The basic principles of sail trim are that to get the best shape from the sail we need to present it at the right angle to the wind, then 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.
Let's begin by adjusting the headsail. We're going to start by sailing just off a close reach, so we don't pinch the wind too much. The first thing to do is to get the headsail to cut the wind at the right angle. This angle is controlled by the headsail sheet.
If you sheet in too hard, the sail will be oversheeted, the smooth airflow will be totally disrupted, and the sail will 'stall', giving massive turbulence off the trailing edge, or leech. But if you let the sheet out too far, the sail will be back winded. The sail will bulge out at the luff, and the leech will flap about as the turbulence increases.
Happily, there's an easy way to get it right. Start by letting out the headsail sheet out until the sail begins to backwind. You'll know when because the leech will start to beat as the turbulence increases. Now pull the sheet in again until the sail settles into a nice, steady shape, and no further! The sail is now cutting the wind at the correct angle. And that's all there is to it.
Next, check the camber. The maximum curve, or camber, should be about 35% to 40% aft. Get yourself into a position where you can look up at the camber - there's no need to lie on the foredeck - you can usually judge it by the seams of the sail.
With the prevalence of roller-reefing headsails, few people touch the halyard once it's up unless they're changing sails, so the temptation is to pull the halyard tight and forget about it until the end of the season. But we're going to adjust it once we're sailing.
If the camber seems too far aft, put a little more tension on the halyard, and watch the camber creep forward until it seems about right. If it's too far forward, slacken the halyard a tiny bit. Watch the sail as you put tension on the halyard.
Once you've got it right, you will see the sail settle into a nice tight shape, and unless the yacht or the wind change directions, you can leave it alone, confident that it's giving you the maximum lift. Experiment a bit. You'll soon get the feel of it.
Unfortunately, adjusting the camber can upset the arrangement of the leech. Some sails have a leech-line, a thin line sewn into the trailing edge of the headsail. If the leech is beating (fluttering) once you've set the camber and the sheet, then go forward and tension the leech-line until it stops beating and no further!
If you over-tighten the leech-line, you'll get a hooked leech, which makes the leech curve inward sharply at the edge and looks horrible. If the leech is already hooked, let the leech-line out until the leech begins to beat, then bring it back just enough to stop the beating.
If you haven't got a leech-line sewn into your headsail, or the cut of the sail is not quite right, then adjust the camber of the sail by using the halyard until you have a good compromise between the camber and a settled leech.
Finally, we're going to put in the right amount of twist. Twist is about the different direction of apparent wind between deck level and the top of the mast. On the headsail, twist is adjusted by the position of the headsail car. If the car is too far forward, the pull downwards on the leech will be too tight and you won't get enough twist.
If the car is too far aft, you'll have too much twist, and the top of the headsail will dump the wind out and lose power. The right amount of twist produces a smooth airflow across the headsail at all heights.
A good way to check that the airflow across the headsail is smooth all the way up is to sew three bits of coloured wool, or 'tell-tales', through the sail in a line just aft of the luff - one near the bottom of the headsail, one half-way up, and one near the top - so that you have about four to six inches on either side of the sail.
If you've got the twist right, you'll see each of these tell-tales streaming parallel to the deck on both sides of the sail. If the bottom tell-tale rises first as you pull in on the sheet to set the sail, your car is too far forward and you don't have enough twist. If the top one rises first, then the car is too far aft, and you've got too much twist.
Adjust the headsail sheet car until all three tell-tales act in unison, and you've got it about right. A trick here is to adjust the windward car while that sheet is slack, then change tacks to test it out.
Once you've got both of them adjusted, and provided you have a balanced rig, you can mark the right place for the car with a bit of insulating tape, because one thing is certain: some crew member is going to move the car just to see what happens.
Don't forget to adjust the twist when you reef the headsail - you can mark those positions, too, if you want.
Remember the three aims: Firstly, maximum 'lift' from the sail, secondly, minimum heeling moment, and thirdly, maximum forward movement. You've got the first one weighed off as far as the headsail is concerned, and it's now giving you maximum lift.
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.
