It sounds as if you need to open the hulls and glass in a relatively light "sub deck" approx' a third of the hull depth down from the deck extending from approx' the shroud mounting point through to the main bulk head under the front beam and then continue it on in front of that bulk head for approx' 600mm forward. This wiil allow the loads generated by the bridle "pulling" the bows together and upwards, and the similar loads caused by the sides way water pressure on the lee ward bow.
As an "A" class these loads would have been minimalised by having a "high" bridle and only one very high aspect mainsail creating only a minor amount of the loads that you are now experiencing, but what you have done when converting to an F16 is to dramatically increase the collective "inward and upwards" loadings on the inside of the hulls forward of the front beam. The front beam/bulk head act as the privit point for those forces and just the "skin" of the hulls with out any structural disipation of those loads will result in this type of failure.
The sub floor does not have to be of any great strength or weight, as it "assumes" much more strength than its own construction would sugggest by its engineering in place.
I know its a set back BUT the man who never made a mistake never made anything, and this problem is relatively easily surrmountable