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The Cruiser’s Guide to Mainsail Control

Mainsail control can be achieved by choosing the right set of tools. The cruising sailor can make use of the traveler, the mainsheet, the boom vang, backstay tension, outhaul or the cunningham to manipulate sail shape. One or a combination of these tools could be the right choice for you, depending on wind and weather conditions as well as your boat’s unique characteristics. 

TRAVELER

The traveler has two functions: it controls the boom's angle to the wind and it helps steer the boat, controlling helm and heel angle in puffs and lulls. Set the twist with the mainsheet then use the traveler to position the boom on the centerline for maximum power and pointing, as long as helm and heel are under control. As the breeze builds and mainsheet tension increases, the traveler will gradually be dropped to keep the boom on the centerline. Think of the traveler as the “fine tune” power control device. As long as the changes in velocity are not too rapid and too great, it will keep the boat on its feet. The beauty of using the traveler is that mainsail twist (controlled by the mainsheet), which is vital to both speed and pointing, does not change, only the total amount of power is affected.

In medium breeze, the role of the traveler will expand to include maintaining control of helm. As the boat heels too far and generates weather helm, you can drop the traveler to depower the boat. The position of the boom, relative to the centerline, will become irrelevant. Dump the traveler down quickly at the onset of a puff, but then be ready to pull it right back up as the initial power of the puff dissipates and turns into forward speed instead of heel. If you leave it down too long you will miss the opportunity to point once you’ve accelerated. The range of adjustment on the traveler will be wide, from the boom on centerline to all the way down to leeward.

In shifty or puffy conditions, or in a lively seastate, the trimmer might have to ease or trim a lot — even constantly. Using the traveler, instead of the sheet, reduces the amount of muscle the trimmer will need to use.

In very puffy conditions when velocity and direction are changing rapidly, the sheet can be a better tool to control power. The mainsheet moves the boom in and out to reach proper trim. Typically the traveler is pulled part of the way back up towards the centerline to keep the bottom of the mainsail working.

The traveler is the perfect tool for balancing heel and controlling helm, especially when cruising. It also makes life easier for the autopilot. Find a good average setting for conditions and fine-tune the traveler to keep the boat from heeling excessively. However, once the velocity has increased to the point that the traveler alone will not keep the boat upright, it's time to shift over to the mainsheet. 

For more guidance regarding how to use the traveler, visit our full article on the topic: What Should I Do with My Traveler?

MAINSHEET

The mainsheet is the “big hammer” when it comes to adjustment for the overall amount of power. On fractional rigged boats with large mainsails (particularly when the boat is on the lighter side), the mainsheet is played somewhat more aggressively and the traveler is usually kept closer to centerline. The bigger sail makes the boatspeed particularly sensitive to small changes in mainsheet tension. The right amount of twist is key.

The sophisticated mainsail trimmer continually makes adjustments to both traveler and mainsheet based not just on the overall amount of power, but also on issues like boat speed, waves, and even tactical situations. For example, the trimmer may ease the mainsheet slightly and drop the traveler to depower if the boat is sailing slower than targets or about to hit a set of waves. Or, if you are lifted or close to fetching the weather mark, it may pay to ease the mainsheet and lower the traveler more aggressively to turn a puff into speed instead of height.

The mainsheet is not always the best choice. When going downwind, with the sheet far out, the traveler or boom vang may be the better tools. 

For more guidance regarding the use of the traveler and the mainsheet, visit our full article on the topic: Depowering the Mainsail with the Traveler and Mainsheet.

BOOM VANG

The vang is the principal tool for controlling twist in the mainsail, which can be a critical part of sail control management. It also assists in controlling the height of the boom, which can provide extra stability and safety. If you don’t use the boom vang in all but the lightest conditions, the boom will rise up as it is eased out and the sail will twist off, spilling power out of the top of the sail as well as limiting how far out you can ease the sail. The golden rule of thumb: Use enough vang to keep the top batten parallel to the boom. If it is too tight, the top telltale will stall. 

When used properly, the vang is synonymous with control. When you need a way to correct the amount of twist in the mainsail without sacrificing power and speed, tighten the vang to maintain the angle of the boom (and so, the twist) and to increase downward pressure on the leech when easing the sheet or traveler. This reduces the pressure on the sail and on the rudder, thereby assisting in helming or reducing heel. Vang adjustments are particularly useful on bigger wind days with puffy breeze — by easing the sheet while using the vang to control the twist, the apparent wind angle will diminish and help you maneuver through the puff.

