Hi KMarshack,
from what I found this weekend, my sail was obviously too full below 5 knots.
May there is something wrong with the sail (the boat is a pretty weird custom combo) or myself, but sitting at the front beam on the windward hull at 6 knots, the best I can expect is to get that hull up in the air, laying my back out would be enough to maintain the proper attitude. Keep in mind the boat is 11 foot wide and I'm 250 pounds.
Thanks for your explanation about diamonds tension vs spreaders rake, It help me to understand a little bit more that mast prebend theory