The author is a retired naval architect. He once testified as an expert witness in a case involving a ship that capsized and sank. This is the fifth of an 11-part series explaining stability to a lay audience. Links to the previous installments can be found below.
What resists heeling forces? The real action happens at the waterline so let's start by looking at a boat that doesn't have any waterline.
We will, from this point on, adhere to the conventional and useful fiction that gravity acts through the single point labeled "CG" and buoyancy upwards through the single point labeled "CB". With our strangely shaped boat at rest in still water and calm wind, the CG and CB are exactly in line and exactly counterbalancing each other. Now let's add a little puff of wind and heeling moment:
Center of Gravity remains in the same place (unless tank liquids have sloshed around or people have moved). The shape of the hole the boat makes in the water hasn't changed so to the CB also remains in the same relationship to the hull. The rotation caused by the slight puff of wind now causes the two opposing forces to be offset. The offset creates rotation in the direction of heel so the boat will continue heeling over, at least until its strange upper works hit the water.
It is like a pencil balanced on its point. A butterfly flapping its wings in China will topple it. For submerged object like submarines to remain upright, the CG must be below the CB so that the offset forces will create rotation back towards vertical. The locations shown are typical for normally shaped surface vessels however.
Now let's change our boat to a conventional shape with waterline.
Note the vessel's level waterline as it might be painted on the hull and the wedge of water on the left that is lifted out and the wedge on the right that is now immersed. The shape of the hole the boat makes in the water has now changed. The CB will be at the center of mass (same as center of gravity for a body of uniform density) of the hole. This center now moves over to the right which reverses the offset of the forces. Gravity and buoyancy now try to rotate the vessel counter clockwise and back towards upright. This is the basic mechanism of stability.
PREVIOUSLY
Next, Part 6: Righting Arms. “The amount of stability a ship has at any particular angle to resist upsetting forces such as wind is the weight of the ship times the length of this arm.”
This is a great series. The concepts of stable equilibrium vs unstable equilibrium, with a marble in a bowl vs a marble on top of a round rock, might help. Vs. pencils and butterflies. :^)
why isnt the CG below the CB? that would seem to be more stable.