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post #29 of Old 11-06-2008
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Originally Posted by southernsmoke View Post
Is there any sort of physics formula or general rule of thumb for calculating what it will take to capsize a boat? I'm talking about a 25'+ keelboat here, not a sunfish, which everyone knows can capsize easily.

I'm pretty new to sailing and I'd like to know what it would take to knock my boat over!
Sup Southernsmoke? Welcome. This is one I can finally answer...kind of. It so happens I'm currently reading the book "Heavy Weather Sailing" (go figure) and it says that

"The simple answer to avoid capsizing is to avoid breaking waves."

It then goes on to say that model tests they carried out showed that

"when the breaking wave was 30% of the hull length high, from trough to crest, it could capsize some of the yachts, while waves to a height of 60% of the hull length would comfortably overwhelm all of the boats we tested. In real terms this means that for a 33' boat, when the breaking wave is 10', this presents a capsize risk; and when the breaking wave is 20' high, this appears to be a capsize certainty in any shape of boat."

It goes on to discuss the variations of full keel, fin, etc. and do all the math on the various configurations (that's where you can get your physics). And it discusses techniques to keep the wind/waves off the beam (drogues, warps, anchors, etc.). It also stresses that the issue is breaking waves - saying that the waves can be extremely high/steep and not necessarily cause a capsize if they aren't breaking. But the above seems to be the rule of thumb.

It's a good book so far.
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