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Ship stability basics
A ship does not stay upright because it is heavy at the bottom. It stays upright for a different reason. When it heels (leans to one side), the point where buoyancy pushes up moves outwards faster than the point where weight pulls down. The gap between the two points pushes the ship back upright. Every stability rule in shipping tries to keep that gap positive, even in conditions that nobody can control.
Three points, and the distance between two of them
G, the centre of gravity, is the point where the whole weight of the ship and everything in it acts downwards. It moves whenever anything on board moves: cargo loaded high raises it, and ballast in the double bottom lowers it.
B, the centre of buoyancy, is the centre of the underwater volume, and it acts upwards. When the ship heels, the shape of the underwater volume changes, and B moves towards the low side.
M, the metacentre, is where the line of buoyancy at a small angle of heel crosses the ship’s centreline. For small angles it stays still. The vertical distance from G to M is GM, the metacentric height. If M is above G, GM is positive and the ship returns upright. If G rises above M, the ship does not.
GM only works for small angles. Beyond them, the correct measure is GZ, the righting lever. This is the horizontal distance between the line of weight and the line of buoyancy at a given angle of heel. Plot GZ against the angle and you get the righting lever curve. The area under that curve is energy, and that energy decides whether a ship survives being pushed over by a sudden storm. The stability criteria in the 2008 IS Code are written mostly as those areas, not as GM alone.
More is not better
Newcomers often believe the opposite. A large GM makes a stiff ship. It resists heeling strongly and returns upright quickly, with short, violent rolls. That motion breaks lashings, moves stowed cargo, injures crew and weakens the structure over time. A small GM makes a tender ship. It rolls slowly and comfortably, but has far too little reserve if anything goes wrong.
Neither extreme is wanted. A loading plan aims for a GM inside a range, which is why the same ship in the same trade takes on ballast differently for a heavy cargo than for a light one. A Capesize (a very large bulk carrier) full of iron ore is naturally stiff, because the cargo is dense and sits low. Part of the loading plan is to reduce that stiffness on purpose.
| Condition | What it looks like | What it costs |
|---|---|---|
| Stiff, large GM | Short, violent rolls; returns upright quickly | Cargo damage, broken lashings, structural fatigue |
| Tender, small GM | Long, slow rolls; pauses at the end of each roll | Very little reserve against a cargo shift, a flooded space or a gust of wind |
| Negative GM | Loll: the ship sits at an angle, and will fall to the same angle on the other side | A dangerous condition that people often mistake for a list |
A list and a loll are different problems
A list is an angle caused by weight that is off the centreline, and the fix is to move weight to the other side. A loll is an angle caused by G being above M. The ship has no stable upright position, so it settles a few degrees to one side or the other. The classic fatal error is to correct a loll by adding weight on the high side. The ship then passes through upright and leans further over on the other side. You correct a loll by lowering G. Start with the free surfaces (part-filled tanks) that usually caused it.
Free surface: the loss you cannot see in the numbers
In a tank that is neither full nor empty, the liquid runs to the low side when the ship heels. The liquid does not have to leave the tank to change the weight distribution: the effect is exactly as if G had been raised. That rise is the free surface correction, and it depends on the width of the liquid surface, not on the amount of liquid. So a shallow, part-filled tank across the full width of the ship is worse than a deep one in a narrow side tank.
This is why tanks are filled completely or emptied rather than left half full, why a ballast exchange is planned rather than improvised, and why a ship taking water into a hold loses stability long before it loses buoyancy.
Cargo that moves
Every rule above assumes the cargo stays in place. Two kinds of bulk cargo do not.
Grain settles and leaves an empty space under the hatch. Once the ship rolls, the grain surface shifts and does not come back. The International Grain Code deals with this in three ways. It sets assumed heeling moments (turning forces) that a ship must be able to absorb. It requires a minimum remaining area under the righting lever curve. And it limits the angle of heel that results. A ship offered for grain must be grain fitted (equipped to carry grain safely) and must have approved grain stability calculations. This is a condition of the charter, not a minor formality.
Liquefaction is worse, because it gives no warning. Group A cargoes in the IMSBC Code, such as iron ore fines, nickel ore and some concentrates, behave as solids until vibration and moisture turn them into a slurry. The cargo then flows to one side, the ship takes a list it cannot correct, and the next roll can capsize it. The control is the transportable moisture limit, which is set below the flow moisture point (the moisture level at which the cargo starts to flow), together with the shipper’s declaration of the cargo’s moisture content. The usual charterparty solution is to make that declaration a condition of loading.
Where a broker meets stability
Brokers rarely talk about stability by name, but it affects their work all the time. It is the reason for the ship’s stability booklet and the loading computer, which must approve every loading plan. It is also why a light cargo that takes up a lot of space can fill a ship before its deadweight is used. It is why a dense cargo can be limited by the strength of the holds and the tank top, not by draught. It decides whether deck cargo is possible at all. Two clauses can look like paperwork: the grain fitting warranty and the moisture certificate. Because of stability, these two clauses decide whether a fixture can be carried out.
References
- IMO, International Code on Intact Stability, 2008, resolution MSC.267(85), 2008. imo.org
- IMO, International Code for the Safe Carriage of Grain in Bulk, resolution MSC.23(59), 1991. imo.org
- IMO, International Maritime Solid Bulk Cargoes (IMSBC) Code. imo.org
- IMO, International Convention on Load Lines, 1966. imo.org
- IMO, Bulk Carrier Safety. imo.org
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