are generally more desirable for marine construction. All of these species easily take a natural handsome finish and have low expansion-contraction rates. These species are all tropical and have no annual growth rings; thus they are free of the early wood - late wood problem. There are excellent quality mahogany-type plywood manufacturers in Netherlands, United Kingdom, Germany, Greece, Israel, South Africa and other countries.
Epoxy Resins: Epoxy resins are high performance thermosetting resins that contain at least one, but usually two or more, epoxide groups per molecule. The three leading producers of epoxy resins are Shell, Dow, and Ciba-Geigy which account for approximately 70% of the world's capacity. In wood/epoxy system, epoxy resin is used to bond and completely seal the wood structure. Epoxy resins are provided either as a large number of premixed and prepacked products or more simply in the form of a base resin with fast and slow hardeners plus various fillers and additives to be used by the builder to modify its viscosity and other properties. The pot life of resin/hardener mixture changes with temperature and the type of hardener used, at 21 C (70 F) it is usually between 10 to 30 minutes and partial cure time is 5 to 9 hours. With slow type hardeners it is possible to extend the pot life up to 3 hours. Epoxy resins are almost 100% solids with no evaporating solvents. The base resins used for boatbuilding have low viscosity for superb penetration and wetting out of wood. They are resistant to saltwater, freshwater and oil. The base resin is not UV-resistant. The surface of the resin is, in most cases, self leveling. The greasy film that is sometimes left on the surface can be cleaned off with soapy water. Mechanical properties of epoxy resins vary widely; representative values are as follows:
Compressive Strength: 20,000 kg/cm2 (284,000 psi)
Tensile Strength: 8,000 kg/cm2 (114,500 psi)
Elongation Break: 10%
Modulus of Elasticity: 280,000 kg/cm2 ( 3,975,000 psi)
The accurate proportioning of the resin and the hardener is important. If prepacked products are not being used, this can be achieved with three different methods. The weight method requires a small scale that weighs accurately between 0 to 500 grams.(A larger batch should not be mixed.) First weight the mixing cup and then pour the resin and hardener into it in the specified ratio. The volume method of measure can also be used, taking into account the different specific gravities of the resin and the hardener. A convenient method is to use a perfectly cylindrical mixing cup and a measuring stick. On the measuring stick two lines are marked which will give the specified ratio - a resin line and a hardener line. Insert the stick into the cylindrical can and pour the resin to the first line on the stick and add the hardener until the second line. The third method is to use a dispensing pump provided by most suppliers. They dispense the exact ratio of resin to hardener in whatever quantity needed, cutting waste. Once properly proportioned the resin and the hardener must be stirred thoroughly, always in a cylindrical pot, moving the stir stick around the outer rim of the pot vigorously until they are well mixed. Immediately after stirring, an exothermic reaction begins which will ultimately lead to the final cure. During this reaction, you can take steps to increase the pot life by diffusing the heat as it builds up. The basic principle is to increase the surface area of the mix by pouring it into a large, flat roller pan. Another method is simply to mix smaller batches that will not have the necessary volume to produce significant heat.
Ferrocement is a highly versatile form of reinforced concrete, constructed of hydraulic cement mortar reinforced with closely spaced layers of continuous and relatively small diameter wire mesh. The mesh may be made of a metallic or other suitable material. Ferrocement primarily differs from conventional reinforced or prestressed concrete by the manner in which the reinforcing elements are dispersed and arranged.
The American Concrete Institute (ACI) Committee 549 defines ferrocement as:
"Ferrocement is a type of thin wall reinforced concrete construction where usually a hydraulic cement is reinforced with layers of continuous and relatively small diameter mesh. Mesh may be made of metallic material or other suitable materials."
A good ferrocement boat is a very good boat. Strong, tough, not easily damaged and easy to repair holes in. It lasts a very, very long time. A poorly built ferrocement boat, however, is not a great prospect in any way. Internal corrosion, rough surface, poor concrete quality are usually the common problems. Ferrocement boats are suitable for amateur construction and being labour-not-material-intensive are usually much cheaper than those built from other materials. Lends itself to the production of one-offs with no limit to the design with freedom to adopt curved shapes. Weight is a main disadvantage, especially in smaller boats. Stress cracking and flaking is sometimes also encountered, particularly in tropical climates.
GRP methods are simple and well within the scope of any amateur. They can be used for the building of all types of boats and yachts from 3 meters to 25 meters.
A mold or core is needed on which layers of fiber reinforcement are laid and saturated with resin to build up the skin-laminate.
Molds cost money and much time to make. Commercial manufacturers, building series of the same yacht can afford the cost of expensive negative or so called female molds.
The amateur, however, has no further use for it when the hull has been made, and has to keep down work and cost. Amateur builders, in search of a cheap way of molding, build their hulls mostly on Positive or male molds that are much simpler to make. The male mold is made upside down, just like building a wooden boat. Inexpensive second hand demolition timbers are generally used. The technique to be followed depends on the GRP method of construction.
The handling of the resin and glass material is not difficult, it is merely a matter of carefully and exactly keeping to the manufacturer's directions. Another and may be the most important condition for success is an absolute dry workshop and constantly keeping up the temperature prescribed.
Cutting the fiberglass material in patterns that can be easily laid over the mold, may give problems if it has not been tried out with sheets of paper before. A tabletop or clean floor of sufficient length is needed to roll out the glass for cutting.
GRP which is fully cured must be correctly prepared before any new laminating takes place or a proper bond will not be produced. The surface must be thoroughly cleaned and roughened or the new laminate will break loose.
