Wood is 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 (20 ft) length; such as dinghies and sailboats). Its abrasion resistance 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 but only marine ply using waterproof glues and even laminates should be used. Cheap construction plywood often has voids in the interior layers and is not suitable to boat building as the voids trap moisture and accelerate rot as well as physically weaken the plywood. No plywood is rot resistant and should be coated with epoxy resin and/or a good paint system. Varnish and Linseed oil should not be used on the exterior of a hull for waterproofing. Varnish has about 60% of the water resistance of a good paint system. Only boiled linseed oil should be used on a boat and only in the interior as it has very little water resistance but it is very easy to apply and has a pleasant smell. Note that used linseed rags should not be left in a pile as they can catch fire. A valuable 200-year-old waka (Maori canoe) caught fire in New Zealand in June 2014 when restorers left rags piled overnight. Raw linseed oil is not suited to boats as it stays damp and oily for a long time. Mildew will grow well on raw linseed oil treated timber but not on boiled linseed oil. 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, otherwise the joint will fail.
Some types of wood construction include:
Cold-moulded 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. The "hot-moulded" method of building boats, which used ovens to heat and cure the resin, has not been widely used since World War II; and now almost all curing is done at room temperature.
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.
Aluminum and aluminum alloys are used both in sheet form for all-metal hulls or for isolated structural members. Many sailing spars are frequently made of aluminium after 1960. It is the lightest material for building large boats (being 15-20% lighter than polyester and 30% lighter than steel). Aluminium is relatively cheap in comparison with wood or steel in most countries. In addition it is relatively easy to cut, bend and weld. Galvanic corrosion below the waterline in salt water is a serious concern, particularly in marinas where there are other conflicting metals. Aluminium is most commonly found in yachts, pontoon and power boats that are not kept permanently in the water. Aluminium yachts are particularly popular in France.
A relatively expensive metal used only very occasionally in boatbuilding is cupronickel. Arguably the ideal metal for boat hulls, cupronickel is reasonably tough, highly resistant to corrosion in seawater, and is (because of its copper content) a very effective antifouling metal. Cupronickel may be found on the hulls of premium tugboats, fishing boats and other working boats; and may even be used for propellers and propeller shafts.
Fiberglass (glass-reinforced plastic or GRP) is typically used for production boats because of its ability to reuse a female mould 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 mould, 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 mould, 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 mould 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 multitude 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 construction" involves a variety of composite materials and methods: an early example was a timber carvel skin attached to a frame and deck beams made of iron. Sheet copper anti-fouling ("copper=bottomed") could be attached to a wooden hull provided the risk of galvanic corrosion was minimised. Fast cargo vessels once were copper-bottomed to prevent being slowed by marine fouling. GRP and ferrocement hulls are classic composite hulls, the term "composite" applies also to plastics reinforced with fibers other than glass. When a hull is being created in a female mould, the composite materials are applied to the mould in the form of a thermosetting plastic (usually epoxy, polyester, or vinylester) and some kind of fiber cloth (fiberglass, kevlar, dynel, carbon fiber, etc.). These methods can give strength-to-weight ratios approaching that of aluminum, while requiring less specialized tools and construction skills.
First developed in the mid-19th century in both France and Holland, ferrocement was also used for the D-Day Mulberry harbours. After a buzz of excitement among homebuilders in the 1960s, ferro building has since declined.
Ferrocement is a relatively cheap method to produce a hull, although unsuitable for commercial mass production. A steel and iron "armature" is built to the exact shape of the hull, ultimately being covered in galvanised chicken netting. Then, on a single day, the cement is applied by a team of plasterers. The cement:sand ratio is a very rich 4:1. As the hull thickness is typically 2.5 to 3 cms, ferrocement is unsuitable for boats less than about 15 metres LOA as there is a weight penalty; above that length there is no penalty. Properly plastered ferrocement boats have smooth hulls with fine lines, and amateur builders are advised to use professional plasterers to produce a smooth finish. In the 1960s and 1970s, particularly in Australia and New Zealand, the cheapness of ferro construction encouraged amateur builders to build hulls larger than they could afford, not anticipating that the fitting-out costs of a larger boat can be crippling.
The advantages of a ferro hull are:
The disadvantages are:
There are many hull types, and a builder should choose the most appropriate one for the boat's intended purpose. For example, a sea-going vessel needs a hull which is more stable and robust than a hull used in rivers and canals. Hull types include:
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