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The word boat describes a vessel that displaces and excludes the water surrounding it, is more bouyant than a floating object such as a log or raft, and is shaped for stability and propulsion.

A large watercraft is properly called a ship; however, the boundary between boats and ships cannot be defined with precision.  Some larger vessels today are called boats, although they exceed in length some ships of colonial America.  In naval architecture, however, a craft less than 65 ft (20 m) in length is considered a boat.

Boats are classified primarily by method of propulsion - for example, sailboat, motorboat, and rowboat.  They are also classified according to function, method of construction and type of materials used, rigging (in sailboats), and other factors.

Only a few basic component parts are common to most boats of traditional style.  The keel is a timber or other element running the length of the center bottom of a boat from the bow, or front, to the stern, or rear.  The keel serves as the "foundation" for the frame, which is covered by a waterproof material to form the hull - the body, or bulwark, of the boat.  These terms are also used to describe modern boats that are not built but are molded in one piece.

BOAT DESIGN BASICS

Boats of some utility can be built with only the most elementary thought for design; a gathering of fallen logs at the edge of a stream, lashed together to make a crude raft, is an extreme example. Over the many centuries of boat use and boat building, such crude beginnings grew by intuition and trial-and-error into an array of sophisticated and highly satisfactory craft.

Bouyancy and Weight

Any object that is to float upon the water surface must sink until it has displaced a volume of water of equal weight. Thus, if, for example, a boat is to carry three people, their fishing gear, an outboard motor, and a supply of fuel—a total weight of 1,000 pounds—then the boat must be made long and large enough to displace 1,000 pounds of water without sinking below standard level. The design task must take into account that the boat has its own weight that also must be displaced; therefore, the greater the weight of the material that is used to build the boat, the larger the size of the boat.

Trim and Stability

After the underwater volume of a boat is designed to meet the requirement that the weight of the boat and its contents equals the weight of the water displaced, the distribution of weights must be suitably arranged. The weight of an outboard motor at the stern of a small boat tends to make it sink deeply in the water at that end (to "trim the stern"). If that tendency cannot be offset by placement of a similar weight in the forward end of the boat, then the offsetting must be obtained by broadening the hull at its aft end so that a greater part of the displacement occurs near the excess weight.

A balance of weights from side to side must also be arranged. Further, to lessen the danger of capsizing, the combined center of gravity of all weights must be sufficiently low in the boat. If other factors make it necessary that the center of gravity be high, then the chance of capsizing must be offset by increasing the width of the hull.

Structure

A boat must maintain its shape in the face of local internal weights, such as an engine or a heavy cargo, and when buffeted externally by waves. If the boat is stiff enough to resist major deflections, it is almost certainly strong enough to resist fractures. On the other hand, since a hull of sufficient strength can be built of thin material, the risk of local puncture can be great in a boat that is otherwise quite strong. A traditional boat built of skins or of bark and a modern inflatable boat are examples of this circumstance.

Watertightness

Part of the structural problem is watertightness—the prevention of leakage through the joints of adjoining pieces. For boats of wooden planking, a caulking between planks was commonly used; the caulking might be fiber threads, pitch, or a combination of these materials. For boats of skin or bark, filling of interstices with pitch was common practice. The contemporary practice of molding a hull obviates several of the traditional problems. Without seams there is no possibility of leakage.

PLYWOOD CONSTRUCTION

Plywood boat construction began in the United States about 1918 and developed rapidly; the two basic types are paneled and molded. Panel construction involves securing flat sheets of plywood to transverse frames and to the keel and other units that supply longitudinal strength; it is used only with chine-model boats, that is, those with flat bottoms or V-bottoms. The amount of compound curve that may be used in a plywood panel is extremely limited.

In molded-plywood construction the form of the boat is established by temporary transverse frames, or molds, and by longitudinal battens (thin, narrow strips of lumber often used to seal or reinforce joints), over which the planking is placed in two or three layers. The first skin is laid on diagonally and secured to the form by staples. Two or three skins are used, and after stapling, the mold is moved into a heating and pressure stage in which adhesion of the skins is accomplished. After adhesion, stiffeners and joinery are added; these usually include keelson, shoe, gunwale, guards, thwarts, decking, outside stem, transom (in a square-stern boat), and centerboard case and mast step (in a sailing hull).