Home . Articles . News . Technique . Mainsail trim

Mainsail trim

2018 September 1

There’s more to getting the most out of your mainsail than just the sheet

It is quite common to hear that proper mainsail trim and setup is only important to those who are racing or trying to go fast. The reality is it is important for everyone. It provides an avenue for the boat to sail at its peak performance, which usually means that it is easier to sail, balanced and therefore more fun.  There is nothing worse than being on a boat that is either heeled too much, has too much helm (lee or weather) or doesn’t have enough power to move. Generally speaking, most people know what to do with the mainsheet (more on this later), but the first step in proper mainsail trim is to have it set properly on the boat and to understand what each control does. Let’s take a look at a quick guide that I use to make sure that the mainsail is set up right so that the sheet is your main focus.

This view of the mainsail—from the center of the sail looking up—is a great way see sail shape and trim. On this nicely trimmed sail, the draft is 40% to 50% back from the luff. Full battens help with the shape and the telltales are flowing



It is amazing to me how often I see boats out with their main halyard not set right. Ninety-nine percent of the time, the halyard is not tight enough. A quick rule of thumb is to set the halyard so that the luff is smooth and has a little tension while the sail is luffing. Once the sail is trimmed in, the sail should have just a hint of wrinkles that are perpendicular to the mast. If there are big wrinkles adjust the halyard to achieve this. It will take more halyard tension when it is windy to get the same results. One thing to keep in mind, the halyard not only affects the luff but the leech as well. If the halyard is too loose, not only is the luff loose, but so is the leech. 

The cunningham also adjusts luff tension, but should only be used after the halyard is set correctly. If the halyard is too loose and you try to fix with cunningham, you may fix your luff issues, but your leech will still not be right. Work hard to get the halyard set right. Pro tip: Place a mark on the halyard once it is up so you can set it there again easily. This will take out a lot of the guess work when you are hoisting the sail each time.


A quick guide for this is to set it so that there is a hint of tension on the sail along the boom. There should never be wrinkles on the sail that are perpendicular to the boom. I always tell folks to error on the tight side if they are not sure what to do. Generally, the lighter the wind, the looser the outhaul can be, but this is the first control I adjust as the breeze comes on. If you have an in-mast furling sail, be sure to look at the shape of the foot of the sail and adjust the outhaul accordingly. Most folks with in-mast furling also do not put enough tension on the foot of the sail. I cant stress this enough, if you are going to make an error, err on the tighter side.


This is an important control. This helps keep the leech tension the way you want it. Again, as a general rule, I tension the vang so that the top batten is parallel to the boom.  This is especially true when running, especially as the breeze comes on. If you are reaching, and have too much power, ease the vang to twist the top of the sail off and spill power. Again, the top batten being parallel is a guide to start with, but this is a critical adjustment, as when you are reaching this will control heel, and when running it will help with the boat rolling and upwind it will help with the twist of the sail.

OK, so now we have the sail set on the rig correctly, now what do we do with the sheet? The easiest and most basic rule of thumb is to ease the sheet until it starts to luff, then trim it until it just stops and you are in the ball park. Generally speaking this is true, and something that I tell folks to look at all the time. Here are some other key hints and tips about setting the mainsheet.


Everyone—racers, cruisers and daysailors alike— should have telltales on their main. The most important one is attached at the leech near the top batten. This shows proper flow. If the sail is over trimmed, this telltale will be stalled and not flying, if the sail is under trimmed, generally the sail will be luffing. This is any easy guide. This is most useful when sailing upwind, but it is also a huge help when sailing off the wind as well.

Heel of the boat

This is a sure sign telling you the boat may not be too happy. If the boat is over heeled, it is time to ease the mainsheet (or drop the traveler).  If by easing the sheet, the sail is luffing all the time, there are two things to also look into: One, the jib may also be over trimmed, and two, it may be time to reef the sail. If the boat is heeled and the main is flogging, something is severely wrong and should be changed (generally it means reefing or a smaller jib or both).


This is the boat’s way of telling you it is not happy. Heel also affects this, but if the heel of the boat is within what you would consider normal, then the helm load will lend a hand about the main trim and set up. If you have too much weather helm, you should consider depowering the main, but either easing the sheet or making the sail flatter (or smaller). If the boat has lee helm, that may mean that you need more power in the main. The jib also affects this situation and should work in tandem with the jib. Keep in mind, the main and jib should be trimmed in unison, so if you are easing/trimming one sail, you generally have to adjust the other as well.

Proper sail trim is one of those things that makes sailing more fun as the boat is smooth, efficient and comfortable. This is great if you are cruising or racing. One thing I recommend is that if you have questions about how to trim on your boat or if your sails look correct, today’s technology is a huge help. Take photos when you are sailing and text them to your sailmaker or local expert. This is the easiest way to get advice on what to do and how to make the boat go better. You can also use a video calling app like Facetime while you are on the water and get immediate feedback from your expert. Also check with your sailmaker. They may have tips, tricks and other bits of info specific to your boat that could help with trim or rig tune. Do not shy away from getting all of the information you can. It will make your sailing more fun. 

Allan Terhune Jr., who has won 11 North American championships, is a sales professional for North Sails.