Sail trim can be a bit of a mystery to the inexperienced sailor. Most of us know the basics - letting out the main and the headsail as we come off the wind, or tightening the sheets as we come closer to the wind. But the rest can seem a bit like magic. In fact simple steps will help you get the best from your sails no matter where the wind is coming from.
Trimming your sails to get the very best from your yacht will help you gain a knot or two, and so save an hour or two on a longer passage. But above all, it will ease the load on your crew, and the helmsman will love you for it.
The most important thing to remember is that the sail isn't a big sheet that the wind pushes along. A properly trimmed sail operates on the same principle as the wing of an aircraft, cutting the air in the direction of the airflow, pulling the yacht forwards in the same way a wing pulls the aircraft upwards.
As an aircraft flies through the air, the curve at the front of the wing, or its aerofoil shape, cuts the air creating a low pressure area on the top surface of the wing, and a high pressure area on the bottom of the wing. So the wing is both 'pulled' upwards by the low pressure above it, and 'pushed' upwards by the high pressure below it, lifting the whole contraption into the air.
Unless you're on a dead run with the wind right behind you (which is usually to be avoided), the sails should be acting in much the same way as an aircraft wing. If you've got them set correctly, they'll have a gentle aerofoil shape, and the air flowing smoothly across them will create a high pressure area on the windward side, and a low pressure area on the leeward side. The keel will stop the yacht from going sideways, resolving the forces into a heeling moment and the forwards movement you want. The trick to good sail trim is to shape the sail so that you get three things:
- maximum lift
- minimum heeling moment
- maximum forward movement
In order to achieve this, we need first of all to create as near a perfect aerofoil shape to each of the sails as we can, and then present it at the right angle to the wind.
To get the right shape, we need to manage two things: camber, and twist. The maximum camber, or the curve that gives the sail its aerofoil shape, should be about 35% to 40% aft from the luff, or cutting edge of the sail, and is adjusted mainly by the halyard.
The tighter the halyard, the further forward the point of maximum camber. On a decently cut suit of sails, it is possible to adjust the point of maximum camber by looking up at the seams of each sail, and putting tension on the halyard, or easing it off.
Twist is a bit more complicated, but ignore it and you'll never get the best from your sails. If you are sailing forward at, say, six knots, with a 20 knot wind on the beam, you will experience two wind forces.
The first is the 'true' wind blowing at 20 knots at right angles to your direction. The second is the wind your forward motion is creating, and it comes from dead ahead - in this case, all six knots of it.
The combination of the two creates the 'apparent' wind, which is what your sail actually experiences. Apparent wind is a combination of the true and the manufactured wind, and it always blows from further forward than the true wind if you are moving.
Because of surface friction with the sea, the apparent wind at deck level is usually a point or two further forward than the wind at the top of the sail. So if you don't put the correct twist into the sail, part of the sail could be stalled.
A sail will naturally want to twist, and all you have to do is control the amount by the tension on the leech. On the headsail, this is managed by the position of the car. On the mainsail, it's managed by the mainsheet or the kicker.
Presenting the aerofoil at the right angle to the wind is a matter of using the headsail sheet, the mainsheet, and the mainsheet traveller in the right way. Think of an aircraft wing: if the pilot tries to climb too steeply, the wing will stall. Equally, if you sheet in the sail too tightly, you'll stall the sail. You can back wind it too, by doing the opposite.
The final basic principle of sail trim is how full, or how flat, your sails are. When an aircraft slows down, the pilot will deploy flaps and slats to increase the camber of the wing. This gives him more lift, but also greater drag. When he's going faster, he reduces flaps and slats, effectively flattening the camber of the wing.
In the same way, when the wind is light, we want the sails to be fuller in order to create more lift; when it's blowing hard, we want the sails to be flatter so as not to overpower the yacht.
So to recap the basic principles: to get the best shape to the sail, we need to adjust the camber so that it's about 35% to 40% aft of the cutting edge, or luff, of the sail. We need to put in the correct amount of twist, to allow for changes in the direction of the apparent wind at different heights, and we need to adjust the fullness of the sail to match the force of the wind. Think of it as the ABC of sail trim: Camber, Twist and Fullness.
Richard Thomas holds a Commercially Endorsed RYA Yachtmaster and Cruising Instructor (Sail) certificate. He runs his own delivery business, www.yachtmovers.co.uk and is available for deliveries, assisted passages, own-yacht tuition, and yacht management.
