Fine-tune the total power of the main with the above controls
The final step in mainsail trim is continual evaluation of the sail's power. The main trimmer must keep track of the boat's heel angle, speed and pointing ability, and how the mainsail may be affecting each.
To get an idea of how to do this imagine that you are sailing upwind in medium air with the main at its most powerful setting. Gradually the wind strengthens. Sooner or later, the main will need to be de-powered, first by flattening it with mast bend and foot tension, later by lowering the traveller in the gusts, and eventually by reefing. How quickly should you depower the main?
The most obvious indication of over-powering is your angle of heel. This is something all crewmembers can feel. Heel isn't necessarily your best guideline, however. Boatspeed and the amount of windward helm are actually more sensitive and accurate indicators.
When you think you might be over-powered, use the following boatspeed test to gauge mainsail power. Lower the traveller until most of the main is luffing, and watch the knotmeter for at least a minute. If your speed increases without an appreciable loss in pointing ability you are overpowered.
Depower by blading out the main. If the main is already as flat as possible (without inverting), put in a reef or change headsails. Once again, check the knotmeter to confirm that your speed is greater after the change than before it.
Another way of testing mainsail power is to measure the rudder angle required to sail in a straight line. We said before that the goal in mainsail trimming is to achieve about 3 to 5 degrees of windward helm. If you have more than this, you are overpowered.
Depower the mainsail by bending the mast, opening the leech, easing the sheet, dumping the traveller, and reefing if necessary. These adjustments are simply changing the total power being exerted by the mainsail. Since most of the main's power is side force, adjusting the amount of this power affects windward helm. You have to get windward helm down into the acceptable range.
To measure your main's depth and draft position, draw chord lines between the ends of the draft stripes. Then measure the shapes as explained in "The Basics.". Note that twist is shown by the difference in angle between chordlines.
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