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post #4 of Old 07-27-2011
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I'm no expert but here is my 5 cents..

every boat can capsize. the question is how easily they pop back up.

stability is objectively demonstrated by to GZ curves and GM (see wikipedia). subjectivey it depends on what you consider stable ie A very wide boat shallow boat (like a cruise ship) may appear stable as it rolls less than a narrow-beamed, deep keeled sailboat but should the wide boat capsize it is far less likely to right itself than the narrow-beamed, deep-keeled boat. The stability curve of a wide shallow boat looks great up to the point of the capsize. In general the centre of gravity of a wide boat is not sufficiently low enough to get the boat to right itself. A sail boat can take knockdowns and pop back up many times (catamarans can't though) as mass of a sailboat is as low as possible and alot is held in the keel.

cruise ships look very unstable to me. They are shallow in order to visit tourist ports like venice and amsterdam) and have a huge air draught. But I wouldn't know what their stability curve looks like. I can't imagine a cruise ship popping back up from a 180 degree knock down simply because the air draught, massive superstructure , and their width is so enormous.

military vessels probably have fantastic stability curves. They are narrow, prbably deep, lots of tonnage in the keel and their speed lends alot to stability.

The stability curves on the internet I've seen don't show the full picture. at some angle of heel GZ must be less than 0 in order for a boat to capsize. The longer GZ remains under 0 wrt angle of heel the less likely it is that the boat will pop back up. The rescue services invest alot of money in boats that capsize and right themselves at the slightest moment. Those boats don't look like cruise ships or aircraft carriers.
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