after reading the forum just could days ago about marginal wind tuneing i wondered about heavy wind sail tuneing ...... is it just the oposite from light wind ??...... moveing boom down same downhaul with more outhaul?? .... mast base forward ???
Boom stays the same always, mast extension 1 extra hole, 1 hole extra on the boom extension.. i always leave the base where it is..
^ what he said. 
nicko this is the way to look at it because there is no perfect setting.
downhaul and outhaul.
decreasing downhaul will move the centre of effort up the sail. once you move it too high you won't be able to counterweight the rig (to much power), so you will either need to lower the CofE with more downhaul or lift your boom to get more leverage.
decreasing outhaul will move the draft forward and increase power. once you move it too far forward you will know as you will feel to much power on your back hand. move it too far back by adding to much outhaul and you kill the power or make the sail twitchy as it becomes too flat.
so in high winds it is most normal to increase downhaul to lock the c of e low so you can control it and add outhaul to reduce the draft and take some of the force off your back hand. but there are lots of combinations of downhaul versus outhaul and adjusting either will also adjust the looseness of the leach.
I pretty much use the same downhaul setting all of the time for all of my sails (nice leech twist) and would usually only increase the outhaul, and often find it useful to move the mast base forward approx 1-2cm to keep the nose of the board down, particularly if the board starts to get a bit flighty (assuming it's not to big for the conditions!). Also agree with petermac, changing down a fin size (relative to board/sail/sailor weight combo) helps a lot with control in testing conditions.
