Main Sail Trim Guide

Main Sheet & Traveler

The two most important guides for trimming a mainsail upwind are to keep the boom on the center line of the boat, and the top batten (three batten main) or the second batten down (four batten main) parallel to the boom. At this point, the leech telltale at the appropriate batten should flow. If it doesn’t, you should twist the sail off more by easing the sheet and pulling the traveler further to windward.

Leach tension

More leech tension closes the leech which allows the boat to point higher. But when you trim past the point of stall, where the top telltale (three batten main) or second telltale down (four batten main) stops flying, the boat will start to slow down. As the boat begins to be overpowered, the traveler should be eased down to reduce weather helm and keep the boat at less than 25 degrees of heel. As the traveler is eased, you will begin to develop back-wind. This is not a problem. In puffy wind, the traveler should be played aggressively. Choppy water usually requires a little more twist in the leech than flat water so you should pull the traveler higher and ease the sheet.

Outhaul & Reef Tension

In underpowered conditions upwind, the outhaul should be adjusted to keep the shelf half open. When all crew members are on the weather rail, the shelf should be closed. If you are equipped with a flattening reef, and are having to ease the traveler frequently in the puffs, you should put in the flattening reef. The flattener is a ring in the leech just above the clew. It is independent of the cunningham and will remove boom droop and fullness from the bottom quarter of the sail. It does not have to be entirely taken up, but rather can be tensioned as more flattening is required. Off-wind, both outhaul and flattener should be eased to fully open the shelf.

Cunningham & Halyard Tension Adjustments

Cunningham and main halyard adjust the luff tension which affects the position of draft or maximum fullness in your main. As a general rule, the draft should be 50% aft from the luff. In underpowered conditions, you should have horizontal wrinkles on the luff to allow the draft to stay in its designed position. As the wind increases, the draft will move aft due to mast bend and cloth stretch, so you need to add luff tension to hold it at 50%. In choppy water, the draft would be a bit further forward (40-45%) for better acceleration. In very flat water and a good breeze, the draft can be allowed to slide aft to 60%.  Off-wind, be sure to ease the Cunningham right off.

Backstay Tension

In traditional masthead rigs with no running backstays, the permanent backstay controls both mastbend and headstay sag. Mastbend is the primary adjuster of mainsail fullness. As wind increases, so should backstay tension. We strongly recommend a powerful and “easy to use” backstay adjuster.

If your masthead rig has running backstays, these are used to control mastbend. They also have an effect on headstay sag. At a given permanent backstay tension, more “runner” will straighten the mast (making the main fuller) and remove headstay sag.

Whatever your rig configuration, you need to make your main flatter as the wind increases. Choppier water requires a bit fuller sail for a given wind strength. You should also mark all adjustments so you can consistently reproduce fast settings.

Vang Tension

If you have a powerful vang, you can use it to induce lower mastbend if you want to flatten the main entry down low. In general, this is not necessary on most big boats. Off the wind, the best rule is to keep the top or second batten down (3 or 4 batten main, respectively) parallel to the boom. Close or beam reaching, you can also watch the top or second telltale down, as appropriate, and keep it flowing. On a power reach where you are on the edge of a broach, keep a hand on the vang to dump it off if the rudder feels as though it is stalling. Downwind in big waves and wind, you can help stop oscillation by “overvanging” and hooking the leech.

Genoa Trim Guide

Wind Ranges

Be sure to follow the recommended wind range for your genoa. Using the sail in too strong a wind will not only slow the boat, but also could damage or stretch the sail.

Sheet tension

The most important variable is sheet tension. You should develop a relationship between spreader tip and the sail. Choppier water and/or lighter air need more leech twist, so the sheet should be eased slightly. In flatter water, less twist will help pointing ability provided the boat has enough power. A careful eye on the knotmeter or other boats around you will help you determine how tight to sheet. You may also put a telltale on the upper leech to check for stalling. If it doesn’t flow, ease the sheet. After tacking, the sheet should be trimmed short of full tension until the boat is up to full speed. In puffy wind, a trimmer should stand by the sheet to adjust it as the wind changes. Also, some communication between the helmsman and the trimmer will help them get the most out of the boat.

Genoa Lead

he genoa lead controls fullness in the bottom third of the sail. It can be thought of as an outhaul for the genoa. The more power needed for choppy water or lighter air, the further forward the lead should be. To set the furthest forward lead, head the boat up and watch to see where the luff backwinds first. It should backwind evenly. The foot should be full and lay almost on the lifeline. This is your powered up setting. As the wind increases, the lead should be moved aft to flatten the foot and depower the top of the sail by allowing it to backwind first. The fore and aft movement can be as much as a foot on a #1 genoa. At the top of its range, the genoa should be trimmed with the foot flat against the shrouds. In choppier water, the lead should be further forward for a given wind strength.

In and out placement of the lead is adjusted less often. Track placement determines the innermost lead. Most modern racing boats sheet the #1 genoas at 10 degrees off center in light-moderate air. To extend the upper range of your heavy #1, you can sheet further outboard by using a short sheet run to a second track or the toe rail. #2 genoas are limited by the shrouds and their shorter LP to about 13 degrees off center. But a #3 genoa that can trim in front of the spreaders can be lead as close as 9 degrees in flat water to allow very high pointing. Fore and aft lead movement is more critical and the range is smaller – 3 to 4 inches. Any time you are reaching, the lead should go outboard. When the apparent wind moves aft of about 35 degrees, the lead should go to the rail and further forward. Again, the rule of thumb is to keep the luff backwinding evenly.

Headstay Sag

While the lead controls the fullness in the bottom of the genoa, headstay sag controls fullness in the middle and top. More sag adds fullness, moves the draft forward slightly, and makes the entry rounder. Whenever your boat needs more power, some headstay sag can be beneficial. This occurs typically with the light #1 up. But if you get caught with a sail in wind below its range, sag will help power it up. Be careful not to use so much sag that the headstay bounces in choppy water. As the wind increases, nearing the upper range of your genoa, you should remove as much sag as possible to flatten the sail. How you do this depends on your type of rig. If you have a masthead rig,

backstay tension controls sag. But remember that overbending the mast can contribute to sag. This may happen if you have a flexible mast and don’t use enough running backstay. In general, the backstay should be well eased in light air (as low as 500 lbs.) and tensioned as wind increases. The maximum backstay tension varies from boat to boat. On a typical 40 footer, it can be as high as 5000 lbs. A typical 30 footer would use about 3000 lbs. On a fractional rig, running backstay controls sag and this adjustment becomes critical, especially in puffy wind.

Small changes in runner tension greatly affect headstay sag, so you should have a crew member assigned to this job. As wind drops, the runner should be eased and taken up as wind increases.

Halyard Tension

Halyard tension controls fore and aft position of the maximum fullness of the draft. The draft should be about 40% aft of the luff. In underpowered conditions (usually with the light or all purpose #1), a good rule of thumb is to leave some small horizontal wrinkles at the luff. As the boat gets powered up (10-12 mph apparent wind) you should barely remove the wrinkles. In stronger breeze with the heavy #1, #2, or #3, key on the draft position. More halyard tension holds the draft forward and makes the head slightly fuller. Choppy water requires the draft to be further forward and the entry rounder. This makes the boat easier to steer. In flatter water, the entry can be finer and the draft allowed to slide aft to improve pointing. Always be careful of using too much halyard, especially in puffy wind. Too much luff tension is slower in light air than vice versa.

Special thanks to Ullman Sails from which we borrowed this great guide.

Now that you’ve finished tacking, why not sign up for our free Digital Magazine Wind & Compass.