Mailsail Trim from Dave Dellenbaugh's Speed & Smarts
While the mainsail provides a good bit of power, it also has a lot to do with the boat's directional control. The sail helps to steer the whole sailplan, functioning a lot like a trim tab on a keel or a flap on an airplane wing.
In particular, the aft part of the main, the leech, is an important influence on the directional tendency of the boat. A closed or tight leech "kicks" the air flow to windward, creating a large side force to leeward at the stern of your boat. This creates weather helm and tends to push the bow to windward. Similarly, an open or twisted leech allows the air to flow easily off the mainsail without developing as much sideways force. So you feel less windward helm.
The main point of the loop cycle was that trimming the main is not such a simple matter. Whenever you adjust a control, it has a primary effect, but also many secondary effects. Therefore, the mainsail trimmer must have a well-rounded knowledge of just about everything covered in this book, as well as a methodical approach to the task.
What's exciting about trimming the main is that it's a job where you can learn tons. The following procedure offers a good, solid approach to mainsail trimming. The five basic steps are:
On a mainsail, twist is controlled by the amount of mainsheet tension, as well as the amount of vang. According to the maxim about trimming the back of the main, we should key off the mainsail leech as our guide for twist. The front of the main is certainly a poor indicator of twist because it sits in the confused airflow of the slot and directly behind the mast's turbulence.
To set proper twist, trim the mainsheet until the top batten is parallel to the boom. If you have a long top batten, your goal is to make its aft end parallel to the boom. Using the angle of the top batten isn't exactly measuring twist (because the angle of the batten is different than the angle of the chordline), but it's a good guide.
When the sheet is eased, the main has a very twisted shape, with the top batten falling off to leeward. As you trim the sheet, the top batten angle narrows until it is parallel with the boom. Trimming harder will take away all the twist, close the upper leech, and make the top batten poke to windward.
The best average setting for the top batten on everything from Lasers to 12-Meters is parallel to the boom. This is one of the golden rules of mainsail trim.
With the batten in this position, the top batten telltale should stream aft between 50 and 90% of the time. This telltale, attached to the aft end of the top batten and extending 8 to10 inches beyond the leech, indicates whether the upper leech is stalling. When the leech is stalled, the telltale curls around to leeward of the main. Twisting the main more will open up the leech and re-establish flow.
When to bend the rule -- Rules, of course, were made to be broken. On a masthead boat, you can sometimes trim the sheet hard enough to tighten the upper leech and poke the top batten slightly to windward. This is fastest in medium air and smooth water, when stalling the top of the main increases the total forward force and lets you point higher.
This works with a masthead rig because the full-hoist genoa steers the airflow around the lee side of the upper leech and reduces the danger of stall. On a fractional rig the upper mainsail has no genoa to steer flow, so its upper leech may need to be opened up slightly by easing the sheet.
There are a number of times when you may want to twist the main enough to let the top batten fall off to leeward. In a chop, after tacks and in light air, ease the sheet to open up the leech slightly and prevent stall.
As we'll explain in the chapter on "Mast Tuning," there are many ways to control how much the mast bends. These include adjusting the partners and mast step for pre-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 a couple more 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.
A well-behaved main should "blade out" when the mast reaches maximum bend. 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.
Inverting the main sometimes works in heavy wind when you really need to depower. What you should look for is just a hint of overbend wrinkles. In general, however, an inverted main will hurt pointing because the leech is not firm. So if you see overbend wrinkles, ease the backstay or tighten the checkstays/runners to straighten 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 the Mast Tuning chapter) until mainsail depth drops to about 14 or 15%.
Outhaul -- 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. 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.
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 will flatten the foot (and open the lower leech) even more than maximum outhaul tension.
The cunningham applies tension to the luff of the main, and this controls draft position. Tighten the cunningham to move the draft forward; ease it to let the draft move aft. In general, the more you bend the mast, the tighter you need to pull the cunningham to get the draft in the right place. You'll also have to pull the cunningham harder on an older main, because a sail's draft moves aft with age.
In light air, keep the cunningham quite loose. A few "speed wrinkles" along the lower luff won't hurt. Remember that you're using the cunningham to control draft position, not make the sail look beautiful. In real light air, you may even have to lower your main halyard (especially downwind) to get the proper luff tension.
When adjusting the traveler, you almost never want to pull the traveler so much to weather that the boom angles to windward of centerline. It's tempting to try this when you're looking for more helm in light air, but it usually stalls the main. On the other extreme, don't ease the traveler to leeward so much that the genoa backwinds the entire main. This may be necessary for a short time to ease helm, but it means you need to depower.
