If you were to take all the boats in the world out of the sea, how much would the sea level drop?
• According to Archimedes’ principle, a body sinks until it displaces its own weight of water. This is why a fully laden ship settles deeper in the water. Each tonne of ship or its cargo displaces a cubic metre of water.
Warships are referred to by their displacement. The combined displacement of the world’s military fleets is about 7 million tonnes.
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The merchant fleet is much larger, but to complicate matters, merchant ships are referred to in terms of deadweight tonnes (dwt). This is the mass of cargo that can be loaded onto an empty ship before it risks capsizing. It takes no account of the water displaced by the ship before the cargo is stowed. The Jahre Viking, launched in 1976, remains the biggest merchant vessel afloat. It can carry a cargo of up to 564,763 dwt and displaces 83,192 tonnes of water when empty.
The world merchant fleet amounts to 880 million dwt. If the empty fleet displaced the same tonnage of water as the cargo it can carry, then the fully laden fleet would displace 1760 million tonnes. Add the displacement of the military fleet and the world’s large ships will displace 1767 million cubic metres of water. When spread over the surface of the oceans (about 360 × 1012 square metres), sea level would rise by a mere 5 micrometres. The slightly higher density of sea water makes little difference to this figure.
Sea freight increases at a rate of about 3 per cent annually, equivalent to emptying the contents of 10,000 Olympic-sized swimming pools into the oceans. But this is, indeed, a drop in the ocean – 25,000 times smaller than the annual rise in sea level.
Mike Follows, Willenhall, West Midlands, UK