Guidelines for Good Mainsail Shape
by David Dellenbaugh

The mainsail must have a very versatile shape. On almost every boat, one main must cover the entire wind range from drifter to howler and from flat water to big chop. So that sail must be capable of changing from full and powerful for acceleration to flat and twisted for depowering. And back again.

Fortunately, there are many ‘tools’ you can use to control the shape of the main. Since the entire luff of the sail is connected to the mast, you have a great influence over mainsail shape by changing mast bend (using backstay, vang, rig tension, mast blocks, butt position, etc.). You also have other tools such as the mainsheet, traveler, outhaul, and Cunningham.

With such good control over sail shape, the most important question is deciding what shape you want. As a very basic rule of thumb, start by setting up the main with the top batten parallel to the boom, the boom trimmed to the boat’s centerline, the position of the maximum draft just forward of the middle and a Guidelines for Good Mainsail Shape by David Dellenbaugh few wrinkles in the lower luff area.

Of course, these ballpark settings need tweaking in different wind velocities, and that’s what I’ve tried to describe in the chart on the next page. But sea state is also a very critical factor that this chart does not fully consider. For example, if you have medium air and big lump, you may actually need to go with settings in the ‘Light Air’ column. If you have light air and very flat water, you may be closer to the ‘Medium Air’ settings.

We are talking only about the mainsail shape here, but remember that everything must be done in concert with achieving good shape in the headsail too. This means your main trim is often a slight compromise.

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MainSail Trim Light-air mainsail trim.

In 5 knots of wind, the mainsail is set up to be quite full overall. The backstay is tensioned just enough to take up the slack (which is evident because the mast is almost perfectly straight). The cunningham is eased so there are slight ‘speed wrinkles’ along the lower part of the luff (though it might be better to have even less luff tension so the sail is fuller and slightly more draft-aft).

The mainsheet is trimmed just tight enough so the telltale at the aft end of the top batten stalls only occasionally (hard to see here). Another guide for sheet tension in light air is that the top batten should be slightly ‘open’ (i.e. it angles off slightly to leeward). You would want even more twist in bigger waves or less twist in flatter water.

Heavy-air mainsail trim.

In 20 knots of wind, this main (same sail as above) has been set up very flat. The main difference between this set-up and the one above is that the backstay has been pulled much harder. You can see there is a lot more mastbend, which pulls the shape out of the middle of the sail and causes the ‘overbend wrinkles’ in the lower part of the luff. The draft is a little far aft so perhaps this sail needs more cunningham or a bit less backstay to bring the draft closer to 45% aft.

The mainsheet is eased to depower the sail by allowing it to twist a lot. Note the upper batten angled off to leeward and the distance between the leech and the backstay. It looks like the traveler has also been eased to depower the sail.

MAINSAIL TRIM Light Air
(<8 knots)		 Medium Air
(8-15 knots)		 Heavy Air
(>15 knots)
Top batten Slightly open (i.e. angled to leeward) unless you have flat water and the higher end of this wind range. More waves require more twist. Parallel to the boom, or somewhat closed (i.e. angled to windward) in ideal pointing conditions. Slightly open. More twist as you get more overpowered.
Top leech telltale Flowing most of the time, if possible. Stalled a lot of the timealmost all the time in ideal pointing conditions. Flowing all the time.
Backstay tension Minimal (just enough to remove slack, loose enough to keep headstay sag and full jib). Moderate-more in flat water as you become overpowered. Maximum (or until you have too many overbend wrinkles).
Traveler Well above centerline-often as far to windward as possiblemust be far enough so boom is on (or above) centerline when main is trimmed properly. Slightly above centerlinejust far enough to keep the boom centered. On or below centerlinefarther to leeward as you get overpowered.
Cunningham Well eased to keep sail fullthere are usually obvious wrinkles along the luff. Tension enough to keep draft at around 45%-usually a hint of luff wrinkles unless the sail is old and needs more luff tension to move draft forward. Pull hard to keep draft forward as mast bends. No wrinkles visible (except perhaps a hint of overbend wrinkles)
Outhaul Moderately eased. Slightly tighter in very light air or very flat water. Tight. A little looser if waves are bigger than the wind. Very tight-take it to max black band on boom to flatten lower third of sail and open lower leech.
Boom vang No tension! (Pull just enough to remove slack so it's easier for crew to move across the boat when tacking). Trim main for sailing upwind and then snug vang. Pull vang tighter if you have to easemain to depower. Very tight to maintain leech tension, mast bend and flat sail shape when easing mainsheet.
Sail shape
(middle draft stripe)		 Sail is quite full overall. Depth roughly 13-16%. Maximum fullness 50% aft. Sail is in middle of designed range with minimal twist. Depth roughly 11-13%. Maximum fullness 45% aft. Sail is 'bladed out.' Depth roughly 9-12%. Maximum fullness 45% aft.
Prebend Maximum. Need to bend mast (to match sail's designed shape) without the mainsheet and backstay. Moderate. Just enough so adjusting the backstay gives you perfect trim simultaneously for both the mainsail and headsail. Minimum. If you have too much prebend the mast will overbend early.
Windward helm
(rudder angle)		 Your goal is a slight pull on the helm (a few degrees of rudder angle) for feel, but you can't always get this. A moderate amount of helmroughly 4-6 degrees of rudder angle is optimum for bigger boats. Less helm for dinghies. You are usually trying to reduce helm and rudder angle so you don't have to fight the wheel or tiller.
Mast rake Maximum. Balance the boat and increase helm by angling the mast aft. Middle range. Minimum. Balance the boat and decrease helm by not angling the mast so far.

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