Mainsail trim made simple - by Bill Fastiggi

This is a quick guide for racers or cruisers to give you the proper mainsail trim for the majority of sailing conditions:

Set halyard tension -  Pull up mainsail halyard and remove 95% of the horizontal wrinkles along the luff of the sail.  The exception is in very heavy air, remove all the wrinkles.  When sailing upwind vs. downwind or light air vs. moderate air, you will have to adjust tension to achieve this setting of "just a hint of wrinkles".

Set outhaul tension - For upwind sailing, pull outhaul tight to remove wrinkles or to close the shelf foot.  Downwind, if you have a shelf foot or loose footed sail, ease the outhaul until shelf is open.  Most boats this will be about a 6" range of adjustment.

Set mainsheet tension - For upwind sailing, trim mainsheet until the back half of the top batten is approximately parallel to the boom.  If you have leech tell tales, the top tell-tale should fly about 50-75% of the time.

Set traveler - Pull traveler up to windward until boom is on centerline.  As the wind increases, gradually slide traveler to leeward to relieve weather helm. 

Vang - For downwind sailing, set the vang much like the mainsheet tension upwind.  Pull the vang on until the back of the top batten is parallel to the boom, and the leech telltale flies about half of the time.

Mainsheet tension downwind - let sail out until it luffs on the forward edge of the sail, then trim just enough to remove the luff.

Cunningham - like main halyard, use to remove all but 5% of the luff wrinkles.

This is simply a quick reference guide that should work well for most boats.  However, you may find through experimentation, that slightly different settings may work better on your boat or sail, especially if your sail is very old or "blown out", or your boat is extremely light or heavy displacement.