we added a jib to a climax 18, its a kinda obscure boat, but super super light, anyhow, when we added the jib, if the wind got up to where we consider it fun the hulls would start to flex to the centerline of the boat, anyhow, we had to take an old FJ mast and cut it down to make a crossbar at the front of the hulls. The hobie 17 hulls might have somekind of similar problem. They are alot smaller that the climax hulls, and although heavier I seem to remember someone telling me that had some flex.

I may be completely off base, but thats my only experientce adding foresail too a boat.