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Join Date: Jun 2011
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Re: Stability information
Sail Calculator pro is a pretty neat site. I like the ability to select a boat from a list and being able to compare two different boats. Sloop was right though, it only showed me a capsize ratio. Maybe it is just a lack of understanding on my part but the capsize ratio really doesn't tell me anything. I read that an offshore boat should have a capsize ratio of less than 2.0 but that number doesn't really tell me anything important.
The site that SloopJon linked did give me an angle of vanishing stability, which is much more important to me. Somebody correct me if I'm wrong but it's my understanding that the capsize ratio is a general indication of a given boats resistance to being capsized where the AVS is the more accurate measurement of angle of heel at which the boat will no longer right itself. In the event of a 90 degree knockdown, knowing the actual AVS, 140 degrees for example, would tell me that I have another 50 degrees to go before some other righting force must be applied to bring the boat back. Again, the capsize ratio tells me nothing.
SloopJon, a couple of questions. On that site one of the parameters it asks for is Hull draft not including the keel. How is that determined? In the list of specs I've looked at, the draft given is the total draft of the boat. Also, some boats have a pretty apparent point at which the hull ends and the keel starts but others have more of a wineglass profile that makes that point a little less apparent. I tried estimating draft by looking for that point in the pictures but it takes very little difference in the value entered to change the AVS drastically. Here are the results of what I entered for an Alberg 35;
Beam 9.75 9.75
Weight 12600 12600
Ballast 5300 5300
Hull draft 2.5 2.0
AVS 182.22 152.45
The AVS given on my second try seems much more reasonable but the variance in the results are great enough that I'm skeptical of trying to estimate the hull draft. Only 6 inches of error produced a difference of 30 degrees. Do you have any suggestions to help me get a more accurate draft entry?
I'm afraid not. The design aspect you refer to could be called "garboard radius" I guess and my last boat - Columbia 43 - had a fairly pronounced one. I ran into the same problem as you because of it. For practical purposes, if you get a satisfactory outcome using the lowest resultant, you are O/K. If you must have the absolutely accurate number you'll need to look further. Perhaps the tangent point of the radius?
Perhaps the maestro can weigh in on this?
I, myself, personally intend to continue being outspoken and opinionated, intolerant of all fanatics, fools and ignoramuses, deeply suspicious of all those who have "found the answer" and on my bad days, downright rude.
Last edited by SloopJonB; 10-07-2012 at 12:58 PM.