Boat hulls, although variously shaped, have only two basic designs: the displacement hull and the planing hull. Any boat lying at rest in the water displaces, or moves aside, an amount of water equal in weight to the weight of the boat. A boat that displaces the same amount of water when moving through the water is called a displacement boat. This type of boat, usually having a curved keel and a narrow stern, actually plows through the water.
The planing hull acts like the displacement hull while it is at rest or moving slowly through the water. However, the faster it moves, the higher it rises until it finally skims lightly over the surface. Many runabouts and all racing craft are of this design.
Conventional hull bottoms are of three general shapes: round, V, and flat. Sometimes two bottom shapes are combined in one boat. This is especially common in runabouts and power cruisers, where the shape forward may be round or V and the shape aft may be almost flat.
In recent years, boat designers have adapted the Polynesian multihull principle. Boats of this design, called catamarans, have two long, narrow hulls parallel to each other and joined by a solid deck structure. The passage of air between the deck and the water surface provides an unusually smooth ride. The separation of the hulls reduces the chances of capsizing, and their narrowness increases the speed.
The methods of constructing wooden boats have changed very little. The three basic methods are carvel-planked: lapstrake, or clinker-built; and strip-planked. In carvel-planked boats the planks are laid edge to edge over wooden frames, or ribs, attached to the keel. The seams are caulked with cotton and putty. In lapstrake boats the plank edges are lapped and fastened through the joint, providing a strong hull of relatively light weight. In strip-planked boats, long, narrow strips of wood are fastened vertically to one another and to the frame. The seams are reinforced with waterproof glue for greater strength and tightness.
The development of waterproof marine plywood has made possible the construction of a different type of wooden boat. The use of plywood panels has simplified boatbuilding because it requires a minimum of framing and fewer seams. However, because of the lightness of the plywood, the gunwales and various joints must be strengthened. Molded plywood boats are made over a form by gluing and clamping together and then by heat-curing several layers of crisscross wooden plies. This method of construction provides each boat with a strong hull that has a minimum of framing.
Aluminum boats are light, durable, and corrosion-resistant. Most of them are riveted; some are welded; and some are stretch-formed over molds and jigs, with plastic-sealed seams. Because small aluminum boats are easily transported on the roof of a car or by trailer and require almost no upkeep, they have become extremely popular.
Another new material, fiber glass, can be molded into almost any shape. The fiber glass is combined with liquid resin under heat and pressure to form a one-piece, leakproof hull, which is durable and resists marine borers and rot. Most fiber glass boats do not require frequent painting, since the color is usually added to the resin or to the final gel coat.
Some boats are constructed of two or more materials, such as plywood sides and bottom protected by an outer coating of fiber glass.
Materials such as aluminum, plywood, and fiber glass have reduced the work and cost of maintaining a boat in good condition. All engines, however, should undergo cleanings and checkups at the beginning and end of each boating season. Paint and varnish finishes must be touched up or redone to preserve the basic material of the boat. A good anti-fouling bottom paint is an absolute must for any boat kept in salt water during the boating season. Lines, horns, lights, and other equipment should be inspected periodically.
In storing a boat for the winter, it is essential that the weight of the boat be properly supported both fore and aft to prevent warping of the hull. Proper ventilation will help to prevent mildew or dry rot. Most trailers are suitable for winter storage of small boats. A covering tarpaulin will protect a boat against inclement weather and will also provide ventilation.