The Fundamentals of Mainsail Trim

Posted by Darrell Nicholson at 10:24AM - Comments: (9)

The article on selecting a cruising mainsail coming up this month (August 2011) offers plenty of advice on choosing a main, but nothing on the important topic of sail trim. Unless you race as well as cruise—and our recent sail survey shows more than one-third of our readers do—you probably take a fairly casual approach to sail trim. 

Opening the mainsail leach in light airs will keep telltales streaming.

While you don’t have to be an incurable sail-tweaker to cover ground on a tradewind passage, assuring your mainsail is well-trimmed will put you safely at anchor sooner and ensure a smoother, more comfortable ride. Good mainsail trim, of course, is paramount when going to windward.

One of the best books on sail selection and trim that I’ve found is Brian Hancock’s "Maximum Sail Power."  I bought mine on Amazon for $14, but you can buy it used for less. Here are a few tips Hancock, a renowned ocean racer who is now a popular motivational writer and speaker, offers on the fundamentals of mainsail trim. If you are interested in getting the best performance out of your sails, Hancock’s book is a good investment.

Telltales: No, you don’t need them, but why go without? Don’t let pride get in the way of these extremely handy devices for checking sail trim. On the mainsail, you can attach them along the leach at the end of each batten. You can also attach them at the luff like you do with your jib, but because of airflow disturbance around the mast, these won’t be as effective as they are on the headsail. You can buy stick-on, nylon telltales about a quarter-inch wide and attach them with some sticky-back Dacron. These are easier to see than basic wool yarn, particularly with a flashlight at night. (If you really want to get fancy for night sailing, the Glow Fast telltale patches look interesting for headsails.)

I use simple wool telltales stitched through the sail, with a couple of overhand knots to keep them from pulling through. Red seems the easiest for me to see. The downside to wool is that they are invisible at night and have to be replaced frequently as they fray. The upside is they are cheap and easy to replace. I also attach wool telltales to the shrouds. Sure, a windex and a digital wind indicator are more sophisticated wind-tracking tools, but, particularly in light breezes, tell tales are more helpful. The skin and hair on the back of your neck are also effective wind indicators, which is why I shun collars and hoods when running downwind in the tropics, particularly in squally weather.

Mainsail trim: As you settle into your desired heading, sheet in the main and watch the top telltale. When the top telltale dips behind the mainsail, followed by the others, ease the sheet until all the telltales are streaming again. Another basic way to check correct trim in light to moderate winds is that the top batten is parallel with the boom. However in very light winds, you will have to open the leach to maintain sail power. This gives the light air a clean path to exit the sail. Over-trimming in light air is a common mistake, even among experienced sailors.

Mainsail traveler: While the mainsheet changes the shape of the sail relative to the wind, the mainsail traveler is the more potent tool for adjusting the angle of attack when the boom is close to the centerline. The mainsail track is particularly helpful in heavier air when the boat is being overpowered. By using only the traveller to find ideal mainsail trim, you can effectively balance the boat by reducing or increasing weather helm. Finding that happy medium of weather helm makes the helmsman's life easier, and it is easier on the autopilot or windvane.

Vangs: The vang’s usefulness comes into play off the wind, when it keeps the leach from opening up and depowering the boat. Apart from sail trim, a modern hydraulic or mechanical vang supports the boom. In this sense it is doing the job of a topping lift, making reefing easier. Hancock makes an important point about vangs that is often overlooked: They should be easy to release. It is surprising how common it is to see a boat sail into port with a broken, or fractured boom that snapped or cracked when the boom dipped in the water, putting tremendous force at the point where the vang attaches to it.

These are fairly fundamental tips. Hancock, of course, goes into much more detail on these and other more advanced sail trim topics. Even if you don't plan to race, and are in no hurry to get anywhere, the book is a helpful guide for anyone considering a new sail purchase.




Comments (9)

Thanks Mark. Someone must have picked up the used ones that were listed. Anyone have another recommendation? I'll see if Brian or Nomad, the publisher, is planning another print run or e-book. I imagine some more used ones will pop up at lower prices.

Posted by: Darrell | July 21, 2011 9:11 AM    Report this comment

My check on Amazon listed the suggested book at $75 to $90 not $14.

Posted by: Mark W | July 20, 2011 10:02 PM    Report this comment

Good eye Chris . . . a closer vetting of blog photos is surely in order.

Posted by: DARRELL N | July 18, 2011 8:45 AM    Report this comment

The photo appears to show that the topping lift needs relaxing. The boom, itself, would then tighten the leach.

Posted by: Chris B. | July 17, 2011 7:33 AM    Report this comment

Good comments all. As I recall, this photo was taken in relatively light air (6-10 knots) pretty flat water. The boat is an International Folkboat. I believe Victor's right, easing the traveler and sheeting the main a bit would help prevent the flow separation that is likely occurring at the bottom of the sail. Any other suggestions? Outhaul? Regardless couple more tell-tales would tell the story--so to speak. Trim is not perfect, I'll see if I can find another pic. It's testing week for us (bottom paint, wood finishes and plotter sounders), so the sweat on my brow obscured my mad search through the files. Send us your own mainsail trim photos!
--- DN

Posted by: DARRELL N | July 14, 2011 5:20 PM    Report this comment

This mainsail looks to have a lot of twist. I would want to tighten the vang,and /or the mainsheet.For an illustrative picture on trim it is not good as I agree completely it needs two more telltales

Posted by: JOHN F | July 14, 2011 12:33 PM    Report this comment

The photo shows only two teltales, on the top two full batens. There needs to be at least two more down lower to properly evaluate sail trim. With the amount of twist, it is very likely that the lower portion of this main is stalled. ... down on the traveller, in on the main.

Posted by: Victor D | July 14, 2011 11:13 AM    Report this comment

In that photo they're showing the leach opened up for very light air, so the top batten is not parallel with the boom. As the breeze picks up you'd sheet in to take out some twist, which would bring that batten in.

Posted by: KEVIN S | July 14, 2011 10:58 AM    Report this comment

Now in this picture, presumably of a mainsail in trim, is the upper batten parallel to the boom? Is this what it should look like? Parallel to the boom does not mean in line with the boom?

Posted by: MICHAEL G | July 14, 2011 10:53 AM    Report this comment

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