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December 3, 2008

Letter: Center of buoyancy must be above center of gravity

To the editor:

I read with interest Paul Brailsford's Wednesday, Nov. 26, letter to the editor ("Minding the center is good politics, safe sailing").

It was very informative, particularly for your readers who may not be versed in the stability of marine vessels. However, I believe his statement that "... her center of gravity is situated on the ship's centerline at a calculated distance above the center of buoyancy so she is in stable equilibrium," is in error.

That relationship would ensure an unstable vessel that would certainly end up upside down! Conversely, the center of buoyancy must be above the center of gravity (the operative word here is "above") by a distance called the "metacentric height." This distance must be a positive number; i.e., buoyancy above gravity, to keep the vessel right side up.

The later statement: "Its distance in feet above the center of gravity ...," is indeed correct and indicates that the earlier statement was only an error in word juxtaposition, albeit a serious one.

Thomas E. Hoffman

Principal, Hoffman Design & Development

Marblehead

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