Boat Building - Construction Materials and Methods

Construction Materials and Methods

• Wood - The traditional boat building material used for hull and spar construction. It is buoyant, widely available and easily worked. it is a popular material for small boats (of e.g. 6-metre length; such as dinghies and sailboats). Its abrasion resistant varies according to the hardness and density of the wood and it can deteriorate if fresh water or marine organisms are allowed to penetrate the wood. Woods such as Teak, Totara and some cedars have natural chemicals which prevent rot whereas other woods, such as Pinus radiata, will rot very quickly. The hull of a wooden boat usually consists of planking fastened to frames and a keel. Keel and frames are traditionally made of hardwoods such as oak while planking can be oak but is more often softwood such as pine, larch or cedar. Plywood is especially popular for amateur construction. More recently introduced tropical woods as mahogany, okoumé, iroko, Keruing, azobé and merbau. are also used. With tropical species, extra attention needs to be taken to ensure that the wood is indeed FSC-certified. Teak or iroko is usually used to create the deck and any superstructure. Glue, screws, rivets and/or nails are used to join the wooden components.Before teak is glued the natural oil must be wiped off with a chemical cleaner, other wise the joint will fail.

  Some types of wood construction include:

  • Carvel, in which a smooth hull is formed by edge joined planks attached to a frame. The planks may be curved in cross section like barrel staves. Carvel planks are generally caulked with oakum or cotton that is driven into the seams between the planks and covered with some waterproof substance. It takes its name from an archaic ship type and is believed to have originated in the Mediterranean. A number of boat building texts are available which describe the carvel planking method in detail.
  • clinkeris a technique originally identified with the Vikings in which wooden planks are fixed to each other with a slight overlap that is beveled for a tight fit. The planks may be mechanically connected to each other with copper rivets, bent over iron nails, screws or in modern boats with adhesives. Often, steam bent wooden ribs are fitted inside the hull.
  • Strip planking is yet another type of wooden boat construction similar to carvel. It is a glued construction method which is very popular with amateur boatbuilders as it is quick, avoids complex temporary jig work and does not require shaping of the planks.
  • Sheet plywood boat building uses sheets of plywood panels usually fixed to a frame. Plywood may be laminated into a round hull or used in single sheets. These hulls generally have one or more chines and the method is called Ply on Frame construction. A subdivision of the sheet plywood boat building method is known as the stitch-and-glue method, where pre-shaped panels of plywood are edge glued and reinforced with fibreglass without the use of a frame. Metal or plastic wires pull curved flat panels into three-dimensional curved shapes. These hulls generally have one or more chines.Marine grade plywood of good quality is designated "WBP" (which stands for water- and boiled-proof) or more usually BS 1088. Both types of plywood construction are very popular with amateur builders, and many dinghies such as the Vaurien, Cherub, Moth and P class (ply on frame construction) and FJs, FDs and Kolibris (stitch-and-glue method) have been built from it. Another variation is tortured ply where very thin(3mm) and flexible (often Okoume)preshaped panels ply are bent into compound curves and sewn together. Little or no framework or longitudinal wood is used. This method is mainly confined to kayaks.
  • Cold-Molding is a composite method of wooden boat building that uses 2 or more layers of thin wood, called veneers, oriented in different directions, resulting in a strong monoque structure, similar to a fibreglass hull but substantially lighter. Usually composed of a base layer of strip planking followed by multiple veneers, cold-molding is popular in small, medium and very large, wooden superyachts. Using different types of wood the builder can lighten some areas such as bow and stern and strengthen other high stresss areas. Sometimes cold moulded hulls are protected either inside or out or both with fibreglass or similar products for impact resistance especially when lightweight, soft timber such as cedar is used. This method lends itself to great flexibility in hull shape.

• Steel (and before that iron) - Either used in sheet or alternatively, plate for all-metal hulls or for isolated structural members. It is strong, but heavy (despite the fact that the thickness of the hull can be less). It is generally about 30% heavier than aluminium and somewhat more heavy than polyester. The material rusts unless protected from water (this is usually done by means of a covering of paint). Modern steel components are welded or bolted together. As the welding can be done very easily (with common welding equipment), and as the material is very cheap, it is a popular material with amateur builders. Also, amateur builders which are not yet well established in building steel ships may opt for DIY construction kits. If steel is used, a zinc layer is often applied to coat the entire hull. It is applied after sandblasting (which is required to have a cleaned surface) and before painting. The painting is usually done with lead paint (Pb₃O₄). Optionally, the covering with the zinc layer may be left out, but it is generally not recommended. Zinc anodes also need to be placed on the ship's hull. Until the mid 1900s, steel sheets were riveted together.

