18SX getting a hull up downwind

I'm sailing an 18sx with the original spin, which is a very full cut. I'm used to the tigers where the windward hull pops out of the water as soon as the spin is up in decent wind. in less then 15knots the 18SX gets up to speed and just when it feels like it should start to fly a hull the spin luffs. this is with the spin sheated as hard as it can.
so in sub 15 knot conditions is it just not possible to get the hull flying with such a full cut sail?
I know that with enough wind anything will fly a hull downwind. what is the critical number for the 18SX?
Thanks
Seamus

I'm not sure you can get the SX chute hot enough to wild thang it. That chute really wants to go dead downwind and collapses when it gets hot. Its just too full to go very high and thats where the power is. Its hard to get the apparent wind to go forward without the chute falling in on itself. At 15kts my 18 with Tiger chute would be double trapped to keep the hull down. 15kts is quite a bit of air for chute sailing.