Wright wrote:

Having noticed that most touring kayak manufacturers recommend shorter
lighter kayaks for lighter weight paddlers (without really specifying why) I
am looking for an explanation of the effect of a person's weight on initial
stability, secondary stability, and performance. Given two identical
touring kayaks and two people of equal abilities, if one weighs 125 lbs and
the other weighs 190 lbs what would be the differences, if any, on stability
and performance?

It all depends on their center of gravity. If the heavier person carried
most of their weight in their legs and butt (as many women do), they
could actually be more stable in a given boat than a lighter person who
carried their weight higher.

On the other hand, a tall, broad-shouldered, barrel-chested 190# man
would feel much less stable in a given boat than a shorter, 125# woman.

Weight ranges specified for kayaks are generally much lower than the
actual carrying capacity of the boat. In many cases, a paddler weight is
specified, but not a cargo weight. The manufacturer assumes that some
amount of extra gear will likely be carried and builds in a margin for
it. Overall, the majority of people end up buying kayaks that have too
much volume for them. It's actually hard to buy a boat that's too small,
since you simply won't fit.

Some disadvantages of a boat with too much volume a

- They will feel less stable, since the bigger boat raises your center
of gravity higher above the water.

- They have more windage and are more easily pushed around by wind and
waves. This can hamper your ability to control the boat.

- Their higher decks make rolling and other advanced skills more difficult.

As a general rule, you want the smallest volume boat that will
comfortably accommodate you and the gear you anticipate carrying.

In terms of performance, there's a trade-off between length and speed.
As a general rule, kayaks are limited to a specific theoretical hull
speed by their wavemaking resistance, which is a function of the
waterline length (THS in MPH = 1.34 x the square root of the waterline
length). The longer the waterline, the higher the theoretical hull
speed. Most touring boats have a THS in the range of 4-6 mph. However,
longer boats have more surface area in the water and therefore more
friction.

At low paddling speeds (~2-3 knots), surface friction is the main source
of drag. As your speed increases, wavemaking resistance becomes the
dominant factor up to the point that you simply cannot paddle any faster
no matter how much effort you put into it. So what does this mean in
practical terms? A shorter boat with less surface friction will be more
efficient at lower speeds, but it will limit how fast one can paddle. A
longer boat will require more effort at low speeds, but it can be pushed
to a higher top speed, provided that the paddler has the horsepower to
do so.

For a small paddler with a proportionately lower power output, a shorter
boat can be advantageous as long as the THS limit is not a problem for
the type of paddling they intend to do.