Trimming your Mainsail-the Six Key Points

by Andrew Dove, Area Manager North Sails Caribbean on 27 Dec 2007 Andrew Dove here offers Sail-World readers the second in his series of helpful advice about the care, handling and use of your sails. This week is is on the critical aspects you need to consider to trim your mainsail to perfection:

Mainsail trim is linked closely to the helm. Depending upon your setting, the boat will be more or less heeled and will have more or less helm. If you sail a ketch, then the setting of the mizzen, though often small in size, can have a great influence on your boat speed.

In the previous section we noted that the Headsail is most probably the principle driving force of your boat and, as this sail will penetrate the air first, the mainsail adjustment always follows that of the foresail. Six main adjustments are available to the crew on most boats

1/ The Halyard and Cunningham tension.
2/ The Outhaul tension.
3/ The Sheet and Vang Tensions.
4/ The Main Traveller
5/ Leech line adjustment
6/ The Back Stay tension.

The halyard and Cunningham excurse a vertical tension through the sail. When setting sail, ensure that the Cunningham is released and hoist your sail until all horizontal creases begin to disappear. Some boats may not have a Cunningham and will need to hoist, sheet in, then check for luff sag and be prepared to ease the sheet, luff a little, and retention the halyard. Those who have read a few of my articles in these pages will know what I think about poor halyards.

It is certain that an inadequate halyard will stretch and render halyard adjust unnecessarily difficult. If you own the boat you are sailing and a Cunningham is absent, do give thought about adding one. Tensioning a sail from the bottom is much easier than tensioning from the head. Do check that your Cunningham lead runs straight and do not encourage the sail to move aft when tensioned, as this can damage the first slide or bolt rope.

The Cunningham is used to maintain and adjust the tension in the luff. If you tighten it, horizontal creases will be absorbed and the draft or shape of the sail will move forwards. When off the wind the tension is released, the shape moves back and the sail shape deepens.

When cruising there is no real reason to sail with too much luff tension; if a vertical crease appears, ease the Cunningham or halyard to avoid damaging your sail. More experienced sailors tend to mark the halyard where a visual reference exists to hoist the sail each time to the correct height with the correct tension. Separate marks are required for each reef.

A number of bigger mainsails have a complex luff curve with the slides positioned on a continual line but with the cloth between the slides slightly cut back. This reduces the need for excessive luff tension to set the sail and often avoids that irritating vibration of the luff slides. Do not be surprised if you’re renting a catamaran and the halyard seems tight that the sail falls away a little from the mast.

Outhaul tension may be considered as less critical for a cruising boat and I have built a number of catamaran sails that have a pin system that fixes the clew ring at a given position. These have an added inconvenience in that they hold the ring vertically and, as the sail is lowered and flaked, the rings twist in its webbings causing unnecessary chafe.

On the sails for the 90m Ponnant sailing ship, I designed a clew ring with a hinge to allow the sail to flake flat either to port or to starboard. Naturally where the outhaul can be adjusted, in light airs and in seas where a little more power is required, then easing the foot of the main will increase draft. Most people will realise from experience the direct relationship between leech flutter, twist and sail shape. Loosening the foot can close the leach and reduce leech flutter.

The main control over the leech twist is from the vang or mainsheet. Photos of yachts of the time of the Second World War show an absence of a kicking strap or vang. Though I’m sure that many may claim to be the initiator of the idea of the modern kicking strap, I believe that it was originally invented on a modern yacht by Jack Holt of Dinghy equipment and design fame.

While the boom is sat vertically above the mainsheet track the mainsheet can be used to simply apply a vertical tension. Obviously as the wind builds or one bears away and the boom is eased beyond the mainsheet track, then the vertical control over the sail comes from the vang tension.

As the apparent wind speed at the base and head of the sail are different, twist is required for the sail to be set correctly over its entire height. Leech telltales are the clear way of estimating required twist. One should attempt to sail with the lower telltales flying horizontally and the uppers alternatively flying and falling off a little. The pitching of the boat through the waves and the acceleration and deceleration of the mast head causes certain instability in the upper telltales.

The mainsheet car determines the angle of the sail with respect to the boat when sailing to windward. Generally in light airs, one can ease the sheet to allow some twist and to bring the traveller car up to windward a little to centralise the sail. As the wind builds the traveller will probably be eased as the sheet is adjusted to maintain the correct loads on the helm. Remember, mainsheet trim fundamentally determines how hard one must work at the helm and how much rudder angle one needs to apply. Well adjusted in a breeze may mean that the luff of the sail backs; this is also the sign that one should consider reefing.

If you have telltales on the body of the mainsail they function in much the same manner as described last month for the headsails. If the windward telltales stall, try sheeting in if the stability of the boat allows. If the leeward telltales stall you should either luff a little or ease the sheet. The telltales on the mainsail should not be positioned too far forward in the sail as they will not readily fly in the disturbed air around the mast.

Once the boat is sailing with a correctly adjusted mainsail, then the leech line can be adjusted. Most sails with a reasonably flat leech require a little leech line to avoid leech flutter. Care should be taken not to over tension as this causes leech hook that will create an air break and slow the boat down.

As much as one will probably need to tension the leech line as the wind increases then one will need to ease the line as the wind weakens. Again leech line adjustment is rendered much more difficult with a stretchy halyard.

Many cruising boats have fixed backstays and no runners. If you do have these controls they have an important influence on the forestay tension of a number of rigs which will affect headsail shape and windward performance. As mainsail shape is generated by cloth shaping and luff curve, straightening the mast pushes cloth back into the sail and fills the attack. If one induces mast bend, the luff will flatten and the leech open to the extent that the roach may invert. Skillful adjustment of the mast controls and Cunningham tension gives a wide range of possible sail profiles.

Leech inversion, particularly in catamaran large girthed mainsails, is generally blamed on poor battens. Though batten quality has an important influence on sail shape, I think at a cruising level that battens enable the design sail shape to be expressed, and that a sail with a major leech full off and hard spot within the sail is caused by poor cloth or faulty design. Good batten tension is probably just enough to remove any vertical creases across thee pockets.

If you are worried about sail shapes, then next time you are sailing, take a series of photos with a wide 30mm lens from the middle of the boom but as low as you can get. Try to hold the camera at an angle so that the tack region is in one corner of the frame and the clew region is in the diagonally opposing corner.

At North Sails, a