Trimming the Headsail
The factors affecting headsail trim are the tension on the forestay, the tension on the luff of the sail, the position of the sheet fairleads and the tension on the leech line.
Many modern yachts are fitted with some form of backstay adjuster that serves to vary the tension in the forestay according to weather conditions. In light winds the forestay should be just firm enough to prevent the headsail from sagging away to leeward. As winds get stronger the backstay should be progressively tightened to increase the tension in the forestay until it is as tight as the strength of the rig and the condition of the hull will allow. On fractional rigs with aft raked spreaders an overtightened backstay will cause the cap shrouds to become dangerously loose. Overtightening may also cause irreparable damage to the hull of the yacht.
Headsail Luff Tension
Variation in the tension on the luff of the headsail has an effect on the position of the Centre of Effort of the sail. A light tension will tend to move the CE further aft, while increased tension will move it further forward. In light winds the luff tension should be eased slightly, and then progressively tightened as the wind increases.
Sheet Fairlead Position
Most yachts are fitted with tracks on which the position of the sheet fairleads can be altered according to conditions. Every headsail has an optimum position for the fairlead, depending on the cut of the sail, the type of rig and the design of the yacht. Any change in this position will also change the direction of the pull of the sheet on the clew of the headsail, and normally the angle of the sheet should subtend a line just above the bisecting seam of the headsail.
In light airs or when sailing off the wind the fairlead may be moved forward a little in order to increase the curvature of the sail, but when sailing close-hauled care must be taken not to stretch the leech out of shape, or to allow the upper part of the sail to touch the outer end of the spreaders.
Leech Line Tension
If the leech of the headsail flutters when the yacht is close-hauled, it means that it has been made incorrectly or that it has been stretched out of shape. When a leech line is fitted this may be tightened slightly to remove the flutter, but overtightening will produce a pronounced hook to windward in the leech, which in turn distorts the smooth flow of wind over the mainsail. In these circumstances the flutter in the leech may be accepted as the lesser of the two evils.
Trimming the Mainsail
The trim of the mainsail is controlled by the tension of the rig, the luff, the foot of the mainsail and the position of the direction of pull of the mainsheet and kicking strap.
Tension on the backstay induces a bend in the mast that tends to flatten the mainsail. this is of some advantage when sailing close-hauled in strong winds.
Tension on the luff in all conditions should always be sufficient to set up the sail so that there are no bags or wrinkles in the luff of the sail. In light winds the luff may be eased slightly and in strong winds it should be tightened. This has the effect of altering the CE and fullness of the sail in the same way as it does with the headsail.
In light airs the mainsail should be fairly full while in strong winds it should be much flatter. Increasing the tension along the foot of the mainsail helps to flatten the sail and make it more efficient in strong winds, but the tension must be released when conditions do not require it.
The Mainsheet Traveller
Most yachts are fitted with an adjustable mainsheet traveller, and this can be used to advantage in various wind conditions. The direction of pull on the sheet can be altered by changing the position of the traveller, and it should be moved to windward when sailing close-hauled, and pushed leeward when sailing off the wind.
The Kicking Strap
When sailing off the wind in strong winds the boom tends to lift in the air, thus creating an inefficient belly and twist in the mainsail. The lift of the boom can be controlled by the kicking strap, adjusting the tension to achieve the correct curvature in the sail. When sailing close to the wind tension on the kicking strap will help to prevent the boom from bending, thus keeping the sail flatter and more efficient for the conditions