: http://www.blueheronkayaks.com/phpBB2/viewtopic.php?f=2&t=1825#p17806

: It shows that it is possible for two kayaks to be the same width,
: but for one to have four times as much righting moment as the
: other.

Stability is complex and subjective. Despite the above reference, width is VERY important in what people perceive as stability. Waterline width is the key variable. You will note in the above examples that the seam width might be the same but the waterline width was not.

The other reason the stability varied is that on the less stable designs, the width was not maintained over much of the boat. If a boat is 25 inches in the middle but immediately (fore and aft) markedly decreases in width it is understandable that it is not "really" 25 inches wide and will therefore not be that stable. The VAST majority of commercial designs do not radically taper from the middle. Very short kayaks HAVE to taper relatively quickly, because the distance to the stern and bow is less, so for a given waterline width the short kayak will tend to be less stable than a boat that is considerably longer. If they are approximately the same length this is not an issue.

If you take two boats of roughly the same length that are commercially available, the one with the wider waterline will have a more "stable" stability curve.

In short, if you know the WATERLINE width of a commercial boat you have a good approximation of its stability, certainly as expressed in its stability curve.

Finally, the classic stability curve is has a lot of problems in terms of understanding why a kayak stays upright. In fact, the righting moment of most kayaks is weak. If someone sits rigidly in a kayak and never leans, they are going to go over. But that is true of standing as well. We stay upright on our feet and upright in our kayaks because 100 million years of evolution have made us very good at shifting our weight around to stay balanced.

Because of the fact that the classic stability curve does not accurately reflect what actually happens with a mobile flexible human (who is a large fraction of the overall weight) in a kayak it is possible to design a boat with a stability curve similar to another boat that people will feel unstable in even though they are happy in the other boat.