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The hull is the watertight body of a ship or boat. Atop the hull may be a deckhouse or some other form of superstructure, like a mast. The line where the hull meets the water surface is called the waterline.
The structure of the hull varies depending on the vessel type. In a typical modern steel ship, the structure consists of watertight and non-tight decks, major transverse and watertight (and also sometimes non-tight or longitudinal) members called bulkheads, intermediate members such as girders, stringers and webs, and minor members called ordinary transverse frames, frames, or longitudinals, depending on the structural arrangement. The uppermost continuous deck may be called the "upper deck", "weather deck", "spar deck", "main deck", or simply "deck". The particular name given depends on the context—the type of ship or boat, the arrangement, or even where it sails. Not all hulls are decked (for instance a dinghy).
In a typical wooden sailboat, the hull is constructed of wooden planking, supported by transverse frames (often referred to as ribs) and bulkheads, which are further tied together by longitudinal stringers or ceiling. Often but not always there is a centerline longitudinal member called a keel. In fiberglass or composite hulls, the structure may resemble wooden or steel vessels to some extent, or be of a monocoque arrangement. In many cases, composite hulls are built by sandwiching thin fiber-reinforced skins over a lightweight but reasonably rigid core of foam, balsa wood, impregnated paper honeycomb or other material.
Rafts have a hull of sorts, however, hulls of the earliest design are thought to have each consisted of a hollowed out tree bole: in effect the first canoes. Hull form then proceeded to the coracle shape and on to more sophisticated forms as the science of naval architecture advanced.
The shape of the hull is entirely dependent upon the needs of the design. Shapes range from a nearly perfect box in the case of scow barges, to a needle-sharp surface of revolution in the case of a racing multihull sailboat. The shape is chosen to strike a balance between cost, hydrostatic considerations (accommodation, load carrying and stability), hydrodynamics (speed, power requirements, and motion and behavior in a seaway) and special considerations for the ship's role, such as the rounded bow of an icebreaker or the flat bottom of a landing craft.
Hulls come in many varieties and can have composite shape, (e.g., a fine entry forward and inverted bell shape aft), but are grouped primarily as follows:
After this they can be categorized as:
The hull is supported exclusively or predominantly by buoyancy. Vessels that have this type of hull travel through the water at a limited rate that is defined by the waterline length. They are often, though not always, heavier than planing types.
The planing hull form is configured to develop positive dynamic pressure so that its draft decreases with increasing speed. The dynamic lift reduces the wetted surface and therefore also the drag. They are sometimes flat-bottomed, sometimes V-bottomed and more rarely, round-bilged. The most common form is to have at least one chine, which makes for more efficient planing and can throw spray down. Planing hulls are more efficient at higher speeds, although they still require more energy to achieve these speeds. An effective planing hull must be as light as possible with flat surfaces that are consistent with good sea keeping. Sail boats that plane must also sail efficiently in displacement mode in light winds.
The hull form is capable of developing a moderate amount of dynamic lift; however, most of the vessel's weight is still supported through buoyancy.
At present, the most widely used form is the round bilge hull.
In the inverted bell shape of the hull, with a smaller payload the waterline cross-section is less, hence the resistance is less and the speed is higher. With a higher payload the outward bend provides smoother performance in waves. As such, the inverted bell shape is a popular form used with planing hulls.
A chined hull consists of straight, smooth, tall, long, or short plates, timbers or sheets of ply, which are set at an angle to each other when viewed in transverse section. The traditional chined hull is a simple hull shape because it works with only straight planks bent into a curve. These boards are often bent lengthwise. Plywood chined boats made of 8' x 4' sheets have most bend along the long axis of the sheet. Only thin ply 3–6 mm can easily be shaped into a compound bend. Most home-made constructed boats are chined hull boats. Mass-produced chine powerboats are usually made of sprayed chop strand fibreglass over a wooden mold. The Cajun "pirogue" is an example of a craft with hard chines. Benefits of this type of hull is the low production cost and the (usually) fairly flat bottom, making the boat faster at planing. Sail boats with chined hull make use of a dagger board or keel.
Chined hulls can be divided up into three shapes:
Each of these chine hulls has its own unique characteristics and use. The flat bottom hull has high initial stability but high drag. To counter the high drag hull forms are narrow and sometimes severely tapered at bow and stern. This leads to poor stability when heeled in a sail boat. This is often countered by using heavy interior ballast on sailing versions. They are best suited to sheltered inshore waters. Early racing power boats were fine forward and flat aft. This produced maximum lift and a smooth,fast ride in flat water but this hull form is easily unsettled in waves. The multi chine hull approximates a curved hull form. It has less drag than a flat bottom boat. Multi chines are more complex to build but produce a more seaworthy hull form. They are usually displacement hulls. V or arc bottom chine boats have a V shape between 6 and 23 degrees. This is called the deadrise angle. The flatter shape of a 6 degrees hull will plane with less wind or a lower horse power engine but will pound more in waves. The deep V form (between 18 and 23 degrees) is only suited to high power planing boats. They require more powerful engines to lift the boat onto the plane but give a faster smoother ride in waves. Displacement chined hulls have more wetted surface area, hence more drag, than an equivalent round hull form, for any given displacement.
Smooth curve hulls are hulls which use, just like the curved hulls, a sword or an attached keel.
Semi round bilge hulls are somewhat less round. The advantage of the semi-round is that it is a nice middle between the S-bottom and chined hull. Typical examples of a semi-round bilge hull can be found in the Centaur and Laser cruising dinghies.
S-bottom hulls are hulls shaped like an s . In the s-bottom, the hull runs smooth to the keel. As there are no sharp corners in the fuselage. Boats with this hull have a fixed keel, or a kielmidzwaard (literally "keel with sword"). This is a short fixed keel, with a swing keel inside. Examples of cruising dinghies that use this s-shape are the Yngling and Randmeer.
Hull forms are defined as follows:
Use of computer-aided design has superseded paper-based methods of ship design that relied on manual calculations and lines drawing. Since the early 1990s, a variety of commercial and freeware software packages specialized for naval architecture have been developed that provide 3D drafting capabilities combined with calculation modules for hydrostatics and hydrodynamics. These may be referred to as geometric modeling systems for naval architecture.
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