Hi Rolf,

I am not less 'simpleton' than others. I sail since 2000 on week-ends in the summertime.

On my Dart 18, the gennaker was too full as well. To understand we have to have a consebe clear about lift, cL and draft (or camber) of the sail. More draft doesn't automatically means more cL or lift.
Lift is a force, cL is a lift coefficient and the relation between the two is lift ~ cL x v², where v is the apparent wind speed.
The luff of a sail without battens collapses in case of small entry angles. A full sail needs greater entry angle than a flat sail. Hence the smallest possible angle of apparent wind depends on the draft of the gennaker. It is rather geometry than L/D.
You can create lift with cL or speed, and due the power of the square, speed has the longer lever.
Take a flat sail and point higher so the apparent wind is stronger, and you have more lift with less cL than the full sail.
Now the lift accelerates the boat. The lift increases even more, but instaed of sheeting out, you bear away and keep the apparent wind speed more or less constant, but sail more deep and fast than the full sail.
So it is the entry angle and not drag, which requires to have a flat gennaker.

Cheers,

Klaus

and right now 1:0 for Germany:England (U21 championship, soccer)