Ship Stability Basics for Yacht Crew

9 min read · Audio Fastlane

Stability is the quiet thing keeping the boat the right way up — and it is a core topic in General Ship Knowledge and the Master exams. It sounds intimidating, but the fundamentals come down to three points and the distance between two of them. Here it is in plain English.

The three points: G, B and M

Every floating vessel has three points that decide how it behaves when it heels:

• G — centre of gravity. The single point where all the vessel's weight effectively acts. Loading and moving weight changes where G sits.
• B — centre of buoyancy. The centre of the underwater volume, where buoyancy effectively pushes up. As the hull heels, B moves.
• M — the metacentre. The point the vessel effectively pivots about for small angles of heel.

GM: the number that matters

The distance between G and M is the metacentric height, or GM, and it is the single best measure of initial stability. If M is above G, the vessel has positive GM: when it heels, it generates a righting moment that pushes it back upright. If G rises above M, GM goes negative — and the vessel becomes unstable.

Stiff vs tender

How big GM is changes the feel of the boat. A large GM makes her stiff: strong righting force, but a fast, snappy, uncomfortable roll that's hard on the vessel and the crew. A small GM makes her tender: a slow, easy roll, but less reserve of stability. Good loading is about striking a balance.

List vs loll — they are not the same

Both look like a boat leaning, but the cause is completely different, and the difference matters:

• List is caused by an off-centre weight pulling G to one side. The vessel heels and sits there. It is still fundamentally stable.
• Loll is caused by negative GM. The vessel is unstable upright, so it flops to an angle on one side — and it can just as easily flop to the other. That's why loll is dangerous and must be corrected the right way.

Free surface effect

Here's the one that catches crew out. A liquid in a partly filled tank — fuel, water, a flooded space — is free to slosh to the low side as the vessel heels. That moving weight effectively raises G, which reduces GM and eats into your stability. It is why slack tanks are a problem and why you press tanks up or run them empty rather than leaving them half full.

Why this matters on a yacht

You apply this every time you transfer fuel or water, take on stores, fill the pool or deal with water ingress. Understanding G, GM and free surface is what turns "we shouldn't do that" into knowing exactly why. It's examined in the OOW route and goes deeper in the Master Master Stability module">Stability module.

Frequently asked questions