09/01/2023
How-to: Mainsail Trim 101
It’s easy to assume accurate mainsail trim is only of concern to racers, but nothing could be further from the truth. Aside from increasing boatspeed, a well-trimmed main will reduce heel, minimize weather helm and decrease leeway. The result is shorter, more comfortable passage times and more enjoyable sailing in general, especially when sailing to windward.
One of the first things many of us were taught is “a flappy sail is not a happy sail.” However, simply keeping your sails full is only part of the story and often leads to over-sheeting, i.e., trimming your sails in too tight, which increases weather helm, or the tendency of a sailboat to want to round up into the wind. This, in turn, makes the boat more difficult to steer. It also makes things more uncomfortable in general by increasing the angle of heel.
Twist
Pretty much every sailor is familiar with the function of telltales on the luff of a jib or genoa, and as a consequence do their best to keep them streaming. Mainsail leech telltales, however, are not as well understood. Ideally, they should stream horizontally as well. In the event they start disappearing around onto the lee side of the sail it’s a clear indication the trailing edge of the main is strapped in too tight, causing it to “stall,” as the airflow separates from the leeward side of the sail in a welter of speed-robbing turbulence.
To fix the situation, it’s necessary to ease out the aft-most portion of the sail. There are two ways of doing this: 1) by changing the sail’s overall angle of attack, i.e., the sail’s orientation relative to the apparent wind, something that is done on a beat by easing the traveler, or 2) allowing the leech of the sail to fall off to leeward, i.e., to increase the amount of “twist” in the sail by easing out the mainsheet.
Note that on boats without a main traveler, the mainsheet by default becomes the means of adjusting angle of attack as well—a situation that is far from ideal. Twist can still be independently controlled, though, through the use of the vang. As is the case with the sheet, increasing vang tension will reduce the amount of twist by creating a downward force on the trailing edge of the sail, tensioning the sailcloth and pulling it to windward. Easing the vang does the opposite, increasing the amount of twist. Even aboard boats with travelers, the vang also becomes the means of adjusting twist when sailing on a reach or run with the boom well outboard.
Read the entire article at Sail Magazine - this link: https://www.sailmagazine.com/diy/mainsail-trim-101
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