Most smaller GRP yachts have a single-skin laminate hull construction, which consists of a few layers of saturated glass material. If this construction is used for bigger boats some sort of cross or longitudinal reinforcement is needed like wooden stringers or cores of foam or other materials that are molded in at the inside of the hull.
The mold consists of a series of building frames set up at regular intervals on a strongback, provided with a transom and a stem-support. Battens bent over this structure from stem to stern are forming the exact model of the hull. Usually, a thin layer of rigid foam sheeting is stitched over the battens. Instead of foam thin straps of diagonally laid plywood are sometimes used. Both foam sheeting and plywood surface can be easily sanded fair and smooth.
Before starting on the laminate the foam or wood surface is covered with plastic sheets to prevent the laminate from sticking to the mold.
When the hull laminate has been finished the surface is sanded down and can be further smoothed with filler and a suitable type of paint.
Once this has been done the whole is turned over or hoisted to an upright position and the molding parts can be broken out. Keel reinforcements, shroud mountings or other interior parts like bulkheads, etc. have to be molded in before the inside is finished with a suitable coat of resin or paint.
This method is specially suited for building just one particular hull and used by both amateurs and professionals. The advantages of it are the stiffness of the hull, whereas the good insulation is also an asset.
The basic idea is first covering the mold with either foam or balsa core on which the outside laminate is built up. After bringing the hull in upright position and removing the molding parts, an inside laminate is put in on the inside surface of the core.
For core material PVC foam or balsa wood mats may be used. PVC foam must have a certain degree of density and be of the type that is resistant to attack by the resin. A well-known brand name is AIREX. The foam sheets are mostly stitched to the mold battens. The foam can be faired and shaped with sandpaper in all forms wanted, such as skegs, rudder blades, etc. Balsa wood core has more compressive strength and is made of end-grain wood that are joined together in a flexible way forming some sort of mat that follow the curve of the hull surface. Compressive strength of the core in sandwich construction is important as the laminates are thinner than that of the single skin.
Stem, keel, and other parts of the hull can be reinforced by molding in pre-shaped pieces of foam-core or wood before the inside laminate is built up.
Horizontally laid strip planking is sometimes used for core material. A lightweight wood with straight grain, such as redwood or western red cedar is the best. The planks are glued together and nailed. Once the shell is completed and sanded flat, the outside laminate is built up on it. In this case special unidirectional glass fiber roving of which 80 % of the strands run in the length is applied diagonally. A second layer of roving is then applied at 90 degrees to the first. It is positioned without overlap and is continuous from board to board so that the shell remains fair. The continuous glass fiber roving leads to an unbelievably strong hull.
The recent advantages of computer lofting and computer hull development have made this building method ideal for those who wish to build a large chine hulled fiberglass boat.
The Ezi-Build technique is a mold construction process that provides a simple way to build a female mold for a one off or limited production boat.
The advantages are, no preliminary plug is required as with most female molding techniques, and no sanding or fairing is needed as the hull is laid up inside the smooth Ezi-build mold.
The key to building the mold is a set of computer generated developable shapes. The term developable means the plates or the panels can be cut from flat sheet and simply laid into place as a lining in the mold structure. There are no compound curves and shape of each panel is such that it fits into place with ease.
Wooden transverse female frames are set up at regular intervals to support a system of 5cm x 1.5cm battens laid lengthwise which in turn accept the plywood panels. These panels are attached to the battens with contact glue so as to minimize any nail holes that would make for extra filling and fairing. The inner surface of the mold is next given three or more coats of non-silicone wax and the mold is now ready for laying up the fiberglass hull.
The decks are built in sections over a simple reverse camber panel mold. The superstructure is laminated either in panels on a flat surface or laid up in simple box molds.
The full text and graphic illustration of the Ezi-Build Fiberglass building technique is available from Bruce Robert's Designs .
Steel and aluminum enjoy a growing popularity.
They allow the building of individual one-off yachts without the need of expensive molds or machinery. The construction of parts such as rudders, engine bearers, etc. are simple; fittings like chainplates, pulpits, liferail stanchions can be simply welded to the hull. Balast, scrap iron or lead, can be simply put into the hollow box-keels saving the costs of molding and foundry. Bulkheads and floors can be made watertight. Water and fuel tanks may be an integral part of the hull structure contributing to the hull's strength. Engine vibration can be minimized.
The Dutch have led the field when it comes to constructing small boats from steel.
Boatbuilding in steel and aluminum involve the same techniques. The main difference being aluminum demands more expertise in its welding.
Before going into different methods of putting a steel boat together, it is proper to give some attention to both welding and cutting techniques. There are three basic cutting tools for steel; oxyacetylene torch, the "nibbler," and the cutting disk in the grinder. For aluminum, most woodworking tools may be used either directly or by changing the cutting blade.
Oxyacetylene Torch is a highly developed and precise instrument that will cut metal quickly, neatly, and inexpensively. Furthermore, some of the shapes are beyond the practical capacity of other cutting equipment. It also has a number of other uses beyond cutting, such as for local heating to facilitate bending, stretching, or compression; for piercing, scarfing, and expanding metal; burning off scale and paint; brazing; soldering; and more rarely for gas welding. With little practice, there is no problem maintaining a cut to a scribed line or within 0.5mm (1/64") of it. When completed, the underside of the cut will usually have a small amount of slag hanging from it. This can be removed simply by stroking it with the flat blade of a chipping hammer or with the edge of a grinding wheel. With proper cutting on clean work, to carefully laid-down scribed lines, very little subsequent grinding, filing, or other conditioning should be necessary before assembly. However, it should be remembered that the mere act of cutting produces heat. Heated metal expands and then contracts when cooled and this may locally alter the stress patterns inherent in the metal. Cutting too much at once or cutting parallel or adjacent surfaces while they are still hot can produce distortion. A good general rule is to allow newly cut metal to cool to room temperature before beginning additional cuts. Cutting, because of progressive contraction of the cooling metal immediately adjacent to the cut, will sometimes curve or "dish" a flat piece of metal. This may be common with thin-gauge stock, 3mm (1/8") or less. Such dishing may be corrected by lightly tapping the edges with a hammer, thus locally expanding the recently cut metal until the plate flattens out. If gas welding is not intended, then, propane that is cheaper then acetylene may be used.
