To many, the America's Cup yachts probably look more like aeroplanes than yachts, flying at an altitude of a couple of metres above the water. In fact, Jimmy Spithill, skipper of Oracle Team USA took flying lessons the better to understand how to control this new craft. 'Flying' is achieved thanks to wing-like hydrofoils (normally just referred to as 'foils') attached to daggerboards (and the rudders) under each of the hulls. The foils generate lift in the water, just as an aircraft wing does in air.
Like an aircraft, the yacht has to attain a certain minimum speed before it can take off, yet the boats and their crews have reached such a level that the boats can remain airborne throughout the entire race. Of course, if things go wrong and the yacht slows down then it can revert to a conventional catamaran, floating on its hulls. If that happens, though, much speed is lost.
Like any yacht the ACs depend on the wind to provide their motive power. Conventional yachts do this through the use of sails. Sails are large areas of canvas traditionally, but more recently they have been made of other materials, such as mylar and kevlar. Sails are usually attached to 'spars' such as a mast and a boom. Sailing - and sailing at maximum speed - is largely about ensuring that the sail is set at the correct angle to the wind direction. The appropriate angle depends on the course of the yacht in relation to the wind, the wind strength and the boat's speed. The angle is controlled through the use of ropes (although, confusingly, sailors refer to these as 'sheets').
A large sail requires a lot of force to control, more than any person could manage just holding on to the rope. Hence, winches are used. Through mechanical arrangements of levers and gears a sailor winds a handle to make the winch rotate and the rope around it is pulled in. The crews of racing yachts (such as previous generations of America's Cup yachts) included 'grinders'. The role of the grinder was to turn handles on the winches to provide the power to trim the sails - a job undertaken by another crew member, a 'trimmer'. In the 1988 America's Cup the Swiss defender Alinghi had a motor to provide power to the winches, but thereafter that was thought to not really be playing the game and hence powered winches were outlawed.
Conventional winches only require power when they are being used to tension the sheet attached to them. Thus, when a yacht is changing course such that the sails need to be sheeted in you would expect to see the grinders providing the power, but once the sail is set, the grinder would stop.
The AC yachts do not use conventional sails, either. They are powered by a wing. Again this is rather similar to an aircraft, except that the wing is mounted vertically, not horizontally. Getting the wing at the right angle is again critical. The wings are not controlled by ropes, though, but rather by hydraulics. The hydraulics have to be pressurized, and that pressure is provided by a team of grinders. As you watch the America's Cup yachts sailing around the course, you will see most of the crew constantly winding handles. They are generating the power into the hydraulics system which can be used to adjust the wing. In fact it is not only the wing that needs to be adjusted. The angles of the foils are critical too and these can be adjusted through hydraulic power.
Emirates Team New Zealand are different in that their grinders do not wind handles. They use leg power, being mounted on bicycle-like winches.
Each yacht has a crew of 6. Four of them are grinders, one of whom will also double as tactician. The other two are the helm and the trimmer - who is mainly responsible for trimming the wing.
The casual observer could be forgiven for thinking that the grinders are in some way powering the boat, that the harder they pedal (or wind) the faster the yacht travels, as if it were some kind of sophisticated pedalo. However, that is not the case. What the grinders are doing is storing up hydraulic pressure to be available to control the wing and the dagger boards. Were they to stop pedalling, then the yacht would not stop. All that would happen would be that the pressure in the hydraulic reservoirs would eventually deplete and it would be impossible to trim the wing or move the dagger boards.
The hydraulic system stores power until it is needed, unlike a conventional winch which has power only while the grinder is operating. Thus on an AC yacht the grinders can - and should - work all the time. That way they keep the pressure up until it is needed. They cannot overpower the system. In the event that it should reach capacity it will just bleed off the extra pressure.
In principle the hydraulics could be replaced by electric motors, powered by batteries. However, as noted earlier, electrical winches were outlawed in 1988. It seems that it was probably felt that if electrical winches were allowed then it would only be one step from having electrical motors attached to propellors, and it would no longer be a sailing race at all. Relying on muscle power does mean that the competition has an athletic element. It also gives the crew something to do. Were there no requirement for grinders then the yachts could probably be sailed by the helm and a trimmer alone.
It would not be true to say that the yacht rely entirely on the wing for power. They do also carry a more-conventional fabric foresail.