This article was first published on SailNet in August of 2000
For most sailboats, the mainsail is the power plant, the figurative engine that makes the boat go. Sure, jibs and genoas are an important part of the total package (don’t try to tell that to a Laser sailor), but aboard most designs, if you don’t have your mainsail working for you, your boat won’t be moving very well. So how do you get the most out of a mainsail? Apart from the mainsheet and the traveler, there are four basic controls that alter the shape and setting of the mainsail. Here’s a rundown on the two most basic of those controls and their functions. If you understand how and why they work, you’ll be well on your way to mastering the art of mainsail trim for performance.
Halyard and Cunningham After you’ve gotten the sail up, you’re ready to fine-tune your halyard. As you do this, make sure the boat is headed into the wind and all the mainsail controls are eased—mainsheet, vang, outhaul, and cunningham. Then, have a look at the luff of the sail and make sure that you’re not creating any unproductive wrinkles by over-tensioning the halyard. Conversely, if you’ve left the halyard too loose, you’ll see scallops along the boltrope or the extreme forward edge of the sail. Both situations can be quickly corrected by adjusting the halyard tension. When you’ve got the right setting for the conditions, many sailmakers recommend marking the halyard near the cleat or jammer (or whatever system your boat has) so that you have a future reference for that wind speed.
Now some boats aren’t equipped with a cunningham, and if this is the case on your boat, you’ll be limited to achieving the proper luff tension with your halyard. Again, the principle of getting it into a ballpark setting for the given conditions is important—not too tight, not too loose. You’ll also find that on some boats—particularly on one-designs—the mast and boom are equipped with black bands (or white bands if the spars themselves are black). The black band at the top of the mast indicates the maximum allowable hoist for the head of the mainsail. You can be sure that the person who made the sail didn’t intend for it to be hoisted above that black band, so keep that in mind as a reference if you’ve got one on your mast.
Now bear off and start filling the sail by sheeting in on the mainsheet. As you sail along, you can fine-tune the luff tension by using the cunningham. On most boats, the cunningham is a purchase system that is attached to a cringle or eyelet in the luff of the main, just a short distance up from the tack. If the breeze increases significantly and you find that the boat is beginning to be overpowered, you can flatten the entry of the mainsail and move the draft forward by tensioning the luff with the cunningham. Conversely, if you find that the wind is getting light and your luff is too tight, causing the mainsail to be too flat and therefore not powerful enough, you can ease the cunningham to give the sail more power.
Generally, you want the draft of the sail (the deepest portion of the sail’s “belly”) to be about 30 percent aft of the luff. In breezy conditions, you’ll probably want it farther forward, so more luff tension, and in light conditions you might want it slightly aft of 30 percent, so less luff tension. Downwind, you generally want the cunningham eased to create a more full shape in the mainsail. On some boats, it’s also important to ease the halyard a half-inch to an inch to get the luff of the sail properly rounded for sailing off the wind. Just remember to re-tension it before heading back upwind.
Outhaul The outhaul is used to alter the tension along the foot of the sail. By pulling the outhaul on, the foot gets taut and the lower portion of the mainsail becomes more flat. By releasing the tension on the outhaul, the foot becomes slack and lower portion of the sail gets more full and consequently more powerful. On smaller boats, the outhaul is often rigged externally, but on most boats 20 feet and larger, the outhaul runs through the boom.
Most sailors understand that in general, you want the outhaul eased when sailing downwind, tensioned when sailing upwind, and slightly eased when on a reach. But there’s a lot more you can do with an outhaul. This control is an essential element for shifting gears in response to changes in wind strength. Say you’re sailing along upwind in 10 knots and you get a sustained puff that jacks the breeze up to 14 knots. More than likely your mainsail will be too full for the new breeze, a condition that will overpower the boat and create excessive weather helm for the person driving. There are several responses to this increase in wind, but essentially you want to depower the sail plan, and you can begin doing this by flattening the lower portion of the mainsail with additional tension on the outhaul.
On most boats, the outhaul is made up of a purchase system featuring line and small blocks that provide a mechanical advantage for tensioning the foot of the sail. Some outhauls, however, are tough to pull on when the sail is under load, and you’ll need to work in concert with the mainsail trimmer or the driver to get more tension. In this case, either have the driver briefly feather the boat up into the wind so the sail is less loaded, or have the mainsail trimmer briefly ease the mainsheet for the same effect, then you can tension the outhaul more easily. Always watch the sail as you adjust the outhaul so that eventually you’ll end up trimming it for a setting that you recognize.
Of course, nothing instructs like experience, so get out there and try these adjustments. If you pay attention to the impact they make, you'll begin to see how they work and they’ll soon become second nature for you. And using these controls in various situations will expedite your education on performance mainsail trim. So log off and get out sailing so you can give it a try.
For Part Two of Mainsail Controls for Performance, including a discussion of the backstay and the boom vang, click here.
Mainsail Trim for Racers Part I by Pete Colby
Mainsail Trim for Racers Part II by Pete Colby
Mainsail Controls for Performance Part II by Dan Dickison
Buying Guide: Cordage Basics