death bear-away

ok went out windsurfing yestersday. Haven't done much with a harness.
When pulled into the harness lines, I found that i immediately started bearing away. I then moved my weight out further onto the rail to steer more upwind, but found that i just pulled the sail further on, and started going squarer downwind. all the time i went faster and faster until i stacked.

I think i had my lines too far back.
Where is a good place for them? Does this vary with wind?

Cheers. i am 6.0 and 70kg.

It does sound likely that your lines were too far back.

In addition to Guy Cribb's sheet, I find it useful to stand the sail up on the beach, or attached to the board on the beach (fin pushed into the sand) and hold the sail by the harness lines. If they are correctly positioned the sail will balance nicely. If it pulls away to the front hand side, move the lines forward. If it pulls away to the back hand side move the lines back. Fiddle with the line position until it is just right. It is worth a little time!
If you don't know where to have the harness lines to start, hold the boom with one hand in the position where it balances (or two hands if that's easier) and place the lines either side of your hand/s.
I don't see many people doing this but I find it works really well for me, especially in strong wind.

It sounds like your rig was too far forward. Try raking it back a bit more.

Might also be an issue of insufficient downhaul. Unless the downhaul is cranked on so that the "floppy " leach extends approx 66% at the top panel and progressively less but still perceptible at the above boom panel,it is likely that you don't have enough. If you are a dinghy sailor you cannot set the sail up so that it looks nice on the beach.
Insufficent downhaul will lead to the problem you describe and is the main cause of catapult with beginners.
Also windsurfer terminology differs to dinghy. Square is 90deg to the wind (a broad reach sailing) and broad is closer to dead downwind (square running sailing).
Get your harness lines set up as described but do crank your downhaul!