Re: [Paddlewise] Stability

>    Having paddled
> lots of different boats with different hull shapes I've noticed a different
> "feel" in the initial stability in different.  Boats with very flat hull
> feel very resistant to tipping.  Boats with a shallow V have a gentle
> rocking feel. Boats with a deeper V start to feel "twitchy",  Rounded hull
> boats feel "slippery" (sort of like sitting on a basketball).  That's not
> very scientific but that's just how different shaped hulls *feel* to me.


I think that you are correct in your impression. But this is not "primary"
stability as defined by Schade (basically the definition that I would
accept)

It is my contention that kayaks are VERY different from normal craft because
the user shifts his/her weight constantly and this represents a substantial
part of the weight of the craft. Stability curves are based on the concept
of the paddler sitting rigidly in the boat as it heels which does not happen
in real life.

Imagine this "thought" experiment. Sit in some narrow 18" kayak. Extending
out is a board 6" wide and 1" thick and it extends out 4 feet in each
direction. It is fiberglassed to the bottom of the kayak.
That kayak will feel "rock stable" as you sit in calm water. You would have
little trouble taking out your camera and taking pictures.

Its "primary stability" as measured on a classic stability curve is little
different than the the craft that has no such board fiberglassed to the
bottom. But it feels ENTIRELY different. The dynamic resistance to tipping
created by the resistance of the board moving through the water gives the
kayaker ample time to adjust his/her weight as the boat starts to tip one
direction or another. That adjustment is practically subconscious. You could
probably stand in that boat.

For that matter, fiberglass a 4 foot dagger board to extend down from the
middle of the hull. Make it out of a slightly buoyant material like wood and
it will reduce the primary stability as measured on a stability curve. But
it will feel very steady at 0 degrees of heel.


I have paddled square logs that are about 16" wide and I can stay upright on
them which I cannot do in a 18" kayak.

My contention is that the square cross-section kayak feels initially more
stable than the round kayak because there is much more dynamic resistance to
the boat rotating around its long axis. It is NOT because of its stability
curve which at 0-2 degrees might be identical to a boat with a rounded hull.

It is like a built in paddle brace.

Rock a flat bottom boat back and forth. You will kick up a lot of waves
because of the resistance to rotation of those flat surfaces against the
water. The rounded hull will rock back and forth with little wave action
created.

In the world of heavy shipping this is not much of an issue because the
cargo weight cannot be shifted quickly and an unstable boat will slowly turn
turtle whether it is square bottomed or rounded. That said, on passenger
ships they will sometimes deploy stabilizers that extends out and resist the
rolling back and forth around the long axis of the boat to increase the
comfort of the passengers.
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