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.

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.

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.

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.

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.

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.

Richard



Your issue was not likely one of absolute weight, but center of gravity.
The only way one can be too heavy for a kayak is if you literally push
it under water. However, the higher your center of gravity, the less
stable the boat will be. Also, the heavier you are, the more effect you
will have when you shift your weight or lean. People who are tall and
heavy will find a given boat to be much less stable than a person who's
short and light.

I think that the extra weight took away the initial stability that the
kayak was intended to have. My guess is that the boat was designed for
smaller people, but usable for medium sized people. My weight was
outside the operating range of that design, making the boat unusable
by me.


That's highly unlikely. With most kayaks, the stability increases as you
push the boat deeper into the water, up to the point that the gunwales
submerge.

I would make a guess that overloading a boat is generally much more
detrimental than underloading a boat. A large boat paddled by a small
person probably will be slower and more subject to wind and wave than
a smaller boat paddled by the same person, but probably still safe and
usable.


Actually, the opposite is true. A heavier person pushes a boat down
farther in the water, increasing the length of the waterline. While this
adds more surface friction, it also increases the theoretical hull speed
of the boat, making it possible to paddle it faster before "hitting the
wall", so to speak.

I paddle low volume boats and am at the upper end of the recommended
weight range for all of them. I also build skin-on-frame boats that are
even lower in volume. All of my boats perform well and have the added
benefits of a better fit and less windage.