Center of Buoyancy

Its relationship to stability; where it’s normally located
A boat’s center of buoyancy (CB) is an important indicator of its stability. I always imagine the hull’s center of buoyancy as having two axes around which the hull rolls. The first axis is the fore-and-aft line, and determines the rolling motion from port to starboard and back; the second axis is perpendicular to the first, and determines the pitching of the bow and the plunging of the stern. This is purely imaginative stuff, of course, but it satisfies my simple mind.In technical terms, the CB, also known as the center of immersion, is the center of gravity (CG) of the volume of water displaced by a floating object. In other words, it’s the center of the total force that is pushing upward on the hull. Together with the boat’s CG, it determines how stable the vessel will be.If the CG is a long way beneath the CB, the boat will be relatively stable; that is, if she’s tipped sideways by wind or waves, she will tend to return to the normal upright position. But if the CB is close to the CG, the boat will be relatively tender and will readily list to one side or the other. That’s why it’s important to stow all heavy objects, including spare anchors, canned goods, and libraries of books, as low as possible inside the hull. It keeps the CG low and helps right the boat when she heels over or even capsizes.Displacement hulls in powerboats and sailboats usually have their CBs about 55 percent of the waterline aft of the bow. In most other powerboats, it’s a little farther back—about 60 percent of the waterline aft of the bow.See also Center of Gravity.