The more I think about the reasons for when to move the traveler and when to ease or pull the sheet, the less sense I make out of it. Is there a simple way to think about the respective roles of the main sheet and the traveler when trimming the main?
Dan Dickison responds:
Don't worry, your confusion is completely understandable. If it seems a little difficult to get a grasp on this area of sail trim, it might be because there's a lot of misinformation circulating in the form of well-intentioned advice. We've published a few articles that might be of help to you here at SailNet, such as "Mainsail Controls for Performance" (see also Part Two) and "Best Mainsail Trim for Racers" (Part One and Part Two). There are also a number of books that do a good job of spelling out what to do regarding the traveler and the mainsheet.
Keep in mind that adjustments to sail trim are made relative to the conditions, the kind of boat you're aboard, and the angle that boat is sailing. As an example, I recently spent the day on board a VO 60 (Volvo Ocean Race 60) where the mainsail trimmer only played the traveler and never the mainsheet once we were up to speed. That was principally because we were sailing in steady, moderate breezes and we were testing sails so we rarely varied the course steered.
Anyway, think of those two controls in this fashion: the mainsheet pulls the boom down and the traveler pulls it to weather. That's not exactly the whole story, but generally it holds true. Usually, if you're sailing upwind, you'll get the mainsheet trim in the ballpark to complement the angle you're sailing, and then you'll adjust the traveler. Let's say you're on board a J/29 and the wind is in the nine to 10-knot region, you want the boom more or less on centerline and thus the traveler a little to weather with the mainsheet relatively tight. Once the boat starts building speed, you can probably crank the sheet in a little bit to flatten the mainsail out somewhat now that there's a flow moving across it.
Should you get a rapid increase in breeze, the appropriate response is to ease the traveler to maintain the optimum angle of heel. Of course some sailors do this as they slightly feather their course upwind to keep the boat upright. If you were to ease the mainsheet instead in this instance, you'd only succeed in allowing the boom to rise, therefore making the sailshape more powerful and promptig the boat to heel more, which all together would be counterproductive. Undoubtedly there are limitations like in big breeze when you can't ease the traveler beyond a certain point but the boat is still overpowered, so you have to resort to the mainsheet.
This is really a very basic explanation. I would encourage you to get a copy of Jim Saltonstall's Essential Sailor, or even John Rousmaniere's The Annapolis Book of Seamanship, which has a good basic explanation of the principles at work.