Main Sail Twist

by Dave Acree

In the Phoenix metro area it is not an uncommon sight to see tall sail rigs on model sailboats. This is because of the light winds which are prevalent in this area and many times you would see these high rigged yacht sail away leaving the rest of the short rigged boats adrift. It is safe to say there is more wind aloft and tall sails capture it, so it is within reason to say wind blows 60 percent at the top of sails.

This varying wind speed affects a sail angle, or twist from top to the bottom. This twist can be shown in the apparent-wind diagram in the figure below. To explain this simply, the wind twist the sail's leech from top to bottom. At any one time there can be 3 to 5 degrees of twist in the apparent wind from the bottom of the mail to the top. The result is the bottom of the main can be beating while the top is close reaching.

Sail Twist

The skipper has several ways to control the twist of his mail sail, the gooseneck and the backstay. When adjusting the main sail, not enough twist and the top will be stalled relative to the foot, too much and the top will luff before the foot.

To adjust the main sail's twist, trim the main to where the bottom batten is parallel to the main boom. Next, take the yacht out to an open area and set the boat on a stand at an angle as if it was it was heeled over beating to windward. Use the backstay line as a straight line of sight so you can measure twist. Next, sight the top of the main sail, it should lie out 1 3/4 inches from the backstay line. You can now adjust the jib so that it too follows the main sail's twist while keeping the slot between the two sails the same all the way along the length.

Another guide to check the main sail's twist is to attach a telltale on the back of the top batten. The telltale should flow aft at least haft to the time. If not, ease the main sail by using the gooseneck.

In speed terms, by having more twist, or more leeward sag to the leech (remember sighting by using the backstay line), gives the sail less power. Less twist, or less leeward sag gives the sail more power. One note to remember, under heavy winds, more twist to the main can be a boost. By applying twist will allow a main to "dump" wind, which causes less heel, hence more speed. This works well until the wind calms down and now the main is not catching enough wind, and the boat goes slower.

To understand this more, take you boat out and experiment. Only by doing will sail twist become understood.
Good sailing!