On most boats you have five controls to properly shape and control your mainsail; the mainsheet, traveler, outhaul, cunningham, and mast bend. For this article we are looking at a typical masthead rig with overlapping headsails. In making trimming the main you need to be well aware of what effect changes in trim will have on performance. A flatter sail will cause less heeling and have less drag which means more speed in conditions where you are over powered, making the sail fuller will produce more power when need. A tighter leech will improve pointing (up to the point where the sail is stalled) while a more open leech will improve acceleration and speed. A fuller sail will inherently be tighter leeched than a flat sail which will help pointing.
In medium air, usually 7 to 10 knots of wind where you aren’t overpowered, the sail should take its natural shape without much adjustment. This condition is what a typical mainsail should be designed and cut for. You want to use a combination of mainsheet tension and traveler position so that the boom is right on centerline and the top of the sail is twisted just enough so that the back half of the top batten is pointing straight aft, parallel to the centerline of the boat. The traveler will have to be pulled well to weather of the centerline to achieve this. The tell tale on the top batten will be streaming back most of the time but the leech should be tight enough so that the tell tale does stall out now and then. The cunningham should be tensioned just enough to remove wrinkles along the luff and the outhaul so be set so the center of the foot is 2” to 4” away from the boom, a little more if the sea conditions are bumpy.
As the wind increases and you start to have more power than you can use flatten the main a little bit. Tighten the outhaul so the foot is pulled up close to the boom and tighten the cunningham a little more if needed to take the wrinkles out along the luff. If you have a bendy rig you should tighten the backstay to increase mast bend and flatten the sail. As you do this the leech will become more open so you will need to tighten the mainsheet enough to keep the leech tight. In this condition the top tell tale should be streaming back most all the time and you want the leech as tight as you can get it before the top tell tale starts to stall. As the wind increases you will have to starting to let the traveler down an inch or two at a time to keep from being over powered.
By the time wind gets over 12 knots you want to get the main even flatter. Pull the outhaul all the way tight and bend the mast to flatten the sail even more. If you are consistently overpowered ease the mainsheet an inch or two to let the top twist open a little more. Generally I let it twist just enough so that I’m not over powered in the average wind and then play the traveler up and down in the puffs. Pull the cunningham tight to keep the draft in the sail forward and the leech open. The top tale should be streaming aft all the time.
In the 5 to 7 knot range you want the main a little fuller to produce more lift in the under powered conditions. Ease the outhaul off so the foot is 6” to 8” away from the boom and ease the backstay to reduce mast bend. This will also make the headstay looser which will help the genoa shape in light air, making it fuller and the entry rounder. Let the traveler down a few inches so that the boom is a little below the centerline to increase speed. In these conditions you need to point slightly lower to develop speed and increase your apparent wind. Ease the cunningham so that you have a few wrinkles along the luff. You want the sail as soft as possible so that it responds to very slight changes in wind pressure. The tell tale on the top batten will be stalled 50 to 60% of the time.
In very light conditions, under 5 knots, you actually want the mainsail to be a little flatter. Too much camber and the flow in light air won’t stay attached. Pull the outhaul out just snug so the foot is up close to the boom and put on enough backstay to slightly bend the mast. This will flatten the sail and open the leech which both help to keep good flow over the sail. The cunningham should be completely loose so you have some wrinkles along the luff.
Below is a quick reference summary.