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Old 10-07-2001
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Pete Colby is on a distinguished road
Basic Mainsail Trim for Racers, Part One

This article was first published on SailNet in April of 2000.


On-the-job training with some great tutors taught the author the nuances of mainsail trim.

The first time that I recall being asked to trim a mainsail was aboard a new 40-foot PHRF racer a few years back. I was a little intimidated because my boss at the time, Ken Read, would be doing the driving. But I had read plenty of articles about mainsail trim, and so, even though I had very little onboard experience with mains, I felt up to the task.

I understood the two primary forces that act on a sail—lift and drag.

  • Lift—the good force—propels a boat forward by creating a low-pressure vacuum on the leeward side, which pulls the sail from high to low pressure. The greater the acceleration, the greater the lifting force.
  • Drag—the evil force—is created when high-pressure air on the windward side of the main tries to escape to the low-pressure, leeward side. (Think of air escaping from the opening in a balloon.) This exodus is not orderly. (Let go of the balloon and watch it do loop-de-loops.) Turbulent swirls disrupt the smooth flow over the sail, causing it to slow down abruptly and detach, or stall, from the sail. And stall reduces lift. 


Altering the shape and the trim of the mainsail as the boat changes heading is the key to optimum sail trim.
So all I had to do was to figure out how to maximize lift and minimize drag and I would be all set.

The other thing I had going for me—or so I thought—was that I had been a sailmaker for years. So I knew how a main should look:

  • The maximum draft (shape) should be located at 38 to 48 percent back from the luff.
  • The lower third should be flat to reduce drag under the boom.
  • The middle third needs a bit more shape. This section promotes lift with its longer chord length (the luff to leech straight-line measurement). The added area accelerates flow, thus increasing the lifting force.
  • The top third is a bit trickier. It is designed with as much depth as it can take. The added depth increases surface area, giving the airflow a longer runway on which it can accelerate. Depth also reduces the effects of drag, by giving the airflow more time and area to generate lift before it reaches the turbulent leech. Further, depth creates a twist. In other words, under a given sheet tension a main with more depth up high will fall off, or twist, to leeward. Seen from behind, the sail looks like a fan blade. The twist flattens the curvature, reducing the chance of separation. And twist also combats surface friction. The air moves faster the higher it is, and a faster-moving breeze approaches the sail at an angle aft of that of a slower breeze. In order to maintain a uniform angle of attack (the angle where the apparent wind direction intersects with a straight line from the leech to the luff), you need to turn off the leech. A greater angle of attack creates more lift.


Of course you need to intimately familiar with the mainsail controls—like the backstay—and how they affect your particular sail.
And lastly, I knew all the controls:
  • The backstay flattens the main and moves the draft aft.
  • The cunningham and halyard pull the draft forward, and flatten the leech.
  • The mainsheet controls the shape of the sail and changes the angle of attack.
  • The outhaul flattens the lower section of the main.
  • The traveler alters the angle of attack and moves the sail along the boat's centerline, and this affects the helm.
  • The vang controls the leech of the main.

With all this information, I thought I would have no problem figuring out the mainsail trim on the aforementioned 40-footer. But this was not the case. I was definitely in over my head, and Ken's frequent "verbal encouragements" confirmed that I needed more on-the-water experience. As it turned out, all the mainsail articles I had read were like a bad course in sex education—they explained what it was, all of its parts, and the technical reasons for using them, but not when, how, or why to make these things work. In either case, there just isn't any substitute for hands-on experience. That said, the following four scenarios should help jump-start novice mainsail trimmers so that they'll begin to recognize when, how, and why to adjust the trim.


Even in strong breeze you still need to pay proper attention to your sail trim to get the most out of the boat.
Finding Speed    There are many times when a boat isn't up to speed. This can occur in light air, or when you sail into a hole, or through bad air, or when you hit a set of waves. It can also happen at the start when you are jockeying for position on the line, or after a tack, or rounding a leeward mark. Anytime the boat is going slow, you are giving up both speed and pointing ability—what we generally refer to as height. You might hear the helmsman or driver say that the helm feels sticky, or sluggish. What do you do? You need to figure out how to get the flow over the sail to accelerate, which is often referred to as "powering up." Accelerated flow essentially increases the driving force that is associated with lift. To achieve this we need to increase the angle of attack and change the trim to create a lift-promoting shape in the mainsail.

When the boat feels sticky, or slow, and your height is off relative to the fleet, ask yourself the following questions:

How is the mainsail's angle of attack to the wind?

  • Is the traveler pulled up so that the boom is on centerline? (The higher the traveler the higher the angle of attack, but be careful not to pull it too high because that will make the angle too great and cause the sail to stall.)
  • Is the main sheet pulled in enough? The sheet pulls the leech in and adds shape to the sail. The added shape and change in leech position also increase the angle of attack. And like the traveler, you have to be careful not to overdo it with the mainsheet. Over-sheeting will increase the depth at the top of the sail and promote stalling. It may also overly increase the angle of attack relative to the rest of the sail. Remember, the breeze higher up in the sail is moving faster and at an angle farther aft. The telltales on the leech can help you determine the correct sheet tension. The appropriate shape will allow air to flow across the main, accelerating as it goes, and streaming off the leech, which should make the telltales fly directly aft. If they are not streaming aft, but are curling around the leech, then you have too much sheet tension, and the sail is stalling.


When the breeze drops your attention to sail trim should intensify.
Is the overall shape of the main a lift-promoting shape?
  • Is the shape uniform and deep enough to accelerate the flow? If the main looks flat and the draft is too far forward, ease the backstay a little. You can also ease the outhaul so the bottom of the sail has a slightly rounded curvature.
  • Is the leech too flat overall, depowering the sail? Try easing the cunningham and halyard.

As you make each new adjustment to the sail, wait a moment to feel its effect. And ask the driver how the boat feels after the adjustment. Also, any time you make an adjustment, do it as smoothly as possible. Abrupt changes can shake any existing flow off the sail and that exacerbates the situation. This happened to me during one of the starts at the SORC this winter. As the driver turned his Mumm 30 upwind at the gun, I wailed on the mainsheet so hard that I stalled the flow. (I think my adrenaline got the better of me.) After we promptly parked on the line and most of the fleet passed us, I got a mainsail-trimming lesson from the skipper throughout the entire first beat. So be smooth, stick to the basics, and sail fast.

In Part Two, Colby discusses basic mainsail trim and three trimming scenarios.


Suggested Reading:

Mainsail Controls for Performance by Dan Dickison

Basic Mainsail Trim for Racers, Part Two  by Pete Colby

Mainsail Twist for Waves by Dobbs Davis


Buying Guide: Backstay Adjusters

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