Open leech vs closed leech – which one’s faster?

When we talk about sail shape, most sailors think about draft depth or position. But one of the most influential features is the leech. Whether your leech is “open” or “closed” can dramatically change how your boat accelerates, points, and handles in different conditions.

What do we mean by open vs. closed?

• Open eech: The back edge of the sail twists away to leeward at the top, creating a “softer” exit for the airflow. You’ll often see the top telltale flying easily, and sometimes even fly upwards. THis shows the direction of flow, helping it off the sail.

• Closed leech: The leech is tighter and when closed, the leech begins to return on itseld, especially near the head of the sail. This traps more wind and resits the twist in the sail. The top tell tail would begin to stall more quickly.

These shapes are controlled by a mix of mainsheet tension, vang/kicker, mast bend, and outhaul, as well as how the sail was cut in some non-one design classes.

Sailing with an open leech...

Pros	Cons
Fast acceleration: The sail “breathes” more, spilling excess wind and making it easier to power up again after manoeuvres or when sailing through lulls.	Lower pointing: An open leech often means you can’t sail as close to the wind, losing height against other boats.
Control through waves: With twist at the top, it generates faster flow in lighter airs which would help in small chop in lighter airs to keep the boat ploughing through. An open leech also helps to add leeward heel which also benefits through small chop.	Reduced drive in light air: Too much twist can “leak” power, leaving you underpowered in marginal conditions. We want to keep flow across the sail, not out of the top.
Less drag: The airflow exits cleanly, which can reduce turbulence as the sail needs three parts of flow in order for us to go forwards quickly: wind flow on, across and off the sail.	More heel in breeze: If wind escapes out of the top, this will generate more heel so if you are unable to keep the boat flat, think about closing the leech.

Open leech in an RS Feva as we can see the twist in the main sail and not a flat boat

Sailing with a closed leech...

Pros	Cons
Higher pointing ability: A tight leech helps you hold a narrower angle to the wind, which is golden when height is the priority.	Harder to control in gusts: A closed leech traps power; so if the wind drops, adjustments need to be made.
Extra power in light winds: Keeps the sail loaded and driving when there isn’t much breeze to work with (working in the gusts).	Risk of stalling: Airflow struggles to exit smoothly, especially if sheeted too hard — leading to a “sticky” feel and loss of speed.
Punch through chop: The additional drive can help carry momentum through waves as we are powering up the 'power area' towards the bottom of the sail.	Stalls the rudder: More weather helm develops, making the boat harder to balance and steer for the helm as a closed leech often leads to a raked back rig.

Closed leech in RS Tera as we can see hook in the leech, but sailor using in high mode as we can see by burgee

Final Thought

There’s no “perfect” leech setting — it’s always being adjusted to the conditions. The best sailors learn to read/feel conditions and adjust: open up to survive and stay fast in breeze, close down to punch higher in light air.

Next time you’re on the water, experiment. Sail two identical upwind legs: one with an open leech, one with a closed leech, and feel the difference!