Monday, April 28, 2008

Stability for Small Boats

As your boat sits at its mooring, two basic forces are at work. Gravity, and buoyancy, which effectively moves your boat upward to the point equal to the weight of the amount of water the boat is pushing out of the way.

The amount of each force exerted is determined by the weight of the boat. A cubic foot of fresh water weighs 62.4 lbs., while a cubic foot of salt water weighs 64 lbs. If you look at a cross-section of your boat's hull, sitting level in the water, you can imagine two theoretical points. The Center of Gravity (CG) will be in the very center of the entire hull space. The other point, the Center of Buoyancy (CB), will be in the center of underwater portion of your boat.

When the CG and the CB are vertically aligned, the boat is level. If your boat is constructed properly, and your gear is stowed correctly, the CG should always stay in the same place. The CB, however, will change position any time the boat begins to list because the amount and shape of the boat under water changes.

The Center of Gravity and the Center of Buoyancy is called the righting arm. The weight of the boat is pushing down at the CG and the weight of the water is pushing up at the CB. This situation creates a rotating force or motion that is called the righting moment.

As long as the force of buoyancy is able to return the boat to an upright position, the situation is called a positive righting moment. If for any reason, the Center of Gravity should shift outside of the Center of Buoyancy, it creates a negative righting moment and the boat is going to capsize.

The ability of any boat to right itself is called stability. It should be evident to anyone that stowing gear and installing equipment on a boat takes thought. Both should be as low in the boat as practical. It's an absolute necessity to make sure that neither can suddenly shift from one side of the boat to the other. Besides careful stowing, another rule to remember is NEVER run parallel to large waves in a boat that is overloaded or too small for the situation. As the waves cause the boat to roll from one side to side, the positions of the GC and the CB are constantly changing. Even a small change during the rolling, such as gear shifting or a passenger moving to the low side, will create a negative righting moment.