March 19, 2024

6 min

Skills & Seamanship

How to trim a sail

sail trim

Sail trim can often seem like a dark art, but in truth getting sail trim 90% there is not that hard and for most cruisers, you really don’t need, or have sophisticated enough sails, to push it much further. By that we mean – don’t beat yourself up about it. Follow these guidelines and you’ll be doing just fine.

How to trim a sail – key steps

  1. Always trim the sail closest to the wind first
  2. Use the sheets to adjust the angle of the sail
  3. Tightening the sheet brings the sail inboard. Easing the sheet moves it out.
  4. Using the sheets, move the sail toward the tell tales that are not flying
  5. The draft is the deepest part of the sail’s curvature
  6. Aim for the draft of the sail to be around 45-50% aft of the luff
  7. Tension the halyard to move the draft forward. Ease the halyard to bring it aft
  8. Adjust the twist in the sail to get laminar flow all the way up – or to spill wind if it’s too strong
  9. Don’t overthink sail trim until you’re racing. Getting it 90% right is easy.

Getting your headsail trimmed well for upwind sailing is important. It’s going to help you keep your speed up, power through any chop and ensure the boat feels balanced on the helm.

If in doubt ease it out

If you do nothing else with headsail trim, go for the old adage ‘if in doubt ease it out.’ It’s so easy to over trim a sail (ie have the sheet too tight) and all this does is reduced the sail’s effectiveness. Yet it often feels like by tightening the sheet we are ‘doing’ something. Do the opposite of what you might imagine. Ease it gently out until it begins to luff, and then tighten it back in just a fraction until it stays firm. At this point, you really are most of the way there with trim, so even if you just do this, you’ll be fine.

Tell tales

It is definitely worth having tell tales on both sides of your sail. These light red and green ribbons show us how the wind is flowing over the sail, so we want them at a few different points. We like them about 12′ aft of the luff, although if you have a roller furler, you may have a few sets or one set in the middle of the sail. You’ll usually want three sets. The first set should be about a quarter way up from the foot of the sail. the next set around the middle of the sail and the final set about three quarters way up. This will give you a really good indication of how the wind is flowing over the sail.

Laminar flow

Ideally, we want the sail angled into the apparent wind so that it blows along the fabric for as long as possible. This is called laminar flow. Once it detaches from the fabric, it is no longer having an effect. When it doesn’t flow along the sail, the usual reason is quite simply that it has been blocked by the angle of the sail. If a sail is over sheeted, the wind will not be able to reach much of the leeward side. We will see this as the telltales will droop. If a sail is under sheeted, the wind will not be able to reach the windward side, and those telltales will droop (the sail will also eventually luff). Think about it. In each instance, the solution is to move the sail toward the tell tales that are not flying. This simple trick is for many students the breakthrough moment in their understanding of sail trim. Trying to get all the telltales flying can be a thankless task. We will look at some more advanced techniques in a moment, but for now, and for most cruisers, getting the tell tales at the base and middle of the sail flying well is plenty good enough.

The draft of the sail

Now we start getting a little more involved. The draft of the sail is the nominal distance from the chord (an imaginary straight line from luff to leach) and the actual fabric of the sail. Where the deepest part of the draft is, from luff to leach, affects the performance of the sail. As a general rule of thumb, we want the draft halfway between the luff and the leach. We control the position of the draft via halyard tension. More tension forces the draft forward. Easing the tension allows it to come aft.  A strengthening wind will gradually force the draft aft, so we should tension the halyard to bring it back into its desired range. Choppier water favours a fuller draft, while flat water and speed favour a flatter sail with the draft forward. Having the draft aft will also enable you to point a little higher. Confused? It is confusing and the best thing to do is get out there, remember these key rules and try playing with the halyard tension and see if you can feel the effect. We are talking maybe an inch of the halyard, so don’t overdo it!

Twist in the sail

Our final trick is to adjust the twist in the sail. We do this by adjusting the position of the headsail cars and, for the mainsail, by adjusting the combination of traveller position, mainsheet and vang. Put simply, twist is the difference in angle of attack between the bottom half of the sail and the top half. The apparent wind will be at a different angle the higher up the sail you go, so to be really effective, we need to adjust the twist of the sail to ensure we get ‘laminar’ flow all the way up.

If you move the headsail’s sheet car forward, the pull of the sheet will be more vertical and downward than lateral and aft. This means the leach will tension more and the top of the sail will come in, but the foot will ease. Move the sheet lead aft and the pull will be more horizontal. The foot will tighten and the leach will ease, allowing it to move out.  In this way we can affect the twist of the headsail and either get great laminar flow or, when needed, allow the wind to spill out of the top of the sail and thus depower it. Much the same effect can be achieved with the mainsail by pulling the sail up the traveller a little and easing the sheet. if you are off the wind, the vang can be used to bring down the boom and tighten the leach.


In summary, sail trim is both science and art and ultimately who cares what you do. If it makes the boat sail better and faster, you’re doing well! Don’t get overwhelmed by sail trim. With a heavy cruising boat and an old roller furler sail, you’ll be lucky to get the telltales all flying and if you do, you’ll be doing just fine. Get out there and have fun. If you want to get out on the water and get practical, hands-on training sign up to the Rubicon 3 sailing school, or join one of our adventure sailing holidays which are packed full of adventure and real-world training.

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