How is it possible that a sailing boat can go faster than the wind? (Engineering, Oxford)

Well my first instinct might be to say, this is a trick question: it isn’t possible. A log carried down by a river can’t possibly go faster than the current. So how could a sailing boat possibly go faster than the wind pushing it? That’s common sense, surely? You can’t fool me! Of course I could be ‘clever’ and say ‘with an outboard motor’, or ‘on the back of a lorry’. But those glib and tricksy answers are rarely interesting for more than a second or two.

And if you think again about how sailing boats work, you may realise that common sense doesn’t always give you the right answer either. Some of the greatest scientific breakthroughs have been made when some genius finally realised that common sense – the obvious answer – was in fact common non-sense. For almost 2,000 years, for instance, people believed Aristotle’s commonsense assertion that unless things are continually pulled or pushed by some force they soon slow down naturally to rest. It took the brilliance of Galileo to fully realise that the role played by friction in slowing things down is crucial. Things don’t have a natural tendency to slow down at all. The opposite is true; a moving object will keep moving at the same speed unless something slows it down, and that slowing force is usually friction. That idea is so ingrained now that this now seems common sense, too – but it wasn’t until Galileo showed it.

With sailing boats, the commonsense answer isn’t the right one, either, as I imagine anyone who has real experience of sailing will know. Sailboats now are actually very rarely driven by the wind pushing the sails from behind, despite what common sense tells you.

That wasn’t true of the first sailboats. They were probably square rigs – that is, square sails hung from a beam or yard from the mast, across the boat at right angles. These were simple and effective. And these did catch the wind from behind the common sense way. The boat was always downwind, and ‘ran’ before the wind as the wind pushed against the sail. Because the wind was behind the boat, the boat would remain stable, despite the area of sail and the mast on top.

The wind didn’t always have to be directly behind the boat. The yard could pivot up to 45° to catch the wind from different angles. And by tacking – that is, zig-zagging – these simple square-rigged boats could even make headway against a wind blowing from in front (though not closer to the wind direction than about 70°). But these simple square-riggers could never sail faster than the wind.

But then, around 2,000 years ago, somewhere in the Middle East probably, fore-and-aft sails were invented. This was a huge and often underappreciated technological breakthrough. Unlike the simple square rig, which stretches across the boat at right angles, the fore-and-aft sail is set up in line with the boat. Fore-and-aft sails can be square in shape, but the earliest were all triangular ‘lateen’ sails, still seen today in Arab dhows. The top edge of the triangle was hung from a yard mounted on the mast and slanting down at an angle from the aft of the boat. The bottom aft corner was left free and secured by ropes.

Lateen sails work in an entirely different way to square sails. With lateens, the sail acts like an aerofoil. In fact, they are driven by the wind blowing across the sails at an angle. When the sail is at the correct angle to the wind, the sail bows out and the sails create ‘lift’ like an aeroplane’s wings because of the difference in air pressure either side of the sail created by the curve of the sails. Of course, the lift is horizontal, drawing the boat forward, not vertical as it is with an aeroplane.

The pressure on the sail tends to tip the boat over sideways, so a keel on the bottom of the boat is essential to reduce the chances of the boat capsizing – and also to ensure the boat maintains its angle to the wind and so keep the pressure on the sails up. It’s the balance between the pressure of the wind and the lateral pressure of the water that keeps the boat skimming forward.

With a lateen sail, a boat can ‘beat’ much closer to the wind – that is, it can sail almost into the wind. Early lateen boats could sail just 40° from the wind direction at very best; some modern yachts can manage less than 20°. Typically, normal cruising yachts sail at about 45° off the apparent wind (the wind relative to the boat); modern performance racing yachts typically sail at about 27°. If the sails maintain the correct angle to the wind, they can create enough lift to draw the boat along faster than the wind. This is what some modern catamarans can do well.

The fastest catamarans can sail twice as fast as the wind, and some sand yachts can sail three times faster. In 2012, Paul Larsen’s trimaran-cum-hydrofoil Vestas Sailrocket managed to smash the world sailing speed record by reaching 65.45 knots – two and a half times the wind speed! Larsen believes it can go faster still.