Anyone who has one of these boats know that the bolts are not only metric, they're some weird metric size -- the bolt wants a 15mm (!) socket, which might make it something like M9. Anyhow, it's amazingly close to a 3/8 UNC, so I figure that perhaps I'll replace it with that along the way. (I've already replaced all the rear-beam bolts with 3/8 UNC, since they and their nuts are all completely removable.)
Having owned and sold a number of Sailcraft Tornados, although all earlier than yours, I can say they all used
(3/8-24) bolts for the beams. If yours has a weird metric size, perhaps someone used them and forced them into the holes.
Having said that, it doesn't really matter, does it? You might be successful with a Heli-coil insert.
Good luck with your drilling. I'm pretty sure it is a plate that is imbedded in the GRP, so you have a little wiggle room.
Should have said 3/8-16 UNC (COARSE THREAD) 
