Richard Ferguson wrote:
I do not buy your argument at all. If the issue was center of gravity,
the trunk of my body, where most of the weight was, did not move when I
pulled my legs out of the water. Also my legs were on the deck, just a
few inches above the waterline, so the change in center of gravity was
minimal. However, the effect on stability was extremely dramatic. (I am
less than 6 feet tall, so my center of gravity is not unusually high.)
In other words, I can't see how a very small change in the center of
gravity could effect a dramatic change in boat stability. There has to
be another cause.

By putting your legs in the water, you did two things:

- You lowered your center of gravity. This is not as inconsequential as
you think.

- You created outriggers. Your legs not only functioned somewhat like a
tightrope walker's pole, but their buoyancy acts as - dare I say it -
sponsons and increases the boat's stability.

Remember that this was a sit on top type of kayak, kind of like a
surfboard with depresions for the butt and feet. Most of the kayak was
underwater even before I pulled my legs out of the water. I think that
part of what happened was that the extra weight of my legs put even more
of the kayak under the water, and that affected the hull design. It
started to look more like a submarine than a conventional boat.

As long as the boat wasn't submerged below the gunwales, the stability
would not have decreased.

One that you said that I might agree with is the idea that weight
increases the stability of a boat, up to a certain point. (You
mentioned the gunwales). It is really a function of the hull design. If
you think of the hull design as kind of an ellipse (or other geometric
shape), then as long as the waterline is below the center of the
ellipse, the boat is likely to be stable. Once the waterline gets above
the center of the ellipse, the effective hull width actually starts to
decrease, and the boat becomes less stable. When the hull leans to the
left, there is less and less boat above the waterline on the left side
resisting the left lean, so the boat rolls. When a boat gets loaded past
the maximum hull width, at that point the boat does start to look like a
submarine.

Conventional canoes are less likely to suffer this kind of instability,
since the width of the boat at the waterline, for most hull designs,
tends to increase as the boat is first loaded, and most people will
never load a canoe to the point where it is close to the gunnels, which
is where the canoe hull sometimes narrows, if only a little bit.

You're basically correct, except that you're assuming that a kayak
loaded up to the gunwales is less stable than one that's more lightly
loaded. That simply isn't the case. If you look at the cross section of
a typical touring kayak, you'll see that it flares continuously up to
the gunwales. That means that the stability will constantly increase as
the boat is pushed deeper into the water. Sit on tops are typically MORE
flared than sit-in kayaks.

If you think about the playboat kayaks, they have relatively low volume,
which means that much of the hull is below the waterline, reducing
stability, which is how they can do some of the tricks that they do,
endos, etc. Even they have to have some width at the waterline, because
the hull width at the cockpit has to be wider than the person, and the
cockpit coaming is several inches above the waterline, so at least the
part of the hull around the cockpit tends to provide stability.

Actually, you're misinterpreting the design. Playboats are actually more
stable in some respects (due to their wide beam) and many people find
them more difficult to roll than a sea kayak. Because they are so short,
they have relatively little fore-to-aft stability, which is something
sea kayaks have a huge amount of. Their maneuverability is a function of
their short length and a hull profile that encourages planing.

Conventional touring kayaks have most of the volume above the waterline,
so they never hit this form of overload instability, which would
probably require a lot more weight than a person and camping gear would
provide. Think about the center of volume of the hull, it would always
be above the waterline.

See my explanations above.

Since the sit-on-top kayaks have relatively little hull above the
waterline, they are more prone to his kind of instability caused by
overload, I would think.

Not generally. Most are wider and have more flare than sit-in boats
(surf skis excepted). That makes them more stable, not less.