The Nibbler for the amateur is a much better bet than gas for cutting steel plates. A sort of electric tin opener, it cuts plate up to 6mm (1/4") thick very rapidly without distortion. It is extremely accurate and leaves clean edges ready for welding.
The Cutting Disks are probably marginally more expensive than either gas or the nibbler and it cannot be used to cut out tightly curved shapes.
Welding is a very important part of the job. Bad or inexperienced welding does not only create deformations with humps and hollows but it also can lead to weakened welds by integrated rust and scoria particles. To the novice it may seem to be a very abstruse and mystifying technique. As in many other industrial arts, good practice is based on a series of relatively simple but important principles. The home builder who does not already posses welding experience, may learn the necessary techniques and practice without too much difficulty, but, of course, to have the welding done by a skilled electrical welder is another option worthwhile considering.
For practical reasons, as well as expense, most steel welding is done by electric-arc stick-type coated electrodes. For welding of aluminum, however, one or other of the shielded gas welding process must be used. The basic principles that should be observed in all welding systems are as follows:
1. The composition of the weld metal laid down must be compatible with the metal being welded.
2. The metal to be welded must be clean, free of extraneous substances, such as other metals, slag, scale, rust, paint or chemicals.
3. The metal to be welded should be fitted so that full penetration of the weld zone is possible, so that no voids or hidden cracks or other stress-raisers are created. The proper spacing for good welding is governed largely by the thickness of the metal involved. As a general rule of thumb, thin material 3mm (1/8") or less, can be spaced a distance about equal to their thickness, or slightly less. For thicker material up to about 10mm (3/8"), it is desirable to bevel one side of the joint. In heavier cross-sections, both sides should be beveled and several passes should be made to fill the beveled joint. When multiple passes are required, through slag cleaning is necessary between each pass to avoid slag entrapment.
4. The method of laying down the weld metal should be such that (a) there is adequate heat to melt and fuse both the weld metal and its joint, (b) the direction of welding is such that there is full penetration of metal to the root of the joint, (c) that the weld is free of entrapped slag, gas bubbles, or other porosity, (d) that the shape of the final weld bead is such as not to promote notch-stress failure or have an excessive contour that will be unsightly or require excess grinding for appearance.
5. The sequence of welding should be such as to minimize or eliminate undue distortion. Shrinkage caused by the contraction of weld metal from the molten state to the solid state is a prime concern and is the main reason why so many steel boats are unnecessarily warped and will "look like a hungry horse". While an individual weld may shrink only a mere hundredths of a millimeter or so upon cooling, the stress created by that shrinkage can be enormous. The cardinal rule in small boat building is that at no time are continuous welds are to be made. The welding is staggered in a sequence such as to oppose and counteract each other. This can be achieved in large part by never laying down more than 5 to 7.5 cm (2" to 3") of weld metal at a given point, by never welding adjacent to these points until they have cooled, by welding adjacent welds in opposite directions so their directional stresses and shrinkages are canceled, and by welding opposite sides in the same manner. In general the hull should be welded from midships out towards the ends, alternating between port and starboard sides.
It should also be a fixed rule not to overweld a boat. A boat should have every weld necessary to insure its integrity under all conditions, but no more. Continuous welds should be avoided where intermittent welds are adequate.
The trickiest job in steel construction of hulls is to get the plating fair and smooth.
Welding of aluminum should be done indoors or under the protection of an efficient mobile enclosure, since, shielded gas welding is necessary. Any draft that passes over the weld area during welding will blow away the shielding gas and cause welding problems. In general, good welding practice is of far greater importance than it is with steel. Weld area preparation and cleanliness of the weld zone cannot be stressed too highly. Even small amounts of contamination from sweaty hands or dirty gloves, release hydrogen and other gasses during welding that become entrapped in the weld deposits, causing porosity, which in turn, will affect the strength and ductility of the weld with consequent cracking. The melting point of pure aluminum is 650 C, whereas, the melting point of aluminum oxide is 2038 C. So, unless the oxide is properly removed or broken up before welding, the aluminum will melt long before the oxide film melts. The entrapped oxide can again cause a reduction in weld ductility and form metallurgical notches or cold laps. Degreasing can be done with such solvents as tauol or taulene. Pencil identification marks can be removed with acetone or alcohol. Wire-brushing by hand with a stainless steel wire brush is sufficient to break up the oxide. Mechanical cleaning of aluminum via power wire-brushing or grinding is not advisable because of the low melting point. All cleaning must be done just prior to welding. TIG welding is usually used for small jobs where the welds will be visible in the finished product. All other welding in hull construction is done with MIG .
