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post #12 of Old 10-07-2012
grumpy old man
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Re: Stability information

Right. The simple capsize ratio is a bit too simplistic to give you any meaningful data. It's good for a quick look and if you did get a number under 2.00 then it might be worthwhile to investigate the stability issue further.

I'm not sure I understand this. If you are using a method that tells you the AVS for an Alberg 35 is above 150 degrees I'd throw that method out. I could be wrong but it seems too high to me. JonB is correct, that could be called the "garboard radius" although it's not a radius in most cases and it changes at each station. In a case where there are hollow garboards and you need to determine a "hull depth" it's going to be tricky. The way I would do it on paper is to draw a straight line tangent to and from the section to the centerline ignoring the garboards, i.e. pretending the keel is not there. But any simplistic approach to determining stability is going to be suspect.

If you contacted US Sailing I am certain they would have accurate stability data for the Alberg.

But why bother? The Alberg will have a high AVS. My guess would be around 138.17 degrees. That's a good number and there is nothing about that design the indicates it would have stability issues. Just look at it. Also keep in mind that critical to any stability study is the VCG. Without an accurate VCG the rest of the equasion is not going to give you an accurate number. If you have found a method that ignores an accurate VCG input then I would call it very suspect.

I'll bet you a dollar that when Carl Alberg designed the 35 he did not do a stability study. I'm sure he drew a shape that he knew was right and in line with other healthy designs of the day. It turns out that he was right and the 35 is a very good boat.

Stability is complicated to most sailors. I think a little understanding of how the elements work can be dangerous. I also think that way too much emphasis is placed on hard stability numbers. AVS numbers only tell a small, static part of the dynamics involved in a capsize. The rest is pretty hard to calculate. If you own a "normal" boat and not some radical design freak or extreme racing boat then your biggest assett in rough conditions will be your own ability to manage the boat in a seamanlike manner. I can't find the formula for that.

Look at Rod Johnstone's exhaustive study on actual, documented capsizes. His results were very interesting. In plotting numbers for the various cases he found that the boats with the best numbers were more prone to capsize. What does that say? Nothing to me. Too many variables to draw any conclusions as far as I am concerned.

I have this image of a couple on their boat in a storm. They get knocked down to 130 degrees. The husband turns to the terrified wife and says, "No worries dear. This boat is good till 132 degrees." I have been in the middle of the Pacific in a taiphoon. I have seen waves that would capsize any boat regardless of the numbers. In all the studies one thing becomes evident, the bigger the boat, the more resistant it is to being capsized.

Hope that helps.

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