On my A Class Cat (see avatar pic) I have put on a new and much fuller sail to get more off-wind and down-wind performance. As expected I have suffered upwind, to some detriment.
Naturally, it needs to be more flat upwind and needs more prebend in the rig, (10m of CF wing) but I don't want to lose shape by cranking it up too much and going back to where I was last season with my old sail.
I can really feel the drag upwind and against other boats it seems 'laggy'. Lots of downhaul/Cunningham helps but the sail gets stretched so much down the luff when you really crank it on.
My reason to post this?
Landyacht sails seem really flat and generate much higher apparent wind speeds, being an upwind machine that they are. I am hoping to get some advice from the LY speed freaks out there to get more speed out of my Cat, from anyone who can suggest something.
Any suggestions most welcome.
It is quite common practice to run a down-haul system that is controlled by the main sheet system, there are several versions, here is just one.
Land yachts need a fuller sail to get up and moving and when get going and increasing speed you need a sail that flattens out as you mentioned perhaps a bit like an automatic transmission in a car. This is normally achieved with mast flex and down-haul
I think this would be one for a sailmaker,. i tend to make my sails to sail upwind then work on a technique to compensate downwind. these days i take 5 or 6 sails to the lake , so if its rough, a bigger sail, light winds, a fuller sail,etc
have you considered having a flatter sail, and a line to force more batten bend downwind. there a few class 5's doing that
Adjustable outhaul is what you need -- forget adjusting the downhaul.
Crank on the outhaul when you're going upwind, let it out (therefore bagging out the sail) when going downwind.
Works like a charm 
I got it from a RC model being sailed by Ross Gamlin. he reckon it was common on RC water yachts. It really works well, but always keepm sheeted, if the mast starts to flap , sheet in