Sandblasting also should be given some attention as it may mean the difference between a trouble-free boat and one that that can be a perpetual headache or an unsightly "rust bucket". In the as-fabricated, as-welded condition, steel hulls are covered with an oxide scale, dirt, grease, and an assortment of other contaminants. These must be removed and a spotlessly clean surface exposed for subsequent finishes. Failure to do this will inevitably result in flaking paint, bleeding scale, and ultimately severe local or general deterioration. Cleanliness in this sense means that it must be sandblasted clean so that every square cm of it is down to raw metal unsullied by the slightest speck of rust, grease, dirt, or scale so that every nook and cranny is bright, every crevice devoid of entrapped soil, and every trace of contaminant gone. There is no other reasonable way to achieve this except by sandblasting. The mechanics of it are simple enough: sharp silica sand is blasted from a tank equipped with flow and pressure controls, or sand is sucked up from an open container such as a large bucket and than blasted. The air is provided by a standard air compressor of generous capacity and the sand is fed by a hose from the tank or container through a metal-carbide or ceramic nozzle from which it is directed at the work. Sandblasting should be confined to an area that can be completely protected with subsequent basecoat within four hours, preferably less.
There are probably as many different methods for putting a steel boat as there are builders, but it is possible to identify the basic structures as follows:
Aesthetics, are of course, a personal thing, and there are many who are attracted to the single-chine shape for metal boats. Single chine has the merit of being a very simple form of construction, permitting the use of mostly flat plates. When building a boat using sheet material, it makes the most sense to think in terms of that material's characteristics and how one may optimize a hull design without incurring too much extra labor. In metal, a single-chine hull is easier and less costly to build. For sailboats, the slight gain in the wetted surface can be offset by slightly greater sail are, made possible by slightly greater ability to carry sail due to the form stability provided by the chine. For powerboats, no doubt, the form will attract more supporters.
The term multi-chine refers to the situation where two or more chines are introduced to the design, the chines adding considerably to the boat's strength. The increased number of chines will enable the use of less acute angles and correspondingly there will be less likelihood of turbulence. The increased number of chines will help to produce a more curvaceous shape and especially sweeter topside curves without requiring that the metal plate be rolled. Additionally, it will be possible to create a deeper bodied vessel, thereby giving greater headroom. The multi-chine form allows the plates to be kept to a reasonable size, and will assist with the ease of handling as there is no temptation to use the large plates that would be used on a single-chine design.
The technique was used by Van de Stadt on steel Doggers and has also been used by Ted Brewer and others in North America for years and more recently is being used by Bruce Roberts extensively. Again, computer aided design have made a great impact on this technique.
Radius-chine hulls, essentially a single-chine hull form employing flat panels everywhere but the strip that joins topside to the bottom, rendering a curvaceous hull without requiring that every metal sheet be rolled. In this way, superior appearance, strength, lighter weight is attainable not to mention a much higher resale value.
The choice of the radius to use is a matter of personal choice. Too small a radius keeps most of the radius below waterline when the hull is at rest and causes a slab sided look. The radius section is best plated first, keeping the edges neat and trim to a fair line so it will be easier to match up the flat plating later. In the areas of bow and stern, the radius panel will go on in one piece. Midships it may be necessary to split the radius panel lengthwise and then trim the centers where they overlap. This will take care of the compound curve in this area. The bottom and topside panels will fall into position without any special bending or forcing the plates.
A detailed treatment of the subject is given in the article Radius Chine Metal Boatbuilding by Dudley Dix.
The traditional round bilge hull feature heavily curved plates at the turn of the bilge. These plates need to be rolled to just the right curve. This is a job for the boatyard. Recently, however, it is becoming possible to order precisely cut and formed plates.
Following the hull form, another major decision that the metal boatbuilder will need to make is whether to build hull upside-down or upright. The main advantage of building upside-down is that it is easier to fix the plating in position on the upturned hull. Some advocates of the upside-down method also argue that most of the welding is then done in a downhand position. However, it must be remembered that when welding the stringers and frames to the plating all of this welding is uphand and it means crawling around inside the inverted hull, which is most awkward when one considers all the bracing that is needed to hold the structure true while this welding is proceeding. Building the hull upright offers easy accessibility during the entire welding operation. The disadvantage of laying on the plating can be largely overcome by using adequate scaffolding. Also there is a little trick of drilling a whole in the plating and pulling this up into position by means of chains, wedges and threaded bolts. A point to remember is, of course, that when the hull is built upside-down there is still the turning over operation, and one saves on this building upright.The building of a metal framed hull may be summarized in the following steps:
Frameless Fairing Technique
A basic design principle is that as the hull plating is thickened frame spacing increases. Carried to its logical conclusion the plate thickness is increased, and the frames spaced ever further apart, till the first frame is the stem and the next is the transom. This is the frameless boat known in other technologies as monocogue construction. However, one must hasten to add that the weight saved by doing away with the frames is not enough to compensate for the added weight of plating. To avoid framing completely brings an enormous weight penalty, hence, the least amount of framing is always there sometimes in the form of bulkheads, interior furniture or other features. Therefore, to avoid any misunderstanding the term "frameless construction" may cause we prefer to use "frameless fairing" instead. The term is more apt, because, incredible fairness is obtained by using full length plates to plate a multi-chined hull without the aid of internal framing.
The plates, computer calculated, can be drawn and cut in advance to millimeter accuracy. All reference lines for bulkheads, floors, engine bed, keel position and reinforcements can also be simultaneously marked on the plates. Because the plate edges match perfectly, putting them together is extremely easy. A simple trestle is sufficient in which to build. Starting with the bottom plates, plates are tack-welded one by one up to the top shell sections. Due to the great accuracy, a shell with fair lines is formed without frames. The strengthening of the shell by welding in frames, floors, engine bed, chain plates, etc. is done later.
