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9 June 1990

Chaos, catastrophes and engineering

IN FEBRUARY 1974, the Gaul, a trawler from Hull, disappeared in heavy
seas off the coast of Norway with the loss of 36 lives. The inquiry into
the disaster found that there was nothing wrong with the ship’s design:
the stability of the vessel against capsize conformed with all required
standards. One possible explanation put forward for the loss of the vessel
was that it capsized as the result of some transient phenomenon when hit
broadside by a short succession of abnormally large waves.

‘Transient phenomenon’ means that the vessel was moving in an irregular
fashion: rocking, swaying, pitching, heaving and rolling. A trawler in a
storm is obviously tossed around, so any capsize caused by waves hitting
a vessel is a transient phenomenon. Strangely, though, the methods that
engineers use to assess how stable a ship is assume that the ship is totally
motionless. The methods simply require that the forces on the stationary
vessel when tilted at various angles should be such as to try to right it.
Various naval engineers at the inquiry pointed out that these tried and
trusted methods of stability design might not always be safe. The motion
of a ship and the way that it rocks change its stability. En gineers suggested
that because of the dynamic nature of capsize a ship’s stability should
be evaluated when it is moving as well as when it is still.

The word ‘transient’, however, has a stronger meaning than just motion:
it is a mathematical term for unsteady motion – motion that has not yet
settled down to a steady and regular pattern. The mathematics of transient

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