Thursday, April 17, 2008

Sailboat Hulls

A sailboat hull is different from any other hull because of its need for lateral resistance surface. For the boat to move ahead the wind forces pushing the boat sideways must be thwarted, and converted to forces that move the boat forward. On a small boat this can be done by extending a board down into the water, and securing it there.
There are several methods, LEEBOARD, CENTERBOARD ­and DAGGERBOARD to name a few.

A low center of gravity will help with the stability of all vessels, but as boats get larger they require an additional righting force in the form of BALLAST. This is usually a lead or iron keel, attached to the boat's hull. If it is lead or iron stowed in the bilge it is called INSIDE BALLAST.
The weight of a boat's ballast may be as much as half of the boats overall displacement, and the keel may extended further below the water than the hull rises above it.
A deep keel, prohibits a sailboat from entering shallow waters, and so a combination KEEL-CENTERBOARD hull was developed where a centerboard operates through a shallow keel. A keel-centerboard boat might draw down below the surface of the water. A FIN-KEEL is one that looks like a fish's fin extending below the boat, and the boat usually has a rudder mounted some distance aft, such as on an additional keel-like exten­sion called a SKEG. A long or FULL-LENGTH KEEL is usually found on cruising boats, because its directional stability is useful on long passages.

Wetted Surface
With a full-length keel, a sacrifice is made in maneu­verability and often in speed through the water, compared to shorter keels with reduced WETTED SUR­FACE-less hull surface beneath the water.
A PLANING HULL is one that lifts out of the water and planes at high speeds. Usually on high­powered motorboats, planing is also possible for boats under sail, such as light-weight, wide, and somewhat flat-bottomed boats, with a lot of sail area to provide the needed power. Higher speeds are possible on these plan­ing hulls, because wetted surface is reduced, eliminating drag, and wave-making resistance of the hull is no longer a limiting factor.

Parts of the Hull That Support Rigging
The hull shape does not necessarily dictate what kind of rig will be above it, but it does determine most of the important handling characteristics of a sailboat. Parts of the hull must be made with extra strength to deal with the strains put on it by the rigging. This means that a certain hull will usually carry a certain rig best suited to it. Here are some parts of the hull that support the rig.
The CHAINPLATES are strong metal straps extending from the rail down toward the keel, on either side of the mast, to accept the rigging holding up the mast. The bottom of the mast usually rests on a part of the keel called the MAST­STEP. On smaller boats, the mast may be stepped directly on the deck or cabin top, where there is often a large hinge-like fitting called a TABERNACLE. This makes it pos­sible to raise and lower the mast on boats that use canals with low bridges, or for trailerable boats, that must have a mast easily unstepped.
A BOWSPRIT may extend forward at the bow of the boat to increase the potential for sail area, and a BOOMKIN (pro­nounced bum-kin) may extend off the stern to accept the backstay further aft enlarging poten­tial sail area.

Self-Bailing Cockpits
A sailboat hull is designed to heel, and will sometimes take waves over the rail and into the cockpit. Any sailboat that will be taken offshore, large or small, should have a SELF-BAILING COCKPIT to get rid of that water. In a small boat this may be holes at the stern that drain the water as the boat moves ahead. There may be suction valves in the boat's bottom, or there may be large drains in a cockpit whose bottom level is normally above the water level, providing natural drainage. The self-bail­ing feature is best for a boat going offshore and valuable on others as well-because the cockpit may fill with water in rough weather, and must be able to drain with­out assistance from the helmsman.