Back to main page


The principle of stability is to many people very complex. I will try to show that the principle is rather easy.
Stability is the tendency of a boat to stay levelled.
With "starting stability" the tendency of the ship to level at small angles is meant.
with "extent of stability" the max angle to what the ship is self righting is meant.
In general there are 3 kinds of stability:

Form stability
Weight stability
Speed stability

Every boat has to some extent to do with this types of stability.
Sometimes one of these types of stability is the most important and one can say that that boat is xxx stabile.

Form stability

When you push a ball underwater you need a force.
A big ball like a skippy ball needs more force to be submerged than a table tennis ball.
The more volume you want to submerge the pore force you need.
This is the law of Archimedes, To submerge 1liter of volume you need 1 kg force.

Now, imagine the following:
I built a small raft of two balls close to each other.
The ball will be submerged a little by the weight (=gravity force)of the raft.
If I then heel the raft one ball goes deeper and one ball goes up.
The ball that goes deeper want to float up more as the ball that goes up.
So, The lower side tends to go up.

This is the principle of form stability.
Now imagine two square rafts. One much wider than the other.
Both having the same heel.

You will see clearly that the lower side is pushed more in the water than the higher side. For the wide raft the blue areas are much bigger, and also spaced much further apart.
Both result in a much higher righting moment.
This shift in floatation force also occurs for round rafts.

Form stability varies with the width of the ship, and less with the form!

How about the saying in some sailing books the form stability becomes less at great heel?
Look at the picture below, and note the distance between the middle and the middle of the blue areas.

The stability as explained here above is the same explanation as in most sailing books, but presented in an other way.
In most sailing books they use "point of buoyancy" "Flotation force" and shift of point of buoyancy.
But where is the point of buoyancy?
Well the point of buoyancy is the middle of the submerged volume.
For a square raft this is easy:
(Note the dotted helping lines)

you can see that the point of buoyancy moves when heeling.
Without the dotted helping lines it will be rather complex.
Drawing a wide raft at a small angle will make it more clear, you can do this yourself.

Weight stability

In the explanation above of form stability we only reviewed the shift of the flotation force.
You also need to look at the shift of the weight due to heeling.
The weight is of course as much as the flotation force.
The flotation force and the weight form a couple, The righting couple.
If you have a big heeling the height of the weight will be very important.
Take a look at the picture below:

Speed stability.

This works like the waterski effect.
If the boat deflect water down at its stern, and then the boat heels, you will get more lift at the lower side.
This is pushing the boat even again.

If you have a boat that deflects water up at its stern, like most tugs, you will get less stability at high speeds.

Back to main page