How to trim a sail
Optimizing Sail Trim
The most common problem for novices (and some experienced sailors as well) is oversheeting. This disrupts the flow of the wind on your sail and causes drag instead of creating power from your sails.
Here is a tip on how to optimize your sail trim without oversheeting your mainsheet.
1) Telltails - You should use the upper telltails as a reference for adjusting your sails for upwind sailing. Try initially undersheeting your sail for a split second by easing your sheet a fraction so that your windward telltail flows up and down (and does not flow back) and your leeward telltail flows completely back.
2) Sheet Tension - When beating upwind, your mainsheet initially should not be vertical but should fall off to leeward - at least 5 or 10 degrees off center. If on a reach, this doesn't apply so you should simply let your sheet out until your sail is slightly undersheeted (windward telltail flows vertically)
Another way you can tell that you have oversheeted is when your leeward telltail does NOT flow back even when your windward telltail appears undersheeted. It is likely that your telltail which is on your trailing edge also does not flow back. When this happens, ease your sheet until the leeward telltail flows straight back.
3) Steering - To complete this sail adjustment and maximize your boatspeed, bear away until both telltails on your sail are trimmed properly. At this stage, you will maximize your boatspeed but may not be pointing as high as possible.
4) Fine Tuning - Now that you've determined that you have not oversheeted your sail and would like to point higher try steering up a few degrees and adjust your sheet tension (in small increments, say 1?" at a time). Wait 10 or 20 seconds until you can feel the real effect of the adjustments. Continue adjusting your point of sail and mainsheet until you are happy with your boatspeed and heading. Remember, stay in the groove and don't try to point too high. Continue this process until you are happy with your boatspeed and point of sail.
It is difficult but possible to oversheet your sail in heavy wind conditions. In heavy winds you can literally disregard your telltails because the windward telltail will seldom point straight back. Instead sail by the heeling angle of your boat and the feel of your boat
You can recognize when you have oversheeted your sail when your feel more than usual weather helm on your tiller. When this happens, try letting your traveller out in increments of about 3 inches at a time but keep your mainsheet tight. Keep adjusting your traveller until the helm or tiller pressure becomes neutral again.
It's really easy to oversheet even after going through this routine. Keep testing whether you are oversheeted by going through the same routine of easing your sheet, bearing away and then fine tuning.