There are various plans in the 9m - 12m range using this technique offered by Van de Stadt Design .
Since the beginning of time, wood has been the traditional boatbuilding material. Ancient ships, and until the last century, trading and naval vessels were constructed of wood.
Carvel has always been the most common form of wooden construction. Generaly, a skeleton of steam-bent oak is formed to support planks from stem to stern. The seams between planks are fitted with chaulking to make the structure watertight. Clinker/lapstrake construction, common for small boats in the past, is a method where relatively thin, shaped planks overlap each other at the seam. Mechanical fasteners (often copper rivets). Modern wooden boats are, however, more often built in wood/epoxy.
The wood/epoxy system can be successfully combined with various methods of construction, from plywood to strip plank and fully cold molded, either with or without stringers. The following basic woodworking and resin application techniques are common to all construction methods.
Scarfing:It is difficult to find lumber and plywood in the lengths desired for boat construction and when it is possible, the extra cost, both for material and shipping, is usually far too high to consider. Thus, it is very basic to the boatbuilder's craft to develop a skill called scarfing. It is the procedure used to join boards, plywood panels or veneers to achieve whatever length of stock is desired without any increase in the width or thickness of the material at the joint. The most common scarfing procedure used in boatbuilding is called the matching bevel. Each piece of stock to be joined has a bevel of the same degree angle machined on one end. These bevels are fitted and then permanently joined using epoxy resin. Usually, an 8 to 1 ratio of stock thickness is the minimum bevel considered adequate. The main object of the bevel is to develop enough surface bonding area to exceed the strength of the wood itself.
Bonding and Laminating Techniques: Bonding with the epoxy resins is essential to wooden boatbuilding and usually the following technique is used: The low viscosity base resin mixed with the chosen hardener is applied by brush to both surfaces. The resin is allowed to soak into the wood and left to cure for some hours. Then more resin (usually with a filler such as microfibers or coloidal silica) is mixed and applied with a spatula to both surfaces and they are brought together and compressed. The filler produces a thickened mass to bridge large gaps between two mating surfaces to form a 100% bond within the joint. After it has cured the bond will be very strong, usually stronger than the surrounding wood. The resin is a very good glue and will join other materials as well. For example, under right conditions wood can be stuck to aluminum.
Frames, ribs, keels, stems and deck beams are structural elements that are usually made by laminating several pieces of lumber together to form the finished parts. The basic reason for laminating these items is to achieve the desired shape of the particular parts as exactly as possible. Traditionally, these shapes were formed by steam bending, by using wood with a natural curve similar to that desired, or by sawing curves on various segments of straight stock and joining them together with fasteners to form the shape desired. Laminating wood has the advantage of providing high strength with low potential for failure due to defect. Any one piece of wood may be compromised with a defect, but if several pieces are laminated together, a defect in one of the pieces is only a minor problem. Furthermore, a curved lamination made up of many thin pieces of wood relieves most internal stresses prior to the bonding of the parts into one unit. Laminating, can be an economical use of material. You can make use of the short lengths of stock by incorporating scarf joints within the lamination. Choosing the proper thickness of the stock to be laminated depends totally on the radius to which the lamination must be made. Even when laminating is not needed to achieve a desired shape, it is good practice to laminate when the stock needed exceeds 2.5 cm in thickness.
Fillet Bonding has become one of the most versatile wood bonding methods, especially for joining parts at or near right angles to each other. Basically, a fillet is a continuous bead of thickened resin mixture applied to the angle between two parts to be joined. It increases the surface area of the bond and serves as a structural brace. Filleting requires no fasteners of any kind and can result in a joint that is as strong as the parts being joined together. A fillet is especially effective when joining parts that meet at difficult angles. A good example would be a bulkhead that meets with the hull skin at varying angles along its perimeter.
Coating: The saturation coating of all wood surfaces is usually done by using resin with no additives. The final shaped and fair wood surface should be as dry as humidity conditions will allow so that a higher than normal moisture content will not be sealed in. If controlled environment is not available, then drier weather conditions to occur may have to be waited for before the saturation coating is applied. Obviously, the surface must be free of any foreign elements such as oil, dirt, or any other substance that would prevent a proper bond from taking place. The most practical method for application of epoxy resin as a coating is the use of the high-density urethane foam roller cover that is approximately 3 mm (1/8 ") thick and bonded on a stiff fiber backing. Other than for touchup, smoothing out or reaching places inaccessible to a roller, the brush is not suitable. The main difficulty with it is to achieve an even and consistent surface coating over a large area. The amount of resin and depth of penetration that takes place is affected by a number of variables. The first and most important is the actual density of the wood itself. Softwoods are more absorbent than hardwoods. Secondly, end grain will absorb much more resin than flat grain. While applying the first saturation coat, often small air bubbles continually appear on the surface. This is caused by the resin filling void areas which were formerly inhabited by air. Working with cold resin applied in a heavier than usual thickness, difficulties may develop in this transformation. The obvious solution is to work in a heated area, but if that is not possible, heating the resin prior to use or applying a heat source to the surface with a heat gun or a heat lamp, after the resin is applied will help. During the initial saturation coating process, those areas that begin to look dry as penetration is taking place should be recoated. A number of coats may be necessary in end grain areas. After the initial saturation coating has cured, the surface is usually somewhat fuzzy and rough because the wood grain has expanded slightly from resin absorption. Lightly sanding removes high spots in preparation for subsequent resin coating or fiberglass application.
