# Trim and roll (or heel) for submersibles

• ENGBIO
In summary, a submarine has three axes (pitch, roll, yaw) that can be adjusted to control its balance. Heel, or the tendency of the boat to tilt in one of these axes, can lead to a decrease in the center of buoyancy, which opposes further heel.f

#### ENGBIO

Hey guys,

Im studying submarines and I was wondering when discussing buoyancy changes for submersibles what is meant by "changes in trim and roll or heel". What is trim/roll/heel? How should I picture it?

Thank you!

Hey guys,

Im studying submarines and I was wondering when discussing buoyancy changes for submersibles what is meant by "changes in trim and roll or heel". What is trim/roll/heel? How should I picture it?

Thank you!

It has to do with whether the sub sits level or tilts in one of the three possible axis. I usually hear the three described as pitch, roll, and yaw.

Cars go down the highway in the direction the wheels point (unless they are skidding.) Boats and airplanes do not necessarily move the same way the nose points.

It has to do with whether the sub sits level or tilts in one of the three possible axis. I usually hear the three described as pitch, roll, and yaw.

Cars go down the highway in the direction the wheels point (unless they are skidding.) Boats and airplanes do not necessarily move the same way the nose points.
Yes, also I think heel is roll induced by a sail or by cornering force.
Yaw is rotation about a vertical axis, as when steering.

When a submarine is on the surface heel can cause the centre of buoyancy to change in a way that reduces heel, eg contributing to stability. This doesn't happen when the submarine is submerged because the hull is surrounded by water.

When a submarine is on the surface heel can cause the centre of buoyancy to change in a way that reduces heel, eg contributing to stability. This doesn't happen when the submarine is submerged because the hull is surrounded by water.

Are you saying that heel can lead to a shift in the center of buoyancy that opposes further heel?

Yes. But only when on the surface.

Thanks, just making sure I understood your meaning.

So for the OP: the submerged boat (hopefully) still has a positive "righting moment" that tends to return the boat to vertical when the boat heels and increases (to a point) with heel angle. However, as CWatters points out, a boat on the surface can gain significant extra righting moment via buoyancy (through the shape of its hull).

Since a sub also has active control over its ballast, it can also adjust its weight/buoyancy (both total amount and the way it is distributed around the boat) to bring the boat "into trim," i.e., to a level flying condition (without additional control inputs from the planes). This is kind of like trimming an aircraft so that it flies straight and level.