Understanding the Parts of Your Power Boat

The hull of a motorboat consists of an internal network of frames that extend from side to side (transversely) and that run the length of the boat (longitudinally). It is covered (plated) by an outer shell that is usually made of fiberglass or metal. 

Think of the keel as the backbone of your boat, the bottom-most structural member around which the hull of the ship is built, running along the centerline of the hull from bow to stern. It is distinguished by a shark-fin-like protuberance at the bottom of the boat. Keels provide stability and generate lift to propel the boat forward. Some motorboats have keels, but many modern-day powerboats do not. Their engines generate sufficient power to propel them through the water.

Motorboats, like most boats in general, are comprised of a series of curves. That's because all these elements combined make for the most buoyant structures. The shape of the bow is designed to 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 the speed at which it can move both depend in part on the chine, which is the shape of the portion of the hull of a boat that sits below the waterline. The shape is determined by the change in angle in the ​cross-section of the hull. If the chine is rounded, or its angles are shallow, it is called a soft chine; if it is squared off, it is a hard chine.  Soft-chined boats usually have more horsepower, while hard-chined boats offer more stability.

The shape of the backside of a boat, or stern, comes into play in rough seas, which can cause a boat to broach (heel to one size and capsize) or to pitchpole, which is when a boat literally summersaults, bow to stern. A flat, square stern has a broader surface for a wave to act upon, compared to a round stern. The round, or cruiser stern, however, is safer in following seas because the wave splits and travels forward along each side of the boat.

The rudder steers the boat, which is driven by one or more screw propellers. These are almost always located at the back of the boat, on the flat section of the stern known as the transom.