Dehler 31 tuning and sailing

Too much rake? But the commonest reason for weather helm on keelboats is not keeping the boat flat enough. So:

Not enough forestay tension (over-deep genoa)?

Old baggy sails? Buy new ones.

Too much sail area in too much wind?

Not enough crew on the rail?

Helm needs to feather slightly?

That's more true of dinghies than keelboats. On a keelboat, the tendency to luff comes largely from excessive heel, especially on designs with fairly wide sterns, which most modern boats have. As they heel, because there is more volume astern, the stern rises, the bow dips and the centre of lateral resistance therefore shifts forwards. The implication is that too much power from the headsail also contributes to weather helm. This is why forestay tension is a key tuning variable on keelboats with simple fractional runnerless rigs.  

It might well be overpowered. It might be something else. Not enough information to tell.

Forestay tension is a very important tuning variable for keelboats. Too little and the headsail is too deep, lots of power but can't point. Too much and the boat lacks acceleration and is very difficult to keep in the groove.

If the boat has a fractional rig and runners, forestay tension is largely controlled by them. For fractional rigs, swept-back spreaders and no runners, it's a black art. I used to own a boat with a single-spreader swept rig and no backstay. Setting forestay tension involved a combination of caps and lowers tension. If you just tension the caps, you increase forestay tension up to a point, then more caps tension starts to compress the mast and it decreases forestay tension. Adding lowers tension straightens the mast and you get more forestay tension. You need to fiddle about with a gauge and spanners and experiment.

If you have a backstay, then generally more tension = more forestay tension. However the backstay is also tending to compress the mast and the bow effect can push the forestay attachment effect forwards = less tension. Lowers tension counteracts this.

It's paradoxical but these simple rigs are complicated to tune because you get these complex interactions.