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How to Trim Sails

Sails are controlled by 'trimming' them, or adjusting the tension on a line known as a 'sheet' that is attached to the sail. Pulling the sheet in, or 'sheeting in' rotates the sail towards the centerline of the boat (closer to you). Letting the sheet out, or 'sheeting out' lets the sail rotate away from the centerline of the boat (away from you). 

Correct sail trim orients the sail at an angle relative to the wind that most effectively drives the boat forward. Getting the correct trim can be tricky because the wind direction is never perfectly constant, and learning to compensate for changes in direction takes practice. 

Trimming sails properly depends on whether or not you're sailing upwind or downwind. Because sails work the same way going across the wind as they do upwind, we trim them the same whether we're going upwind or across the wind.

Trimming sails upwind

In order for a sail to work like a wing, it must maintain a relatively consistent shape to allow air to
flow smoothly from the front edge of the sail to the back. See how these sails are curved in the pictures.

If your sail is trimmed incorrectly--"sheeted" too far in or out--it will not have a good shape and will not drive the boat forward effectively. A poorly trimmed sail has a ruffling, inconsistent shape and will flap in the wind ("luffing").

Learning to trim sails upwind correctly is simple: just let the sail out ("sheet out") until the sail is flapping in the wind like a flag, and then gradually sheet back in until the sail develops a nice, smooth curved shape. We describes sails in this ideal state as "on the verge of luffing".

Trimming sails heading 90 degrees (perpendicular) to the wind

Remember, because sails work exactly the same going both upwind and heading 90 degrees relative to the wind direction, trimming a sail on a course perpendicular to the wind is exactly like trimming a sail when going upwind.

Trimming sails downwind
Trimming sails downwind is a little trickier. Because the sail is acting like a bag or net to 'catch' the wind and push the boat along, going downwind your sail will almost never be luffing

So how do you tell when your sail is trimmed properly going downwind?

The short answer is it's not easy! Ideally you want your sail oriented perpendicular to the direction of the wind. This exposes the maximum amount of sail area to the wind to make your sail the most effective it can be. If you're having trouble visualizing this, imagine how water dams are typically oriented on rivers--perpendicular to the water flow. 

In the picture, the wind is coming from behind the sailor.