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post #3 of Old 01-21-2005
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Steel masts???

I have only recently been running into discussions of using steel masts. It is based on the idea that steel is much cheaper to purchase than aluminum if purchased by the pound and that a dollar''s worth of steel is a lot stronger if measured as a simple unit of cross section than aluminum. But it is not that simple. In any given verssel, the items that control the strength of the structure of a mast, require a minimum wall strength and a minimum resistance to buckling, And there is the rub. To achieve a suitable wall strength and resistance to buckling, the steel mast ends up being somewhat heavier than the aluminum spar. This is a problem because it raises the center of gravity and can greatly increase the moment of inertia. That combination can result in what is referred to an ''excitation capsize''. This was very much a problem with older IOR era boats and was a major contributer to the problems encountered during the Fastnet Disaster. In effect what happens when you have a heavy spar is that the boat stores a lot of knetic energy when it rolls. This kinetic energy causes the boat to roll past the point that it would have if it had less inertia carried high. In a wave train that is close to the sympathetic harmonic of the boat the boat is likely to roll through every increasingly large angles until a knock down occurs. This higher moment of inertia can also contribute to a roll over in a single large wave incident.

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