In many cruising and even some racing boats, the standard vang that comes with most setups isn’t actually large enough to handle the loads that need to be leveraged for its best effectiveness. The more powerful you can make your vang – say, upgrading from a 4-to-1 to an 8-to-1 purchase ratio, or adding a fairlead or extra block — the more helpful it will be. Talk to your sailmaker about increasing the power of your vang. These seemingly small adjustments can pay off big time when it comes to cruising performance and comfort.

For more guidance regarding the use of the boom vang, visit our full articles on the topic: It's All In The Twist: Back to Boom Vang Basics, and Do I Really Need a Boom Vang?

BACKSTAY TENSION, MAST BEND AND LEECH

If you have a mast that bends when you apply backstay tension, you have a powerful tool for controlling mainsail shape. Assuming you start with a little bit of pre-bend (mast bending forward at the middle), the compression induced by the backstay will increase the bend forward at the middle of the mast. This has a few of effects:

First, as the middle of the mast moves forward, the luff of the mainsail is pulled away from the leech. This flattens the sail just as pulling on the outhaul and moving the clew away from the tack flattens the foot of the sail. It just does it over a much larger area since the whole mast is moving forward to some extent. The more bendable the mast, the easier it is to add backstay tension and flatten (de-power) the sail. On small boats that don’t have a backstay, pulling on the mainsheet has the same effect. Tensioning the mainsheet tensions the mainsail leech. The leech acts like a backstay to induce compression and mast bend. The boom vang can also help. Pulling on the vang pulls down on the boom to control leech tension, but it also forces the boom forward, pushing bend into the lower sections of the mast.

Second, when you compress the mast with the backstay, the head of the sail gets closer to the clew. This opens the leech of the mainsail which induces twist. Normally, whenever you add backstay you need to add mainsheet (or vang) to keep the twist the same, unless of course you want to depower even more by adding twist.

Finally, mast bend also shortens the distance between head and tack, making the luff go soft. You will see the telltale sign of horizontal wrinkles perpendicular to the mast appear as you add backstay. To compensate, add luff tension with either the halyard or cunningham. Conversely, when you ease the backstay, you need to remember to ease luff tension and ease the mainsheet or vang. 

Don’t forget the role the backstay has in controlling power. If you put on a lot of backstay, it will also tighten the forestay. This straightens not only the jib’s luff, but also flattens the main. Together, these will impact power and provide a little more stability.

For more guidance regarding the use of backstay tension, visit our full article on the topic: How Does Backstay Tension Affect Mainsail Trim?

UTILIZING OUTHAUL AND CUNNINGHAM

The outhaul controls depth in the lower third of the mainsail. Easing it adds depth and power; pulling on it flattens and de-powers. If the boat is heeling too far and developing weather helm, add outhaul. Upwind, the mainsail should generally be flatter than when sailing off the wind, so it is better to use more outhaul. Easing the outhaul will round up the lower leech and help pointing in smooth water and light to moderate conditions. The outhaul is usually only eased all the way off (approximately 100mm from maximum tension) when sailing off the wind. In light to moderate conditions when you need power and helm, ease the outhaul until the sail is 50mm-75mm from maximum tension and the foot shelf is partially relaxed. Increase tension gradually as the breeze builds.

The tension required along the luff of the sail is a function of apparent wind velocity. In more wind more tension is needed—and vice versa. This means that you need to vary tension not only when the breeze changes velocity, but also when you change point of sail. Since there is more apparent wind when you sail upwind, you will need more tension than when sailing off the wind. Tension halyard or cunningham so that just a hint of wrinkles appear in the lower third of the sail. When more power is needed, use the Cunningham over the halyard. It’s two connecting points provide a down haul with more strength in high wind conditions. 

Look to our Trim Guide for more fundamental principles of mainsail trim.

BE FLEXIBLE

You will notice each of these tools are mentioned interchangeably and in cohesion with each other throughout this article. 

Utilizing the traveler, main, vang, backstay, outhaul and cunningham each have different impacts on different boats, depending on their weight, size, setup and maneuverability. Every boat has its nuances, so familiarize yourself and choose what strategy is right for you. The best road to identifying the right method is always practice. 

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The Discussion

Paul Weiner
Paul Weiner

I typically control my vang by hand on my 37-foot masthead rigged sloop without using a winch. I'm wondering if I should be using a winch to tighten the vang hard enough for it to be effective.

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