Use a foam sponge roller to apply the second coating of epoxy resin. The important goal now is smoothness of the coated surface. Roll the resin in several directions using vigorous long rolling strokes. A minimum of two coats - one saturation coat and one secondary coat - to every surface within the boat structure are recommended. For surfaces that will require considerable sanding, a minimum of three or more coats are recommended. Pigments may be added in secondary coatings to provide a visual control of the film thickness, to prevent excess sanding or to provide protection to the base resin from ultraviolet rays. Sometimes, a slight wax-like substance appears on the cured surface that is a byproduct of the resin and hardener reaction. This residue is water soluble and easily removed. To eliminate any potential contamination problems between coatings, it is sometimes easiest simply to recoat while the previous coating is still in its "green" state. The disadvantage of this system is that no sanding can take place in between coatings, which may cause the final surface to be rougher than desired.
General sanding of the hard resin surface can be a lot of hard work.
Glassfibre Fabric Application: Glassfibre reinforced coating provide several benefits. The first is increased abrasion resistance. The second is that it acts as a screed to guarantee a minimum coating thickness on the wood surface. Also, it can contribute strength and stiffness to a boat structure in some situations. During the final finishing process of laminated veneer hull exteriors, to fill the many hundreds of tiny staple holes may present a real problem. A lightweight fiberglas fabric helps as it acts as a wicking agent that continually transports new resin over the top of the staple holes. When Douglas fir plywood is used, it will almost always check unless it is sheated in fiberglass fabric. Just plain epoxy resin coating generally will not be sufficient to prevent it.
Fiberglass materials intended for use with polyester resin are not suitable for use with epoxy resins as they may contain binders that dissolve in polyester styrene but not in epoxy. Unless they are specifically manufactured for use with epoxy resins, they may float in the resin without absorbing it.
To achieve a generally smooth surface over which fiberglass fabric can be applied, the saturation coat depressions and voids should be corrected with a puttying application and subsequent sanding. Whenever possible, the fabric should be laid on the surface with the dry method. This involves applying the fabric without any resin. Masking tape may be used to hold it in position. The advantage of it is that there is no rush with the squeegeeing process because resin is poured in a small area and the squeegeeing is completely finished before moving on to another section. The resin never gets a chance to start a cure before the squeegeeing is finished, as it does with the wet method. However, on vertical surfaces or overhead positions, the dry technique will not work and wet method will have to be used. First a light coat of resin is applied to the surface. Then the glass is laid into this wet resin and positioned by hand. It is usually best to apply the fabric directly from a roll instead of trying to position one large unrolled piece of glass over an entire surface. One person handles the roll of fabric, slowly unrolling it unto the surface, while two more people immediately position the fabric and smooth out the wrinkles. As soon as any part of the fabric is in place, more resin is applied, wetting out the glass completely and removing any air, using the squeegee. In either system, the squeegeeing process should completely saturate and fill the fabric with enough resin to properly wet it out, while removing all excess resin that might tend to cause the fabric to float up from the bonding surface.
Either single or multi-chin shapes are in fact one of the most suitable construction forms for amateurs with a limited skill. The use of epoxy resin saturation and fiberglass sheating has renewed the interest in plywood construction which is the closest rival to steel for speed of hull and deck construction and it continues to be the most economical material for building a boat.
The conventional method is to apply plywood panels over a frame and stringer assembly. For frames a standard size stock is usually used (2.5cm x 7.5cm - 1" x 3" is typical). The dimension of the stringers and chines that will be notched into the frames is an important point in determining the frame stock dimension. A good joint system for frame joints at a chine or keel is to simply butt fit the two frame pieces together and then covering each side with a gusset of plywood. Either clamps or staples may be used to provide temporary pressure while bonding the frame pieces. It is a good practice to perform all possible saturation coating operations on the frames prior to set up. Setting up of hard chine frames are easier as the chines serve as easy reference points. Fairing is also considerably easier than fairing round bilge hulls. Following the setup and final fairing of the frames the stem, keel and transom is installed. A chine log is an important structural element in hard chinned plywood hull. It gives additional structural support to a potentially vulnerable part of the hull area. It also provides a large enough bonding surface area to join two pieces of plywood adequately. The resultant sharp edge at the chines is an obstruction to hydrodynamic flow and this is one of the major disadvantages of the hard chine concept. Rounding the sharp edges minimizes the problem. Fiberglass tape reinforcement on the outside is often used to provide a large radius without compromising the strength of the joint. With smaller boats, it is common to apply a complete prescarfed panel over one section of the hull in one bonding operation. With larger boats, it will be easier to install smaller panels and perform the scarf joint assembly right in place on the boat hull. To bond the panels to the framework use an adhesive mixture with a viscosity slightly thicker than heavy syrup. If there are large voids to fill, a thicker, peanut butter consistency may be needed. Apply enough adhesive to all joints so that a slight excess squeezes out. A total, 100% bond is desirable to guarantee structural integrity. With precoated panels, the adhesive may be applied to the framework surface only. After successfully bonding one panel to the framework, excess adhesive should be cleaned up immediately. Performing final fairing is the easiest part of the hard chine construction. If the framework was a fair one to begin with, plywood should follow this fairness precisely, producing a smooth, even surface. Most of the fairing work will be concentrated in the areas of the chines, keel and splices where two panels of plywood have been joined.
An even faster and easier method of building a hard chinned plywood boat is the stitch and glue technique. There are four steps in this building method :
The radius chine hull shaping principles described above for steel and aluminum hulls may be adopted to plywood to produce an economical round bilge hull mainly from sheet plywood epoxy resins. The radius usually takes up about 1/3 of the total hull surface area. That means that 2/3 of the hull can be skinned at a relatively fast rate because it is applied in sheet form. The radius is cold molded using two or more layers of plywood.
