How-to: Mainsail Trim 101

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A well-trimmed main makes for a more comfortable ride and an easier helm

A well-trimmed main makes for a more comfortable ride and an easier helm

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.

Pulling the boom down with the vang or mainsheet decreases twist (left); When thinking about mainsail draft it’s important be aware of both the amount of draft and its fore-and-aft position (right).

Pulling the boom down with the vang or mainsheet decreases twist (left); When thinking about mainsail draft it’s important be aware of both the amount of draft and its fore-and-aft position (right).

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 air flow 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.

Again, a mainsail set with the correct amount of twist will have all its telltales streaming neatly aft. A good starting point for most boats when sailing to windward is to tension the mainsheet or vang so that the second batten from the top of the sail is parallel to the boom. To get the correct angle of attack in light air the boom should then be set either on centerline or close to it. Once you have these basic settings dialed in, you can fine tune your sail trim by playing the sheet, vang and traveler as necessary.

Note that in moderate conditions and flat water, attentive mainsail trimmers aboard fully-crewed racers will sometimes squeeze in the sheet a little more, reducing the amount of twist so that the top telltale disappears behind the lee side of the sail anywhere from 50 to 75 percent of the time. The reason for this is that doing so increases the overall drive of the sail and helps squeeze out a little extra pointing ability. This performance increase, however, comes at the price of it being more difficult to steer, since having the sail strapped in tighter also makes it easier to stall the sail’s trailing edge.

Cruisers should, therefore, follow the lead of doublehanded and solo racers, who tend to set their sails with more twist. This creates a wider and more forgiving “groove” when sailing upwind, with less need for constant sail trim and careful helming.

In terms of overall trim, ideally, the amount of twist in the headsail will match that in the main. If you’re twisting off the top of the latter, therefore, it’s usually worth moving the jib car back as well to match.

Easing the main traveler decreases the angle of attack (left): Taking up on the traveler increases the angle of attack (right).

Easing the main traveler decreases the angle of attack (left): Taking up on the traveler increases the angle of attack (right).

Draft/Depth

Draft, or depth as it is often called, refers to the fullness of a sail’s curvature. To calculate draft, start by drawing an imaginary line from the luff to the leech. The point of maximum draft will be the point at which the sail is farthest away. The amount of draft is typically expressed as a percentage and is determined by dividing the fullness of the sail at the point of maximum draft by the chord length (the length of the line you drew from the luff to the leech) and then multiplying by 100. Typical draft, or depth, aboard a cruising boat will be around 15 percent.

Underway, less draft, or a flatter shape, results in less power or pressure in the sail, making it a better shape in strong winds, where greater power would also result in increased heel. In lighter winds, on the other hand, you want all the power you can get, which means more curvature—though be warned this same increased curvature means you won’t be able to point quite as high, as the leading edge of the sail angles off to leeward.

The “location” of a sail’s point of maximum draft (or depth) refers to its fore-and-aft position. Ideally this should be around 40 to 50 percent of the distance from the luff. If, on the other hand, the draft is too far forward or too far aft, the airflow is more likely to detach from the sail and stall.

Draft depth and location are controlled through a combination of luff tension, foot tension and mast bend. Luff tension is controlled by adjusting the tension in the halyard and/or cunningham (assuming your boat has the latter). Increasing luff tension stretches out the material comprising the leading edge of the sail, thereby moving the point of maximum draft forward. Decreasing the amount of luff tension allows the draft to migrate aft.

Foot tension is controlled with the outhaul. Increasing outhaul tension flattens out the lower part of the sail reducing the amount of draft there while leaving its location unchanged. Easing the outhaul allows the sail to bag back out again.

Finally, especially on a fractionally rigged boat, backstay tension can be used to increase mast bend, thereby reducing the depth in the middle portion of the sail. Doing so also pulls the point of maximum draft forward.

The best way to assess the depth of a sail and its draft position is to look up at it from a point immediately below the boom just forward of its midpoint. Many sails also carry horizontal “draft stripes,” which make it that much easier to determine the amount of curvature they’re carrying. A number of sailmakers even offer apps that allow you to automatically measure the position of maximum draft in a sail.

