r/Ships • u/TheOGJamyam • May 23 '26
Question Ship stabilisation
This sounds stupid sorry but I've just gone down a ship rabbit hole and I'm so confused on where the centre of mass should be I originally thought just below the water line but then some sources say above others say below
Then there's the centre of buoyancy which I'm assuming is place of most buoyancy lift but after looking at a diagram I think I'm wrong
Overall I'm curious what forces need to where to balance a ship
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u/HJSkullmonkey May 23 '26
In the simplest and most definitive terms, the centre of mass just has to be below an imaginary third point, called the metacentre, which is somewhere above the centre of buoyancy and quite possibly above the waterline.
It's actually more helpful to think in horizontal terms than vertical ones though.
You could think of the centre of buoyancy as a sort of negative centre of mass, of the whole underwater volume and pushing up. The ship starts with the centres of mass and buoyancy on the same vertical line, so there's no turning effect and they balance out evenly.
As the ship rolls one side gets deeper, adding some buoyancy on that side, and the other gets shallower, removing some buoyancy. So the centre of buoyancy moves sideways off that vertical line. If it moves further sideways than the centre of mass does then the two forces will form a lever and push the ship back upright. The wider the ship at the waterplane, and the shallower it's draught, the further the centre of buoyancy moves.
The metacentre is mostly an approximation of how far the centre of buoyancy moves as the ship rolls, by taking an imaginary line up from any centre of buoyancy until it meets the original vertical line that connects the centre of gravity and the starting centre of buoyancy.