Site updated:

20th August 2019

Longitudinal Stability - calculating draft increase.

I've always known that boats turn over when they are top heavy, but never really understood the science behind it. Having a boat capsize is not my idea of a good day out. So this little corner of the site is Dr. Holt's attempt to understand model boat stability - without getting too scary or geeky.

Its always nice to know how much the draft will increase when you add a kg of weight. This page should allow you to calculate this information.

The formula to calculate kg per cm immersion is (Water plane Area x Water Density) x 10.

So the figure changes with draft!

To simplify the calculation of water plane area for any given draft I divide the shape into squares and triangles.

The calculation for my model Triton at 70mm (loaded) draft is below:

The calculation for my model Triton at 10mm (ballast) draft is below:

From the figures above you can see that for Triton, at ballast draft it takes 1.200kg to increase the mean draft by 1cm, while that figure increases to 1.550kg at loaded draft.