Ship stability basics come down to three points and the distance between two of them. Stability is the quiet thing keeping the boat the right way up, and it is a core topic in General Ship Knowledge and in the higher deck certificates. Here it is in plain English, the way I would explain it on the bridge.
Key points
- Every vessel has three key points: G (centre of gravity), B (centre of buoyancy) and M (the metacentre).
- GM, the distance between G and M, is the main measure of initial stability. Positive GM means she comes back upright.
- Stiff and tender describe the feel of the roll: large GM is stiff and snappy, small GM is tender and slow.
- List and loll look alike but have opposite causes, and loll is the dangerous one.
- Free surface effect from slack tanks raises G in effect and eats into GM.
The three points: G, B and M
Every floating vessel has three points that decide how it behaves when it heels:
- G, the centre of gravity. The single point where all the vessel's weight effectively acts. Loading, using fuel and moving weight all change where G sits.
- B, the centre of buoyancy. The centre of the underwater volume, where buoyancy effectively pushes up. As the hull heels, the underwater shape changes and B moves towards the low side.
- M, the metacentre. The point the vessel effectively pivots about for small angles of heel. It is a useful reference only for small angles; at larger angles of heel the picture gets more complicated.
GM: the number that matters
The distance between G and M is the metacentric height, or GM, and it is the best single measure of initial stability. If M is above G, the vessel has a positive GM: when she heels, she generates a righting moment that pushes her back upright. If G rises above M, GM goes negative and the vessel becomes unstable upright.
Remember that GM only tells you about initial stability. How she behaves at larger angles of heel depends on the righting lever (GZ) curve, which is why the stability booklet for your vessel matters as much as the single GM figure.
Stiff vs tender
How big GM is changes the feel of the boat. A large GM makes her stiff: a strong righting force, but a fast, snappy and uncomfortable roll that is 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.
| Stiff (large GM) | Tender (small GM) | |
|---|---|---|
| Roll | Fast and snappy | Slow and easy |
| Righting force | Strong | Weak |
| Reserve of stability | Plenty | Less |
| Main problem | Hard on crew, gear and structure | Easier to upset or to take a large heel |
🎧 Study on watch: Stability is much easier to learn by ear than from a diagram. The Audio Fastlane app has narrated courses built around the MCA syllabus for yacht crew. Get the courses in the Audio Fastlane app →
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. She still has positive GM, so she is fundamentally stable.
- Loll is caused by negative GM. The vessel is unstable upright, so she settles at an angle on one side, and she can just as easily flop to the other. That is why loll is dangerous and must be corrected the right way.
The classic clue is what happens when you change the heel: a listing vessel stays where the weight is, while a lolling vessel may flop across to the other side. If you suspect loll, do not start moving heavy weights across the deck or filling tanks on the high side. Textbook practice is to lower G, for example by ballasting low tanks (if the tanks are divided, fill the low side first, accepting that the angle may increase slightly at first) so that she does not flop violently to the other side. Follow your vessel's stability guidance and get advice from the Master.
Free surface effect
Here is the one that catches crew out. A liquid in a partly filled tank, whether fuel, water or a flooded space, is free to move to the low side as the vessel heels. That moving weight effectively raises G, which reduces GM and eats into your stability. The effect grows with the cube of the tank's breadth, so one wide slack tank does far more harm than several narrow ones, which is why slack tanks are a problem, and why you press tanks up or run them empty rather than leaving them half full.
Why ship stability basics matter on a yacht
You apply this every time you transfer fuel or water, take on stores, fill the pool, load a tender or deal with water ingress. Understanding G, GM and free surface is what turns "we shouldn't do that" into knowing exactly why. It is examined on the route to becoming an officer of the watch (transverse stability is part of the MCA's General Ship Knowledge written exam for OOW (Yachts)), and goes deeper as you move up towards Master. See how to become an OOW on yachts for where it fits in.
Frequently asked questions
What is GM in ship stability?
GM is the metacentric height, the distance between the centre of gravity (G) and the metacentre (M). It is the key measure of initial stability: if M is above G, the vessel is stable and returns upright when she heels.
What is the difference between list and loll?
List is caused by an off-centre weight and leaves the vessel stable but leaning. Loll is caused by negative GM, which makes the vessel unstable upright so she flops to an angle and can flop to the other side. Loll is the dangerous one.
What do stiff and tender mean for a ship?
A stiff vessel has a large GM and a strong but fast, snappy roll. A tender vessel has a small GM and a slow, easy roll but less reserve of stability.
What is free surface effect?
Free surface effect is the loss of stability caused by liquid moving in a partly filled tank as the vessel heels. The shifting liquid effectively raises the centre of gravity, reducing GM.
How do you reduce free surface effect?
Keep tanks either full (pressed up) or empty where you can, minimise the number of slack tanks, and use divisions in tanks where fitted. Because the effect rises with the cube of the tank's breadth, wide slack tanks do the most damage, and dividing a tank down the centreline cuts the free surface effect to a quarter.
Related guides
- How to become an officer of the watch on yachts
- How to pass your OOW oral exam
- Yacht crew qualifications explained
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