Dudley Dix gives a detailed description of this method in his article Radius Chine Plywood Boat Building
All plywood is capable of being compounded, that is, of being bent in two directions at the same time; the amount being limited by the thickness of plywood. This property is used to produce a limited type and shape of boat hull such as catamaran and trimaran hulls and small dinghies. The advantage of it is that strong, lightweight hulls may be produced, eliminating the time consuming processes of lofting, setup.
To begin construction of the compounded shaped hull, you need to develop two identical flat plywood hull panels of the proper shape and size to make up the two halves of the eventual hull skin. The amount of rocker cut along the bottom of the panels where they will be joined together to form the keel determines the amount of compounding. Then accurately position the two panels on top of one another and drill wiring holes along the keel line and bow (and stern if the hull is to be a double-ender). The next step is to install a sheer clamp on the interior side of each panel using epoxy resin adhesive and either staples or clamps to hold them in place temporarily. Then position the panels in a large arc in the longitudinal direction while the adhesive is still uncured. This tends to introduce a bit of a laminated curve between these two wood parts and helps to promote some compounding into the plywood panels, especially in the upper regions around the sheer area. After the adhesive has cured you will have to plane the sheer clamps to a point at a proper angle in the bow so that they meet perfectly at the same time the plywood panels touch at the bow area. For a double-ender this is repeated for the transom. Before the panels are joined, slightly radius and bevel the facing inside edges along the keel and stem areas. If the edges are left sharp, they will misalign themselves. The panels are now ready for assembly. Stack them on top of each other so that the predrilled holes line up and insert a piece of copper wire in each set of holes twisting the ends together. Set the panels upright and separate the sheers from one another. Precut-to-length sticks are inserted between the two sheer clamps to hold the panels apart. When the two panels are spread at their desired angle, the next step is to snug up on the copper wire joining the two panels so that the edges of the panels are not only drawn tightly together, but are centered properly so that the keel line is relatively straight. When the two keel edges of the panels are in correct position and lashed tightly with the wires, you can begin building the fillet bonding of the keel joint and reinforcing it afterwards with glass tape. But before starting the keel joint, scrible a line on the inside of each panel approximately 5 cm (2") up from the center keel joint for reference later on when the exact centerline of the keel is lost under the fillet bonding. A deck jig cut out of plywood and reinforced with stringers and cross ties is used to hold the sheer clamps in their exact final position. As in other methods, bulkheads are used in compounded plywood construction as a major tool to distribute high loads over the skin area. They are installed using fillet bonding on each side of the plywood to join it with the hull sides. You can further support the skin by laminating in place ribs manufactured of thin strips of wood which will bend easily around the hull. Depending upon the hull being built, you may have many other interior items to install. These might include centerboard or daggerboard cases, special compartments and in larger hulls, interior accommodation. Some of these may be left until after the deck jig is removed; however, you must install large items before the deck beams that are needed to hold the sheer in final position are installed. Simple plywood deck beams are adequate in many situations, provided they have enough depth so that they have a large enough area for proper filleting where they join with the sheer clamp and the hull skin. Once the deck beams are installed and all of the interior support system is completed the deck jig is no longer needed and can be removed. You can turn the now rigid hull upside down at this point and snip the copper wires flush with the surface, complete the fairing at the keel line and stem, rounding over to whatever radius desired. The final step is to apply a minimum of two layers of glass tape over the outside of the keel area.
The hull is formed, in this method by diagonally laid veneer or plywood strips which are stapled and bonded using epoxy resin to each other and to the keel and stringers. The strips of the second layer are stapled and epoxy bonded at 90 degrees to the first layer, and the third at 90 degrees to the second layer. There are three basic methods of laminating: the mold method, the strip plank method, the stringer-frame method.
The Mold Method: Wood veneer or plywood strips are laminated over a mold or plug in the shape of the hull desired. The mold provides a sound base upon which you can exert pressure to facilitate a good bond between laminations until the adhesive cures. Usually staples supply this pressure. The biggest advantage of the mold method is reproducibility. The disadvantage is that it is difficult to absorb the time and materials required to construct a mold if only one boat is to be built. Another disadvantage is that interior frames, bulkheads or stringers must be installed in the interior of the hull after it is removed from the mold. This limits the size to smaller boats which will have hulls thick enough to support themselves with very little need for interior framework.
The Strip Plank Method: As pointed out above, the mold method does not economically justify itself when building larger, one-off custom boats. To solve this problem, the strip plank method of building laminated hulls have been developed in the attempt to utilize the mold to become part of the hull. Edge glued strip plan hulls have been built successfully for many years but it never received the attention that it should have. The only problem with it was that these hulls still relied on intricate interior framework to provide athwartship strength and stiffness and lacked any real diagonal strength and stiffness. Using the basic strip plank hull as a form over which to laminate diagonal veneers solves this problem. Thus an exceptionally rigid and strong monocoque hull structure can be produced and, at the same time, eliminate most of the normal interior framework associated with wooden boats. An advantage of it, especially for larger boats, is that a great deal of internal structure such as bulkheads, frames and furniture items, can be included during the setup. The resulting minimum skin thickness is over 22mm (7/8") and its associated weight may be too heavy for a smaller boat. Another disadvantage is that edge glued strip planking is not a particulary fast building method in comparison to laminated veneering techniques.