Tensioning the outhaul flattens out the lower portion of the sail (left); Tensioning the backstay flattens the middle of the sail and moves the draft forward (middle); Tensioning the halyard or cunningham moves the draft forward (right).

Tensioning the outhaul flattens out the lower portion of the sail (left); Tensioning the backstay flattens the middle of the sail and moves the draft forward (middle); Tensioning the halyard or cunningham moves the draft forward (right).

Changes in wind strength

Most of the above apply to ideal conditions, with a boat nicely powered up on a beat in 8-12 knots of true wind. As the wind begins increasing, though, and excessive heel and weather helm start to become a problem, the first thing you’ll want to do is start flattening the sail, by tensioning the halyard (or cunningham) outhaul and backstay. These adjustments will also bring the draft forward, which will result in a more balanced helm and better speed.

In years past, a good raceboat main trimmer would likely then ease the traveler in the gusts to “blade off” the sail as well, increasing its angle of attack while maintaining a constant amount of twist. Today, however, more and more trimmers are also easing the mainsheet, with the aim of reducing heel by depowering the top of the sail. Cruising yachts without a traveler are therefore not necessarily at as much of a disadvantage as was once thought.

That said, aboard a boat without a traveler it can also be difficult, or even impossible to sheet the main into near centerline without cupping in the leech so much that airflow in the aftermost portion of the sail becomes stalled. These crafts, therefore, need to be sailed fast and free even when upwind.

Again, in a building breeze, the key to depowering a sail are to progressively increase backstay (assuming it’s adjustable), luff and outhaul tension. If you find yourself in a situation where you’ve got things tightened down and the boat is still feeling overpowered, it’s time to reef.

Of course, in a fading breeze, you’ll want to do just the opposite, i.e., increase the amount of draft and reduce the amount of twist in the hope of getting as much power out of the sail as possible. Note, even knowledgeable sail trimmers are often caught out by a decreasing wind. Angle of heel and helm balance are good guides here, as the boat becomes progressively more upright and the helm becomes more neutral. If nothing is done, the sails will become as flat as a board and boatspeed will drop accordingly.

Another common mistake is strapping in your sails too tight to keep them “looking” full as things go lighter still, causing them to stall. Watch your telltales and do your best to keep them streaming, even in a drifter.

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Reaching and running

So far we’ve been looking primarily at what to do when sailing upwind. But what happens when you bear away? A raceboat will start out by easing the traveler to keep the sail at an optimal angle to the wind without increasing twist. On a boat without a traveler, increase vang tension to achieve the same effect as you ease the mainsheet. This is often referred to as “vang sheeting.”

When bearing away the apparent wind will also begin to decrease, so power up your sails by reducing luff and outhaul tension to create a fuller shape. As you do so, also to try and keep your telltales flying, at least until the apparent wind starts to go aft of abeam, at which point they become pretty much useless.

Eventually, as you bear further and further away to the point where you are on a run, the shrouds will be in the way of the boom and you won’t be able to ease it out anymore. The question then becomes, how far to ease the sheet? To my mind there’s only one answer here—all the way. Sheeting in to keep the sail off the shrouds will only add to weather helm, as it moves the sail’s center-of-effort aft. This makes the boat slower and more difficult to steer. It will also contribute to a corkscrewing motion in a seaway.

In my experience, the potential for chafe on shrouds and spreaders is a problem that’s vastly overstated, especially on a sail with decent spreader patches. If you’re really worried, use some DrSails adhesive (drsails.com) to attach a bit of sacrificial webbing onto your batten pockets. I’ve successfully done this on a 21-day trade-wind Atlantic crossing, with no damage to the sail, despite its having spent most of the passage resting against the shrouds and spreaders.

Similarly, my partner and I got 24,000 miles, many of them downwind in big breezes, from the last mainsail aboard Zest, her 36ft Rob Humphreys sloop with few, if any issues. This was a membrane sail predominately used for long-distance racing. Maintaining good chafe protection—using standard self-adhesive patches in way of the shrouds and spreaders—meant chafe was not in any way a factor in the sail’s eventual demise 

Rupert Holmes has raced and cruised more than 80,000 miles on a wide variety of boats in Northern Europe and the Mediterranean, as well as the Atlantic and Southern oceans

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