The Fine Art of Mainsail Trim
"Hey Krabby, I just started racing beer cans this summer and I need some advice on how to trim the mainsail,” asked a newbie racer.
“When in doubt - let it out,” I quickly replied.
“Thanks for that, but …well, I was hoping for a few more pointers…I know the drill, a rum and tonic is your drink, right?” He asked as he ordered a round from the bar.
“Ok, now you’re talking and so am I. My advice is still the same; if you are not sure of the sail trim, then you are probably sheeted in too tight - so let it out. Look up from under the boom and make sure the leech is not hooked to weather. You should have telltales along the leech of the main - make sure they are streaming out straight and not stalled. The top batten should be parallel to the boom. Starting the mainsheet (easing it out) will open up the leech and let it fall away to leeward. Tightening the sheet will make the leech hook to weather and stall the sail.” I said as my drink arrived.
“Ok, it is starting to make sense, but I usually look at the luff of the sail and keep tightening the mainsheet until the bubble is gone from the luff,” responded the up-and-coming sail trimmer.
“You’re right, you don’t want the main to luff - however when you are sailing close-hauled, the shadow of the mast and the jib will back-wind the main making a bubble - it almost impossible to sheet-in far enough to get all of the bubble out of the luff before the main stalls from over-sheeting. You need to work in concert with the jib trimmer to make sure the headsail is not over trimmed. Again: when in doubt, let it out. Nothing is slower than over-trimmed sails. Once you have the mainsail trimmed as I described, then you can make some fine adjustments that will get rid of most of the bubble in the luff without stalling the leech.” I said taking a long drink.
“That is starting to make sense, but there is more to it, right?” inquired the eager learner.
“Well, of course - but we can’t cover all of that or your bar tab will run you aground and we will be too drunk to have a conversation.
The most important thing to keep in mind is that the mainsail is key to steering the boat. It is similar to the rudder in the water but in the air. Pull the mainsheet in too tight for your point of sail and the boat turns upwind; ease it out too far and the boat falls away to leeward. This is because the mainsail is your rudder in the air. When you tighten it and the mainsail carries more load than the jib and it turns the boat into the wind because it is aft of the center of effort. The same is true when you ease the main and let the jib carry more load the bow goes to leeward.
On small boats, the helmsman controls both the main sheet and tiller. On bigger boats, the main trimmer works in concert with the helmsman to balance the helm so that he or she can steer effectively on the desired course. If you notice the helmsman fighting to keep the boat from rounding up into the wind, then you need to ease the main a little until he has better control on the helm. This changes constantly as the wind speed and direction changes and also the point of sail. An occasional glance at the helm’s position, to see the position of the rudder relative to center, will give you an idea of how the trim of the main is affecting the steering. The helmsmen will ask for fine adjustments to get the feel they want to sail the boat fast.” I explained reaching for my drink.
“Wow, that’s a lot of information. It will take some practice to get it all down. Now, what about the traveler?” asked the mainsail trimming student.
“That’s the next step. First, get the basic idea with the traveler centered. Once you have the concepts down that I have explained, then you can use the traveler to adjust the angle of the boom relative to the centerline of the boat without adjusting the mainsheet - this allows you to control the shape of the mainsail, independent of the angle of the boom. For example, you can let the leech twist, but keep the angle of the sail relative to the centerline of the boat the same by bringing the traveler to weather and easing the sheet. This allows you to have the boom on centerline without sheeting the main in so tight that is over trimmed, which can happen when you are sailing upwind in light air. Conversely, you can keep the leech from twisting but change the angle of the main by easing the traveler and keeping the main sheeted in. this is a good way help keep the boat a little flatter when the breeze freshens. Before you start moving the traveler - it is important to get the basic idea of trimming the main sheet firmly in your head. As I said at the beginning, there are many things to learn and I only gave you the basics.
One last idea, since my cup is approaching low tide - get involved with some of the club events with the J 22 fleet. There are some excellent sailors that can show you some of what we talked about on a small boat, where you can see and feel the results of your actions and learn quickly.” I concluded as I saw sandy bottoms in my cup.”
“Thanks, Krabby! That was well-worth a drink. I’ll do my best to absorb all that info. Thanks for the advice about the J22s - I’ll get out there the next time at the next opportunity to get involved with the club boats!” exclaimed the newly educated sailor.