The trimmer must continuously fine-tune the traveler to keep the boat on her feet and the helm in the groove. Just remember to keep an eye on boatspeed to help you find the fastest settings.
The relationship of the mainsheet to the traveler is sometimes tricky to understand. Consider the following analogy to the genoa: When you move the genoa lead aft, you twist the genoa. This is just what happens when you ease the mainsheet; it twists the main. Moving the jib lead inboard narrows the genoa's angle to the centerline. The same thing happens when you trim the traveler; it narrows the main's angle to the centerline.
Note: On many boats, adjusting the traveller automatically changes the mainsheet tension, and not always for the better! Ideally, you should be able to play the traveler without affecting sheet tension.
To get an idea of how to do this, imagine that you are sailing upwind in medium air with the main at its most powerful setting. Gradually the wind strengthens. Sooner or later, the main will need to be depowered, first by flattening it with mast bend and foot tension, later by lowering the traveler in the gusts, and eventually by reefing. How quickly should you depower the main?
The most obvious indication of overpowering is your angle of heel. This is something all crewmembers can feel. Heel isn't necessarily your best guideline, however. Boatspeed and the amount of windward helm are actually more sensitive and accurate indicators.
When you think you might be overpowered, use the following boatspeed test to gauge mainsail power. Lower the traveler until most of the main is luffing and watch the knotmeter for at least a minute. If your speed increases without an appreciable loss in pointing ability, you are overpowered.
Depower by blading out the main. If the main is already as flat as possible (without inverting), put in a reef or change headsails. Once again, check the knotmeter to confirm that your speed is greater after the change than before it.
To test mainsail power with windward helm, measure the rudder angle (see Preparation chapter) required to sail in a straight line. We said before that the goal in mainsail trimming is to achieve about 3 to 5 degrees of windward helm. If you have more than this, you are overpowered.
Depower the mainsail by bending the mast, opening the leech, easing the sheet, dumping the traveler, and reefing if necessary. These adjustment are simply changing the total power being exerted by the mainsail. Since most of the main's power is side force, adjusting the amount of this power affects windward helm. You have to get windward helm down into the acceptable range.
Recut -- Another suggestion is to take photos of your main in action, and show these to your sailmaker. Often he can perform an inexpensive face-lift that will move some of the draft from the back to the front again.
Sail care -- Treat your main well so it will stay fast as long as possible. Don't let it flog, and be sure to fold well after each day sailing.
From a speed point of view, battens should support the roach (extra area in leech beyond a straight line from clew to headboard) without distorting sail shape. This means they must be very stiff at their outboard end At the same time they must be quite flexible up forward, where they enter the more curved area of the sail.
Use stiffer battens (at the top one or two pockets) in heavy air to take curvature out of the leech. They are too soft if they allow much curvature in the upper leech or fail to control the lower leech in strong winds. Battens in the bottom two pockets should be very stiff (you can tape two together if necessary).
Use softer top battens in light air to avoid a hard edge at the forward end of the battens. The battens are too stiff if they create a kink running from the inboard end of one batten to the next. Leech Cord -- One more word about the mainsail leech. The leech cord should be set just tight enough to eliminate flutter. This may mean you get a slight hook in the leech. This is more unsightly than significant. It's better to reduce flutter, which destroys the sailcloth and tends to get worse with age.
The area that separates the main from genoa is called the slot. The width of the slot can be controlled by a number of different factors:
Ease the traveler to induce lifting, then readjust the sheet as follows: If the top lifts before the bottom, the sail is too twisted, so trim the mainsheet. If the bottom lifts before the top, ease the mainsheet to induce more twist.
Lots of backwind in the mainsail means that the slot is too narrow. Correct this by:
Mainsheet -- Downwind the sheet controls the overall angle of the main and has very little to do with adjusting twist. Ease the sheet until the front of the main almost luffs. It can be helpful to put a telltale half way up the sail and a couple feet aft of the mast. This will help you know when the back of the main stalls.
Vang -- The vang is not used much upwind except on smaller boats that vang-sheet in heavy air. Downwind, however, you must use the vang to control leech tension (twist) because the mainsheet no longer pulls down on the boom.
One of the biggest mistakes that sailors make is to over-vang the main in lighter air. Often the weight of the boom provides plenty of leech tension; use the vang only when it gets windier to keep the upper batten roughly parallel to the boom.
Sail depth -- In general, you want a fuller main shape downwind than upwind. This means you should ease the outhaul, cunningham and backstay when you go around the windward mark. In light air, you may have to drop the halyard slightly as well. One exception is an overpowered reach, when you want to depower the main as much as possible to reduce helm.