The line that stops a ship being overloaded
On the side of any large ship there is a circle crossed by a horizontal line, with a little ladder of shorter lines beside it. That is the Plimsoll line, and it marks how deep the ship is allowed to sit in the water.
It exists because of one man's campaign. In Victorian Britain, some owners sent out rotten, overloaded ships that were insured for more than they were worth, and sailors who refused to board could be jailed. Samuel Plimsoll spent years in Parliament fighting about it, and in 1876 the load line became law. The extra marks beside the circle allow for different water: a ship floats slightly lower in warm fresh water than in cold sea water, because that water is less dense.
Why a steel ship floats
A steel bar sinks, a steel TANKER does not, and the difference is entirely shape. A hollow hull shoves aside a huge volume of water, and the water pushes back with a force equal to the weight of what it displaced. As long as ship plus CARGO weighs less than all that water, it floats.
This is also why loading is such an exact business. Add weight and the ship settles until it has pushed aside enough extra water to match — which is precisely what the Plimsoll line keeps watch on. A canal BARGE is built wide and flat for the same reason: it has to displace its load in water only a couple of metres deep.
The box that changed the world
In 1956 an American haulier called Malcom McLean loaded fifty-eight identical metal boxes onto a converted ship. Until then, cargo went aboard as barrels, sacks and crates, handled piece by piece by gangs of dockers, and a ship could easily spend longer tied up at the JETTY than at sea.
The standard container ended that. A crane lifts a sealed box straight from a lorry or a train onto the deck, and the largest vessels now carry more than twenty thousand of them. Shipping became so cheap that where a thing is made stopped mattering very much, which is why a toy, a T-shirt and a phone can all arrive from the far side of the planet. Around ninety per cent of traded goods still travel by sea.
Odd boats with good reasons
Some designs look peculiar until you know the job. A GONDOLA is deliberately lopsided, with the left side built longer than the right, so that one rower standing on one side drives it in a straight line instead of endlessly correcting. Venice settled a separate argument about showing off in the 1500s by ordering that they all be painted black.
A TUGBOAT is mostly engine: a stubby hull, enormous power and propellers that swivel, so it can shove a LINER many times its size sideways into a berth. A CATAMARAN splits into two narrow hulls, which slice through water far more easily than one wide one and resist tipping. And a PUNT has a flat bottom and no keel at all, because the river it works is often knee deep.