boat-building

Boat building is the design and construction of boats and their systems. This includes at a minimum a hull, with propulsion, mechanical, navigation, safety and other systems as a craft requires.

Construction materials and methods

Wood

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. 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.

Traditional wood construction techniques can be classified into the "shell-first" method (also called "planking first") and the "frame first" method. With "shell first", the form of the hull is determined by joining shaped planks that are fastened together, followed by reinforcing the structure with the frames (or ribs) that are fitted to the inside. With "frame first", the hull shape is established by setting up the frames on the keel and then fastening the planking on the outside.

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.
* Clinker is a planking-first technique originally identified with the Scandinavians and Ingveonic people in which wooden planks are fixed to each other with a slight overlap that is beveled for a tight fit. The planks are 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 longitudinal long wood such the chines, inwhales (sheer clamps) or intermediate stringers which are all bent around a series of frames. By attaching the ply sheets to the longwood rather than directly to the frames this avoids hard spots or an unfair hull. 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 drawn together then edge glued and reinforced with fibreglass without the use of a frame. Metal or plastic ties, nylon fishing line or copper 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. Australian plywood manufacturers and suppliers have issued warnings that some Asian nations are selling ply stamped BS 1088 which does not meet international standards. Specifically, they say outer plies are too thin (should be minimum) or are very thin (less than ) or high-grade surface ply such as Okoume is combined with a much heavier and wider inner cores. Most high-grade marine Okoume (Gaboon) ply uses lightweight poplar inner cores. Often the 1088 stamp is blurred in the poor Asian ply so it is not clear. In Australia and New Zealand a higher-grade marine ply than BS1088 is AS2272. It requires both faces to be "A" quality, with even-thickness plies. The most common plywood used for this grade is plantation-grown Hoop Pine which is fine grained, very smooth, moderately light (at it is the same weight as Meranti ply and about 13% heavier than genuine poplar cored BS1088 Okoume). Hoop pine has a very high stress rating of F17, indicating high strength. Meranti (Lauan) ply has a stress rating of F14 and Okoume ply F8. Okoume ply is commonly coated with epoxy to increase strength and impact resistance as well as to exclude water. Both types of plywood construction are very popular with amateur builders, and many dinghies such as the Vaurien, Cherub, Tolman, 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() 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 moulding is a composite method of wooden boat building that uses two or more layers of thin wood, called veneers, oriented in different directions, resulting in a strong monocoque structure, similar to a fibreglass hull but substantially lighter. Sometimes composed of a base layer of strip planking followed by multiple veneers. Sometimes just veneers are used. Cold-moulding is popular in small, medium and very large, wooden super-yachts. Using different types of wood, the builder can lighten some areas such as bow and stern and strengthen other high-stress 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.
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.

*Lashed-lug, is a boat-building techniques of the Austronesian peoples. It is characterized by the use of sewn holes and later dowels ("treenails") to stitch planks edge-to-edge. The planks are further lashed together and onto ribs with fiber ropes wrapped around protruding carved lugs on the inside surfaces. This is a shell-first (or planking first) method of construction The seams between planks are also sealed with absorbent tapa bark and fiber that expands when wet or caulked with resin-based preparations.

Metal

Iron and steel

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

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.

Cupronickel

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

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 material

"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.

Ferrocement

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:
* cannot burn, rot, or rust; & no osmosis
* good insulation: cool in summer, warm in winter
* tougher than GRP, and almost as tough as a steel vessel; (and if damaged, easily repaired almost anywhere in the world)
* properly built, a ferro hull is as fair as a GRP hull.
* they may be cheap to buy (see disadvantages, below)

The disadvantages are:
* many home-built ferro boats are lumpy, overweight and ugly.
* some early builders, realising that their creation proved to be disappointing, scuttled their vessels and fraudulently claimed insurance.
* accordingly, ferro yachts may be difficult to sell and nigh impossible to insure.

''See also'': concrete ship, concrete canoe.

Hull types

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:

* Smooth curve hull: these are rounded and free of chines or corners.
* Chined hulls: these hulls have flat panels (typically plywood) which meet at a sharp angle known as the "chine". Chined hulls range from simple flat-bottomed boats where the topsides and bottom meet at about 110 degrees (such as banks dories and sharpies) to skiffs where the bottom is arced rather than flat. Multi-chine hulls allow an approximation of a round hull shape.
* Flat-bottomed hull: flat-bottomed hulls are suitable for canals and non-tidal rivers. They are often shallow-draft, and may operate in shallower water. They are cheap to build but, being less stable and having no keel to give directional stability, they are unsuitable for sea-going vessels. (However, large ships are almost always flat bottomed, having a suitably large draft to overcome the problems of small flat-bottomed boats).
* Displacement hulls: a displacement hull always remains partly submerged. Such a hull has a maximum "hull speed" which is a function of its waterline length. An exception is the catamaran, whose twin hulls are usually so fine that they do not engender a bow wave.
* Planing hulls: planing hulls have a shape that allows the boat to rise out of the water as the speed increases. Sail boats that plane are typical V-shaped at the bows and flat-bottomed aft. Hydroplanes are very light, flat bottomed, high powered speed boats that plane easily on flat water, but they become unstable in rough water. Powerboats designed for rough water are usually deep V-bottomed with a deadrise angle of about 20–23 degrees. The most common form is to have at least one chine to allow for stability when cornering and for a supportive surface on which to ride while planing. Planing hulls allow much higher speeds to be achieved, as they are not limited by the waterline length the way displacement hulls are. Planing hulls need sufficient power to "reach the plane", after which the lack of drag allows high speeds yet with reduced power consumption.

Glossary

*Anchor: a device for holding a boat to the seabed, typically with chain and rope. Traditional anchors include the fisherman, Danforth, and plough types (such as the "CQR" and "Delta"). Recently, far more effective anchors with more reliable holding include the "Rocna" and the "Ultra", both of which are spade anchors.
* Bitts: a pair of short strong posts of wood or steel on the deck of a boat intended to take mooring lines. Also called "bollards".
*Bilge: the lowest part of the interior of a hull. Water, fuel tanks, ballast, and heavy stores are variously placed in the bilge to lower the craft's centre of gravity.
*Bilge keels: a pair of short keels fitted on either side of the hull. Less hydro-dynamically efficient than a fin keel, they have a shallower draft. Full-length bilge keels add rigidity to a hull. Bilge keel craft are found in areas with large tidal ranges to keep the vessel upright when dried out.
*Bilge pump: a manual or electric pump for draining the bilge. Set at the lowest point, its inlet is protected by a screen to minimize blockages.
*Block: a pulley used to give a line a fair lead and multiply its hauling power. Single and double sheave blocks are common, triple rare.
*Bobstay

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