North Sails Fast Course
By nature, a mainsail is the most durable and flexible sail in any inventory. It has to cover an incredible wind range, which means stretching into almost every shape possible.
While the mainsail provides a good bit of power, it also has a lot to do with the boat's directional control. The sail helps to steer the whole sailplan, functioning a lot like a trim tab on a keel or a flap on an aeroplane wing.
In particular, the aft part of the main, the leech, is an important influence on the directional tendency of the boat. A closed or tight leech "kicks" the airflow to windward, creating a large side force to leeward at the stern of your boat. This creates weather helm and tends to push the bow to windward (See below).
Similarly, an open or twisted leech allows the air to flow easily off the mainsail without developing as much sideways force (See below). So you feel less windward helm.
Mainsail trim procedure:
In the last edition of the Fast Course book, this chapter included an extensive trim loop diagram to explain mainsail trimming. The loop cycle was helpful for some people, but quite confusing for most, For this reason, we dropped it. However, we found the basic five-step trim procedure very helpful, and we have expanded on it here. The main point of the loop cycle was that trimming the main is not such a simple matter. Whenever you adjust a control, it has a primary effect, but also many secondary effects. Therefore, the mainsail trimmer must have a well-rounded knowledge of just about everything covered in this book, as well as a methodical approach to the task. What's exciting about trimming the main is that it's a job where you can learn a great deal. The following procedure offers a good, solid approach to mainsail trimming.
The five basic steps are:
» Set twist with mainsheet tension
» Set depth with mast bend and outhaul tension
» Set draft position with luff tension
» Set helm balance with traveller position
» Fine-tune the total power of the main with the above controls