The Stringer-Frame Method: It is probably the most widely used method for producing laminated hulls. It has the advantage that you can install interior members, such as bulkheads and frames, during the setup. It can be used successfully to build just about any size boat from a 3m (10') pram to 18m (60') ocean racer. The method also has the potential to produce the best strength and stiffness to weight ratio hulls. The stringer-frame method, however, also has disadvantages. The main problem is that you have to begin laminating the hull skin with what in reality is an inadequate mold. Another disadvantage is that it results in a cluttered interior.
Wood for Laminating and Application: A basic requirement for all wood stock used in the laminating process is that you are able to bend it easily over the most severe curves present in the hull. Large hulls may permit using stock up to 10mm (3/8") thick; in very small boats perhaps you can only use 1.5mm (1/16") thick veneer. Usually 3mm (1/8") thick material is the practical minimum thickness when staples are the only method of applying laminating pressure, and this thickness is used in most boatbuilding projects. The amount of compound curvature in the boat hull determines the width of the laminating veneer. 20cm (8") is the common width in most boatbuilding situations. The length of the stock is not critical, but you save time if the stock is long enough to reach around one half of the hull at a diagonal angle. You can produce laminating stock yourself, but you will be wasting 50% to 70% of your stock in sawdust. Another problem is that it is difficult to resaw more than 10cm (4") thick stock. Commercially sliced veneers are available in 20cm to 25cm (8" to 10") width and 3.5 meters (12") long. The problem with sliced veneers is that it is very difficult to slice veneers any thicker than 3mm (1/8") without causing breakage within the wood fiber. Commercial plywood, mainly because of its availability, is widely used as a laminating material. The main benefit is that it is already a laminated material and very stable dimensionally. A disadvantage is that it is likely to cost quite a bit more than veneer. It also tends to be heavier for its volume than plain wood of the same type. Even with these disadvantages, plywood is the most practical solution in most cases.
Whatever the material, it is absolutely necessary that the edges of the stock be perfectly straight and true. Plywood already has a manufactured straight edge which is accurate enough to need no further trimming. Commercially available veneers come with rough-sawn edges that must be straightened before use. Begin applying by placing the first master veneer in the middle of the mold. Because of the compound curvature of the hull, the next veneer will not fit perfectly flush against the master veneer and you must trim or spile it so that both edges of the veneers will mate perfectly. Due to the shape of most hulls, the veneers will start to taper out toward the ends that approach the keel or sheer. Begin spilling by lightly tacking the new veneer as close as possible to the one just positioned. Usually the two veneers will be close to touching at the ends, with a gap in the middle. You can mark the veneer for trimming using a pair of common dividers or a compass. Then remove the marked veneer and trim it to the marked line. Besides bonding this first layer of veneers to a permanent piece of the boat where available you can also edge glue the veneers together. This prevents any further resin from running down into the inside of the first layer. When you apply the second layer, you will exert a good deal of pressure which may cause excess resin to run out in gaps between the edges. It is the use of structural, gap filling adhesives that permits making laminated hulls using the minimal pressure that staples are able to supply. By increasing the viscosity of the adhesive and applying slightly more than is actually needed, any developing voids are avoided. Standard foam roller or a notched trowel may be used to spread the adhesive evenly. In stapling the maximum grid spacing is usually 5cm (2"). However, the curves present in the hull drastically affect the laminating pressure. In the midpoint of a heavy curved area, very few staples are needed; at either side of this sharp curve where the veneer will want to move away from the surface you will need to apply more staples. Begin stapling pattern in the central area of the veneer; this helps work the excess resin out towards the edges of the veneer. In laminating the second and succeeding layers begin by properly placing a master veneer midpoint on the hull at the proper angle. Proceed to spile and position all of the veneers over the entire hull surface using a few staples. Then remove one veneer at a time, apply sufficient adhesive to the bonding surface, and replace the veneer in position and apply pressure with an adequate stapling pattern before moving to the next veneer. This way you will be able to apply adhesive and laminate the veneers all in one operation and eliminate much of the mess which results when the spiling and bonding operations are done simultaneously. Remember to fair and smooth up the hull prior to applying the last layer of veneers. Remove serious humps and fill hollows. Overall fairness of the hull should be good enough at this stage so that no more than half thickness of the last veneer layer will have to be removed during final fairing.
Ferrocement boats are suitable for amateur construction. Lends itself to the production of one-offs with no limit to the design with freedom to adopt curved shapes. Due to weight problems it is used for larger boats. Also the economy of the method is more pronounced in large boats.
The first stage involved is fabricating cross sectional frames from steel rod. To increase stiffness of the frames each is temporarily reinforced with horizontal and vertical angle bars, fixed to frames with u-bolts. The frames are then secured to the building berth by bolts that allow for vertical adjustment. Once the frames are in position the next job will be to shape the stem and keel pieces usually from galvanized pipe and weld to the bottom of each frame, thereby fixing the horizontal spacing of the frames. The transom is usually also constructed from shaped pipe overlaid with square mesh. The tops of the frames are joined together with steel bar, from the point of the stem, along the deckline, to the top of the transom. The next stage is to tie mesh reinforcement to the now reasonably rigid framework which is finally overlaid with layers of chicken wire both on the outside and inside of the hull. The next step is to plaster the outside of the hull in one continuous operation. Once the outside sets, the plastering of the inside may start, one frame section at a time. The decks, cabin sides and roof are constructed in the same manner.
For a detailed account of the building of a 57' ferrocement ocean sailing cruiser with illustrative pictures see How to Build a Ferro-Cement Boat in Easy (?) Stages by the Wood Family.