You can't deal with the outhaul -- that's tension in the foot of the sail -- in isolation from the other controls. It's just one control for shaping the mainsail
As a general rule, you want foot and leech tension to be balanced (this is analogous to how you set your jib
cars), so outhaul is usually hardened or eased together with the mainsheet or vang to make the sail flatter or fuller. But you also have to deal with twist, so when you get the shape approximately the shape you want, then you fiddle with the vang (or mainsheet when close-hauled on many boats) to get the twist you need, then re-adjust the outhaul.
Flatter or fuller generally means less and more power, but with many caveats. Fuller adds power up to a point, but sailing upwind where you are sensitive to drag, you get diminishing returns faster and may need to trim flatter for less drag. Also in very light wind
, air flow may not be strong enough to stay attached around a very full sail and you may need to trim a bit flatter, however much you may be yearning for more power.
Twist is adjusted to keep the airflow attached along the sail top to bottom and you will need varying amounts depending on the wind
strength and sail shape.
You also need to deal with angle of attack, which you control with the traveler to the limits of its travel, then mainsheet after that (transferring leech tensioning function to the vang at that point, if you weren't already using that).
Then there is draft
position and halyard
tension -- but these are relatively fine points compared to the above. Whole books
have been written about mainsail
trim and it's not really possible to cover the whole subject in one post.
None of these adjustments can be done without affecting the others, so you have to play with various combinations until you figure out the right combinations for different conditions and points of sail. Headsail trim also profoundly affects air flow over the main, so it goes without saying that none of these controls will work right if the headsail is not trimmed.
Hope this helps. These are just a few hints and by no means any comprehensive guide -- sail trim is a rather black art which just takes lots of practice and experimenting.
Someone no doubt will add more information about reading telltales -- there are many useful rules which I have not gone into.