The hullof a boat consists of an internal network of frames that extend from side to side (transverse) and that run the length of the boat (longitudinal), covered by an outer shell plating, usually made of fiberglass or metal. Each boat also has a keel, the backbone of the boat which runs along the center bottom of the boat.
Bow, Deck, and Gunwale - Curves That Matter
Ever notice the curves of boat? The shape of the bow is designed to the lift the boat with the waves, rather than cutting into them. The curvature of the deck from stem to stern, known as sheer, along with the flare and tumblehome also determine the boat's displacement and buoyancy. Flare increases displacement, and is the outward turn of the hull as the sides come up from the waterline. Tumblehome is the reverse of flare. It is the shape of the hull from the gunwale, the upper edge of the side of a boat, to the waterline. The curvature of the deck from beam to beam, or camber, allows water to flow off the deck.
How your boat handles and its speed depends in part on the chine – the part of the boat below the waterline – and the shape of the stern or transom. If the chine is rounded, it is a soft chine; if it squared off, it is a hard chine.
Following seas can be dangerous, causing a boat to broach or pitch pole (the stern is lifted higher than the bow, which can cause capsizing). A flat, square stern has a broader surface for a wave to act upon compared to a round stern. The round, or cruiser stern, is safer in following seas because the wave splits and travels forward along each side of the boat.
Rudder and Propeller
The rudder steers the boat, which is driven by one or more screw propellers.