Join Date: Dec 2004
Location: Portugal, West Coast
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Re: Full or fin keel?
Slocum's boat SPRAY (a beamy boat).....had a "point of vanishing stability" of only 90degrees.....I have a narrow boat, but according to calculations, the point of vanishing stability is pretty extreme...in the other way.
As far as difficulty in capsizing, a wave can capsize just about anything. What is important is how fast (if at all) the boat will come back up.
Yes, a huge breaking wave can capsize anything but there are a proportionality between beam and the size of a wave needed to capsize a boat. A smaller breaking wave can be enough to capsize a narrow boat but not enough to capsize a beamy boat.
Regarding vanishing stability a beamy boat has not necessarily an AVS worse than a narrow boat. The Avs has nothing to do with beam but with how deep the CG is. Being an old design I don't think your boat will have a very big AVS, I mean it has not a big draft and the weigh is not all on a bulb on the bottom as in a modern boat. There are many modern beamy boats with AVS around 130º but probably that is not more important than the force that they are making to right itself up at 90 0r even 100º. After all it is way more frequent a knock down then a complete roll and at that position the boat is vulnerable: You want it up again as fast as possible. A small wave that hit the boat at that position can capsize the boat.
Regarding what you say it is more important ( "What is important is how fast -if at all- the boat will come back up"
) I guess that what is more important, providing the boat has an AVS superior to 110º is the proportion between the positive and the negative part of a stability curve. Meaning the proportion that exists between the energy needed to capsize a boat and the energy required to right it up from inverted position. Saying it in another way, the proportion between the size of a wave capable to capsize the boat and the size of a wave capable to re-right the boat again from the inverted position.
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Last edited by PCP; 03-07-2013 at 05:20 PM.