BoatBanter.com - Kayak stability

On 11-Jun-2004, (Gene Cosloy) wrote:

Slightly different question: If you're familiar with SeaKayak magazine
reviews, they supply some interesting numerical data with respect to
the subject of stability. As an example, if given the same or equal
conditions, if it takes say only 10 foot lbs of force to heel a boat
25 degrees which represents the point of imminent capsize, that should
also mean that it requires the same amount of force applied opposite
to right the boat?

No. The righting moment is the force that is pushing you back up. When
you heel a boat, it will resist the heel up to the point of collapse.
If you take away the heeling moment, the righting moment will push it
back to a neutral position.

Now if another boat requires 20 foot lbs to heel the boat the same 25
degrees and with the same result i.e. imminent capsize, and the same
amount to resist or right the boat, which boat is more stabile?

The greater the righting moment at a given angle of heel, the more stable
the kayak will feel.

Which is easier to control? Which would you rather paddle?

It is difficult to say. You can probably discount a kayak from these
graphs (since it would be wildly different than what you're looking
for) they say nothing that should make you choose a kayak. The final
judgement is only made in the water.

while the charts
and numerical values can be wildly divergent, the reviews narrative
descriptions regarding stability frequently appear to be similar.

The flaw in the graphs is in the way the data is determined and what
it means. The model is a kayak with a static paddler. Real kayaks
have live, moving paddlers. If the kayak starts to heel, a real
paddler will react and that changes the geometry and hence the behavior
of the heeled kayak. Remember that the reviewers are not beginners
or even intermediate paddlers. They can make any kayak perform and
can make up for differences that may be noticable (or difficult
to overcome) for beginners.

The way to read the graph in a useful way is as follows.

The slope at the origin (degree of heel near zero) indicates the initial
or primary stability. The slope here can be positive (inherently stable)
or negative (inherently unstable). The more positive it is, the higher
the initial stability. Beginners tend to like high initial stability,
while experienced and adventurous kayakers prefer low initial stability.
Racers will accept negative initial stability.

The maximum height of the graph indicates the degree of secondary stability.
A kayak with a very high graph will be more stable on edge than one with
a lower value. However, a key feature is what angle of heel this occurs at.
If the peak is at a relatively small angle of heel, the kayak will be
difficult to edge and will be (likely) harder to handle. If the peak is
further to the right and at a larger angle of heel, the kayak will roll
on edge easily and will sit well over solidifying its stability.

An example of a kayak with very low or negative initial stability is one
with a deep V hull or with a V and rounded hull. Some racing kayaks
cannot stay upright in the water without the active participation of
the paddler.

An example of a kayak with high secondary stability is a fat, wide kayak
like the Solstice GTHV, Necky Pinta or many of the foldable kayaks like
Kleppers. They are difficult to edge and tend not to carve turns
easily.

An example of a kayak that has moderate initial stability and solid
secondary at a higher angle of heel are many of the Greenland-inspired
designs like the Anas Acuta, Pintail, Explorer, Chatham etc. They are
easy to get on edge and can sit there comfortably. My Ellesmere is
twitchy at zero degrees of heel and is _more_ stable on edge. These
kinds of kayaks tend to be popular with advanced paddlers.

Mike