Mainsheet Trim

Mainsheet Trim: Angle of Attack vs. Leech Tension

By Dave Kirkpatrick

One of the fundamental concepts of mainsail trim is the distinction between leech tension and angle of attack. Understanding how to manipulate these two variables is key to boat speed in all conditions.

The Concept

Imagine a boat with both a mainsheet and a traveler:

• Traveler: Controls the angle of attack (how close the boom is to the centerline).
• Mainsheet: Controls leech tension (pulling the boom down).

In a boat like the Vanguard 15, which lacks a traveler, we must use other controls—specifically the vang—to achieve similar effects.

Light Air: Powering Up

In light air, your goal is to generate power.

• Technique: Sail with a loose vang.
• Effect: When you ease the mainsheet, the boom rises. This decreases leech tension, twists the sail, and makes it fuller.
• Result: The sail powers up, helping you punch through lulls and chop.

Heavy Air: Vang Sheeting

In heavy breeze, your goal is to depower while maintaining a flat, efficient sail shape.

• Technique: Pull the vang on hard ("vang sheeting").
• Effect: The tight vang takes over the job of controlling leech tension. When you ease the mainsheet, the boom goes out (changing angle of attack) rather than up.
• Result: You spill wind to keep the boat flat without losing the leech tension that drives the boat forward. This is critical for pointing and speed in big breeze.

Summary

• Mainsheet: Primary control for angle of attack (in/out) when vang is on.
• Vang: Primary control for leech tension (up/down) in heavy air.

Mastering this interplay allows you to shift gears seamlessly as conditions change.