Basics of Boat Design

Parts of a Sailboat
Although sailboat design varies widely, all sailboats share a few basic components. The boat’s main body is called the hull. The front of the hull is referred to as the bow, while the rear of the hull is called the stern. The rudder extends from the stern and is used to steer the boat. The centerboard, under the hull, helps sailors maintain a steady course by limiting the boat’s movement from side to side. The mast and the boom support the boat’s sails. The mainsail, the largest sail on a sailboat, is fastened to both the mast and the boom. The triangular sail in front of the mast is called the jib.

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

I. Buoyancy and Weight

For an object to float on the water’s surface, it must sink enough to displace a volume of water equal to its own weight. For example, if a boat is to carry three people, their fishing gear, an outboard motor, and a supply of fuel—a total weight of about 500 kg (1,100 lb)—then the boat must be made long and large enough to displace 500 kg (1,100 lb) of water without sinking below the water level. Boat designers also have to take into account the weight of the boat itself. The heavier the material used to build the boat, the larger the boat has to be.

II. Trim and Stability

In addition to considering the total weight of the boat and its contents, boat designers must also consider the distribution of weight. The weight of an outboard motor at the stern of a small boat tends to make the boat sink deeply in the water at that end (to “trim the stern”). If that tendency cannot be offset by placing a similar weight in the bow of the boat, then the offsetting must be obtained by broadening the hull at its aft, or back, 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.

III. Structure

A boat must maintain its shape in the face of local internal weights, such as an engine or a heavy cargo, and it must be strong enough to resist the force of battering waves. Because 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. For example, a traditional boat built of skins or of bark, or a modern inflatable boat, are sufficiently strong and buoyant, but all are vulnerable to puncture or perforation.

IV. Watertightness

Boats must also be watertight—that is, invulnerable to leakage through the joints of adjoining pieces. Boatbuilders made wood plank boats watertight by caulking between planks with fiber threads, pitch, or a combination of these materials. For boats of skin or bark, filling interstices with pitch was common practice. The contemporary practice of molding a hull eliminates the problem entirely. Without seams there is no possibility of leakage.