Small Boat Design

Discussions around designing small boats including kayaks and canoes.

Stability curves *LINK* *Pic*

While I was struggling with a cold and couldn't do much building, I added a feature to my S&G design software that computes stability curves of my designs. Given the combined weight of the kayaker and the weight of the kayak (I took 90 and 25 kg, respectively), the center of submerged volume is computed at different depths of submersion, and angles over which the kayak is tilted relative to the vertical. This results in a table of values in which I can interpolate (based on the weight) the center of volume in function of angle. The difference in X direction of the center of volume and the center of mass leads to a torque that is then plotted in function of angle.

In simple terms, assume you can grab the kayak by its center of mass and push it down corresponding to the above combined weight, the torque that you feel when rotating the kayak along its length axis, is what's computed above.

I have 2 designs, one of which has a flat bottom at the center, and another one that is V shaped (see members.cox.net/aling13). I took these models and scaled them in 2 different ways: 19' x 20" and 17' x 22", and ran the program. The result is depicted in the added graph.

Two things are clear: (1) the V shaped designs have better initial stability (meaning: larger torque) but the flat bottom is more stable in a relatively short region beyond that, and (2) the difference in stability (torque) between the 2 different sizes is much more than one would expect based on a simple beam comparison (22" vs 20"). So yes, narrower boats are relatively much tippier than one would expect just by looking at it.

This may be common knowledge to most kayakers but it's nice to be able to look at these curves with kayak design software and predict stability before actually building it.