By Ian Walker GBR Challenge's skipper explains how his crew work the boat

TACKING (bow passes through eye of wind).

The tactician decides when to tack after consultation with the navigator, helmsman and skipper.

The helmsman chooses the exact moment and finds a flat spot on the water to avoid waves.

He calls the tack and starts to turn the boat up into the wind.

The grinders - four on the headsail, three on the main - will keep the sails angled to the wind until it begins to transfer from one side to other.

As the boat comes head to wind the sails start to flap and the trimmers cast off the sheets from one side.

The new trimmer tails (pulls in) the new sheet and the grinders work the winches to pull in the sails on the other side.

The mainsail and headsail trimmers - with the help of the grinders - then set the sails to the best possible shape for accelerating out of the tack.

Even a simple tack uses 14 of 16 people on board.

The helmsman steers the boat round the mark and turns downwind.

As they adjust the sails for the turn the crew are getting ahead of themselves, creeping the spinnaker up to the top of the mast.

The sail is banded in wool until it is ready to fly at which point a rope is pulled to break the wool and burst the spinnaker open, filling it with wind.

The headsail is then lowered to the deck to allow the spinnaker to fill properly.

The skill is getting it to happen as soon as possible after the boat has turned downwind but not before.

A good set is under 10 seconds, or the equivalent of two boat lengths past the windward mark.

When gybing you have the spinnaker up so instead of transferring the headsail you have to transfer the spinnaker and its pole from one side to another.

The mastman releases the pole at the mast end - it either comes off completely for an asymmetrical spinnaker, or shoots up a track on the mast for a conventional spinnaker.

The bowman releases the spinnaker guy - the rope that adjusts the angle of the pole - from the outboard end.

The pole is swung under the forestay to the other side where he attaches the new spinnaker guy to the pole.

The sail is then trimmed in and constantly adjusted by the trimmers and grinders.

The helm judges the perfect angle for the fastest course around the mark as the crew prepare to drop the spinnaker and get ready to hoist the headsail to point back upwind.

When the boat is round the mark and heading upwind the crew hoist the headsail and drop the spinnaker, collapsing the air out of it and gathering it into the boat.

If the spinnaker is inadvertently dropped into the water, which is likely to happen at some point during the series, it poses a huge risk to the boat as well as slowing it down.

The sail acts like a huge fishing net and is impossible to hang on to.

If this happens all the ropes connecting the sail to the boat may have to be cut free to prevent the mast being ripped out.