Set depth with mast bend and outhaul tension


MAINSAIL - Target depths and draft positions


















These numbers are general targets only. Even with the best sail-measuring tools, it is typical to have errors plus/minus 1-2 percent for depth and plus/minus 2-3 degrees for draft position.

The depth of a sail is important for performance, especially with the main, which must change shape radically to cover a wide wind range. The primary means for adjusting depth in the upper two-thirds of the main is mast bend. Bending the mast moves the luff away from the leech, which does several things simultaneously – it flattens the sail, opens the leech and moves the draft aft (see below).

© North Sails
As we'll explain in the chapter on "Mast Tuning," there are many ways tocontrol how much the mast bends. These include adjusting the partners and mast step forpre-bend, tensioning the backstay, runners, jumpers, vang and so on.
The lesson of the trim loop was that bending the mast changes more than just the depth of the main. If you want a flat main that maintains the same twist and draft position as you had before, you'll have to make two adjustments.
First, trim the mainsheet. Bending the mast brought the mast tip closer to the end of the boom, which allowed the leech to twist more open.

Second, pull harder on the cunningham. When the mast bent, it pulled all the fullness out of the front of the sail and left you with a draft-aft shape. The cunningham will reintroduce curvature to the luff (see below).

A well-behaved main should "blade out" when the mast reaches maximum bend (see below). With the sheet trimmed hard and cunningham tight, the depth of the main should be reduced to about 8%. This shape, ideal for heavy wind and flat water, will stream quietly behind the mast without flogging or creating drag.

If the bend of the mast exceeds the sail's designed luff curve the shape of the main will go a step beyond blading out. In this case, the mainsail shape inverts as the leech falls away from a hinge created by the long diagonal creases, You'll see large overbend wrinkles running from the clew toward the middle of the mast (see below).
© North Sails

Inverting the main a bit sometimes works in heavy wind when you really need to depower. In general, however, an inverted main will hurt pointing be cause the leech is not firm. So if you see overbend wrinkles, ease the backstay or tighten the checkstays/runners tostraighten the mast.

Mast bend is sometimes necessary in very light air as well as in heavy air, For aerodynamic reasons, slow-moving air remains attached to a flat, open leeched sail more readily than to the deep sail that intuition suggests. Since there isn't enough wind power to bend the mast in light air, many mains will be too full. You have to artificially "pre-bend" the mast with rig tension (as described in Mast Tuning) until mainsail depth drops to about 14 or 15%.


The best way to control depth in the lower third of the main is with the outhaul. Basically, the tighter the outhaul, the flatter the bottom of the sail (see below). If the waves are big for the wind, ease the outhaul slightly to give more power. If the waves are small for the wind, as in an offshore breeze, pull on the outhaul to flatten the sail and reduce drag.

© North Sails
Besides depth, the outhaul also changes the tightness of the lower leech. Easing the outhaul adds depth to the foot, which in turn closes the lower leech. Conversely, tightening the outhaul opens the lower leech. You can see this change by sighting forward from the backstay, or by looking at the angle of the lower batten from under the boom.

The tighter the lower leech, the more windward helm you have. That's why it makes sense to tension the outhaul in heavy air to open the leech and reduce helm. If you have a flattening reef, this flattens the foot even more than maximum outhaul tension.


» Set draft position with luff tension