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Ship stability basics
A ship does not stay upright because she is heavy at the bottom. She stays upright because when she heels, the point through which buoyancy acts moves outboard faster than the point through which weight acts, and the offset between them pushes her back. Every stability rule in shipping is an attempt to keep that offset positive under conditions nobody chose.
Three points, and the distance between two of them
G, the centre of gravity, is where the whole weight of the ship and everything in her acts downwards. It moves whenever anything aboard moves: cargo loaded high raises it, 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 that underwater volume changes and B shifts towards the low side.
M, the metacentre, is where the line of buoyancy at a small angle of heel cuts the ship’s centreline. For small angles it sits still, and 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, she does not.
GM is only valid for small angles. Past those, the honest measure is GZ, the righting lever: the horizontal distance between the lines of weight and buoyancy at a given heel. Plot GZ against angle and you get the righting lever curve, and the area under that curve is energy, which is what actually decides whether a ship survives being knocked over by a squall. The stability criteria in the 2008 IS Code are written mostly in terms of those areas rather than in terms of GM alone.
More is not better
This is the point a newcomer gets backwards. A large GM makes a stiff ship: she resists heel strongly and snaps back with a short, violent roll period. That motion is what breaks lashings, shifts stowed cargo, injures crew and fatigues the structure. A small GM makes a tender ship: long, slow, comfortable rolls, and far too little reserve if anything goes wrong.
Neither extreme is wanted. A loading plan is a search for a GM inside a range, which is why the same ship in the same trade will ballast differently for a heavy cargo than for a light one. A Capesize full of iron ore is naturally stiff because the cargo is dense and sits low, and part of the loading plan is deliberately giving that stiffness away.
| Condition | What it looks like | What it costs |
|---|---|---|
| Stiff, large GM | Short violent roll period, snaps upright | Cargo damage, lashing failure, structural fatigue |
| Tender, small GM | Long slow roll, hangs at the end of the swing | Very little reserve against a shift, a flooded space or a gust |
| Negative GM | Loll: she sits at an angle and will flop to the same angle on the other side | A dangerous condition often mistaken for a list |
A list and a loll are different problems
A list is an angle caused by weight off the centreline, and the fix is to move weight the other way. A loll is an angle caused by G being above M, so the ship has no stable upright position and settles a few degrees to one side or the other. Correcting a loll by adding weight to the high side is the classic fatal error: she will pass through upright and go further over on the opposite side. A loll is corrected by lowering G, starting with the free surfaces that usually caused it.
Free surface: the loss you cannot see in the numbers
A tank that is neither full nor empty has a liquid surface that runs to the low side as the ship heels. The liquid does not have to leave the tank for the weight distribution to change, and the effect is exactly as if G had been raised. That rise, the free surface correction, depends on the width of the free surface rather than the amount of liquid, so a shallow slack tank across the full beam is worse than a deep one in a narrow wing.
This is why tanks are pressed up 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 she loses buoyancy.
Cargo that moves
Every rule above assumes the cargo stays put. Two families of bulk cargo do not.
Grain settles and leaves a void under the hatch, and once the ship rolls the surface shifts and does not come back. The International Grain Code answers this with assumed heeling moments a ship must be able to absorb, a required residual area under the righting lever curve, and limits on the resulting angle of heel. A ship offered for grain must be grain fitted and must have approved grain stability calculations, and that is a chartering condition rather than a technicality.
Liquefaction is worse because there is no warning. Group A cargoes in the IMSBC Code, iron ore fines, nickel ore, some concentrates, behave as solids until vibration and moisture turn them to slurry. The cargo then flows to one side, the ship takes a list she cannot correct, and the next roll finishes it. The control is the transportable moisture limit, set below the flow moisture point, and the shipper’s declaration of moisture content. The standard charterparty answer is to make that declaration a condition of loading.
Where a broker meets stability
Rarely by name, and constantly in effect. It sits behind the ship’s stability booklet and the loading computer that must approve any plan. It is the reason a light, high stowing cargo can fill a ship before her deadweight is used up, and the reason a dense one can be limited by hold strength and by tank top loading rather than by draught. It decides whether deck cargo is possible at all. And it turns two clauses that look like paperwork, the grain fitting warranty and the moisture certificate, into the terms that decide whether a fixture can be performed.
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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