• Aluminium - either used in sheet for all-metal hulls or for isolated structural members. Many sailing spars are frequently made of aluminium after 1960. The material requires special manufacturing techniques, construction tools and construction skills. It is the lightest material for building large boats (being 15-20% lighter than polyester and 30% lighter than steel). Aluminium is very expensive in most countries and it is usually not used by amateur builders. While it is easy to cut, aluminium is difficult to weld, and also requires heat treatments such as precipitation strengthening for most applications. Corrosion is a concern with aluminium, particularly below the waterline. It is most commonly used in small pleasure and fishing power boats that are not kept permanently in the water.

• Fiberglass (Glass-reinforced plastic or GRP) - Typically used for production boats because of its ability to reuse a female mold as the foundation for the shape of the boat. The resulting structure is strong in tension but often needs to be either laid up with many heavy layers of resin-saturated fiberglass or reinforced with wood or foam in order to provide stiffness. GRP hulls are largely free of corrosion though not normally fireproof. These can be solid fiberglass or of the sandwich (cored) type, in which a core of balsa, foam or similar material is applied after the outer layer of fiberglass is laid to the mold, but before the inner skin is laid. This is similar to the next type, composite, but is not usually classified as composite, since the core material in this case does not provide much additional strength. It does, however, increase stiffness, which means that less resin and fiberglass cloth can be used in order to save weight. Most fibreglass boats are currently made in an open mold, with fibreglass and resin applied by hand (hand-lay-up method). Some are now constructed by vacuum infusion where the fibres are laid out and resin is pulled into the mold by atmospheric pressure. This can produce stronger parts with more glass and less resin, but takes special materials and more technical knowledge. Older fibreglass boats before 1990 were often not constructed in controlled temperature buildings leading to the widespread problem of fibreglass pox, where seawater seeped through small holes and caused delamination. The name comes from the multiude of surface pits in the outer gelcoat layer which resembles smallpox. Sometimes the problem was caused by atmospheric moisture being trapped in the layup during construction in humid weather.

• Composite - Originally "composite" referred to a timber carvel skin fastened to iron frame and deck beams. This allowed sheet copper anti-fouling to be employed without the risk of galvanic corrosion of the hull fabric. It was employed for fast cargo vessels so that they were not slowed by marine fouling. This use is now obsolete. While GRP, wood, and even concrete hulls are technically made of composite materials, the term "composite" is often used for plastics reinforced with fibers other than (or in addition to) glass. Cold-molded refers to a type of building one-off hulls using thin strips of wood applied to a series of forms at 45-degree angles to the centerline. This method is often called double-diagonal because a minimum of two layers is recommended, each occurring at opposing 45-degree angles. "Cold-molding" is now a relatively archaic term because the contrasting "hot-molded" method of building boats, which used ovens to heat and cure the resin, has not been widely used since World War II . Now almost all curing is done at room temperature. Other composite types include sheathed-strip, which uses (usually) a single layer of strips laid up parallel to the sheer line. The composite materials are then applied to the mold in the form of a thermosetting plastic (usually epoxy, polyester, or vinylester) and some kind of fiber cloth (fiberglass, kevlar, dynel, carbon fiber, etc.), hence the finished hull is a "composite" of fiber and resin. These methods often give strength-to-weight ratios approaching that of aluminum, while requiring less specialized tools and skills.

• Steel-reinforced cement (ferrocement) - Strong, long lasting and very heavy. First developed in the mid 19th Century in France. Used for building warships. Extensively refined in New Zealand shipyards in the 1960s and the material became popular among amateur builders of cruising sailboats in the 1970s and 1980s, because the material cost was cheap, although the labour time element was high. The weight of a finished ferrocement boat is much higher than most wooden boats. As such they are often built for slower, more comfortable sea passages. Hulls built properly of ferrocement are more labor-intensive than steel or fiberglass, so there are few examples of commercial shipyards using this material. The inability to mass produce boats in ferrocement has led there to there being few examples around. Many ferrocement boats built in back yards can have a rough, lumpy look, which has helped to give the material a poor reputation. The ferrocement method is easy to do, but it is also easy to do wrong. This has led to some disastrous 'home-built' boats. Properly designed, built and plastered ferrocement boats have smooth hulls with fine lines. Amatuer builders are advised to use a professional plaster to produce a smooth finish. Most ferrocement hulls are designed as heavy displacement. See also concrete ship, concrete canoe.