Proper mainsail trim is important for boat balance and speed and the importance has increased over the years as many newer designed sailboats have reduce the size of the headsail while increasing the size of the mainsail. Proper mainsail trim takes into account wind speed as well as wave conditions.

Many sailboats have a host of mainsail control lines – mainsail sheet, Cunningham, traveler, backstay, boom vang, mainsail halyard and outhaul. All these control lines change the shape of the mainsail.

Before understanding adjustment of mainsail control lines, you need to see sail shape and how these control lines will adjust the shape of the sail. Sail shape has three attributes, twist, draft and position of draft. Twist is the amount, little, medium or a lot, of the mainsail leech, particularly in the upper portion of the sail falling off to leeward. If one can stand behind the boom and look up the leech of the sail, line up the top batten with the boom, it should be parallel or falling off a little. Draft is the curvature of the sail is relationship of the distance from the luff to the leach (chord). Extremes are flat draft to full deep draft or a rather large curve in the sail. A finally, where is the draft positioned in the sail. A desirable location is 40% to 50% aft of the luff. Some sailmakers put draft stripes on the mainsail to help you visualize the position of the draft.

There are two ways to tension the mainsail’s luff, mainsail halyard and Cunningham. The mainsail halyard pulls up on the luff increasing luff tension and the Cunningham pulls down on the luff increasing tension. The amount of tension on the luff moves draft position, tight luff tension moves draft position forward, while less tension moves the draft position aft.

The traveler moves the angle of the boom, so changes in the position of the mainsail in relationship to the direction of the wind without adjusting the mainsail sheet, which will adjust the twist on the upper portion of the mainsail. If you are excessively heeling close-hauled in heavy air, ease the traveler moving the boom to leeward. This changes the mainsail to wind angle relationship, thereby reducing heeling. You should adjust the traveler in relations to the wind – close-hauled, beam reaching and downwind.

Adjustable backstays allow sailors to change the bend in the mast, as a result the shape of the mainsail. Tightening the backstay induces bend in the mast decreasing the draft as the mast bows forward lengthening the luff cord in the mainsail. At the same time, the distance from the top of the mainsail to the end of the boom decreases, freeing the leech of the sail to twist to leeward. The combination of mast bend and a freer leech creates a flatter mainsail.

The mainsheet changes the angle of the mainsail to the wind as well as the tension of the leech of the sail, twist. When you ease the mainsheet, the boom rises, increasing twist in the upper portion of the mainsail. The amount of twist you want in the mainsail is determined by wind and wave conditions.

The outhaul mainly affects the draft in the lower part of the sail, near the boom. Easing the outhaul reduce the distance from the tack-to-the-clew of the mainsail (foot), thereby creating greater draft in the lower portion of the mainsail. The boom vang creates a downward force on the boom. When you ease the mainsheet then the force of the wind will lift the boom, reducing the distance from the boom to the top of the mainsail causing the top portion of the mainsail leech to fall off to leeward, inducing twist in the mainsail. To prevent the boom from lifting, tighten the boom vang.

Mainsail controls interact differently in creating sail shape, twist, draft and draft position and you should adjust each accordingly to create proper sail trim. In the beginning to understand and identify the controls adjustments on the shape of the mainsail, I went to extremes on the adjustments, so I could see how shape of the mainsail changed. After I identified how the shape of the mainsail changes per control adjustments, I was able to make slight to moderate adjustments in mainsail controls to create proper sail trim.

In a later blog, I will discuss mainsail shape per wind and wave conditions.

See you out on the water and remember Dress for the Elements to enjoy the waterlife.