This is a GREAT post on stability from the Paddlewise email list...

~d

Date: Sun, 10 May 2009 22:59:02 -0700
From: Robert Livingston
Subject: Re: [Paddlewise] Advice on boats

I will throw an arrow because I think that you are wrong.

I think that this argument that "stable" boats find themselves at 45
degrees on 45 degree wave faces to be wrong in the family of kayaks
24" or less.

First of all, I have paddled lots of kayaks including "stable" ones
and I do not detect the problem. This is a perceptual issue and other
may disagree.

The second point has to do with stability curves.

Many have seen stability curves of kayaks in Sea Kayaker or elsewhere.
These curves are interesting and valuable but subject to confusion.

First of all, most kayaks 24" or less are actually unstable. The
curves do not reflect this because they do not reflect the actual
height of the center of gravity of a typical kayak (which for this
conversation is a craft 20 - 23" wide) with an adult in it. But these
curves (which really are created for real ships that have immovable
centers of gravity) do not reflect reality for kayakers. Unlike ships,
kayaks have moveable centers of gravity.

Stability curves as classically calculated are EXTREMELY sensitive to
where you assume the center of gravity to be. Raise that center of
gravity up a bit an that "stable" looking curve goes away. Those
curves are based on an unrealistically low estimation of the center of
gravity.




The curves imply that you can lean such a boat out say 45 degrees and
you will bob back to the midline.

You won't. At least a dummy rigid paddler with the weight distribution
of a human will not. That kayak is going over. In fact, that kayak is
going over if it is leaned out 20 degrees etc. As an experiment, try
it in a warm pool. Sit rigidly in your kayak and have a friend lean
you out 15 degrees and then let go and see what happens if you do not
move a muscle but simply sit rigidly in your kayak. You are over.

The fact of the matter is that we bipeds are stability machines.
Imagine the stability curve of a human standing on one foot. Most of
us can do this. But this is HIGHLY unstable. People can stand
"rigidly" but they are not completely rigid. They make subtle
adjustments very early so as to stay upright. We are VERY sensitive to
starting to tilt and correct immediately.

We instinctively, and very successfully, keep are selves upright
whether while standing on land or sitting in a kayak. This is before
you have learned to use your paddle to brace. When the kayak starts to
tip you immediately shift your center of gravity to counteract this.
Sometimes you will see beginners shimmering in their kayak as they
compulsively remain completely vertical.

This behavior is so ingrained in us as bipeds that we do not give it
any conscious thought unless the kayak gets very unstable.

An experienced kayaker knows that he can lean out at 45 degrees
without difficulty and become upright again using his paddle. But if
you have no paddle, when you lean your boat, you will lean your body
the other way to compensate. You can only lean your body laterally at
the waist so far. If you go beyond this limit, you are over.

My contentions are that kayaks as most of us know them on this list,
are "unstable". We will exclude the rubber ducky 30" wide kayak
category. We will exclude 34 inch canoes. So we are talking about sea
kayaks which as we commonly use the word are "18 - 24" wide. These are
all unstable. You might think of a 24" kayak as a fat stable dog boat
but it is really not all that stable. It is stable for a human because
of our skills, honed by evolution, at keeping ourselves upright. We
can sit in those boats and take photo etc without difficulty. But
remember we can do that standing up also. Stick a frozen sitting
corpse in that boat and it is not going to take much of a wave or a
tilt to get it to flip over.

And I would argue that it is not hard in that 24" boat to get it to
tilt 45 degrees if the water surface becomes tilted 45 degrees. It is
easy. You lean a bit (which is instinctive) and the boat will tilt.

I do think that there are "shapes" of kayak cross-sections that can be
desirable or undesirable in terms of feelings of "comfort" when it
comes to being sideways in heavy seas. But I think that the
explanation that we frequently see in publications with the kayak so
"stable" that it cannot be leaned into a wave so as to remain in the
"upright" position is fiction.