Mugrace72: This isn't a sailcraft of canada boat...I used to have one of those, and this is quite different. But your conjecture about the weird bolts could be correct. At any rate, you're certainly right that it doesn't really matter. I'm never going to find a tap this size, or a replacement bolt!
Erice: These bolts are definitely a little odd. My micrometer says the head is 14.98mm, which is more than 9/16. The shaft is 9.49mm or .3735"...so it could be 9.5mm metric, or 3/8 standard. The thread spacing seems to be 1.61mm or .637". Yeah, I know. I don't believe those numbers either. Maybe they're Whitworth (!).
Catman: the attachment for the REAR beam is all done through the inspection port. Two short HORIZONTAL bolts that go through a vertical stainless plate attached to the rear beam, and one long vertical bolt though the beam-end and saddle into the hull. I just looked under the saddle, and there's SOMETHING that's about 1" wide by 2" long by 1/2" thick glassed to the bottom of the saddle, but it looks more like a piece of fiberglass than like metal. There ARE washers between the nuts and this reinforcement, but perhaps I'll stick in a piece of 1/4" aluminum to help better distribute the load.
Up forward there are two threaded rods sticking up from the deck on the inner edge of the hull, and a stainless strap that goes over them held down by a couple of nuts to hold the beam on. On the outer end of the beam, there's the bolt I've been talking about...which was intended to screw into this metal backing, whatever it might be.
Anyhow, now that I've had a better look into that bolt-hole, it appears that up forward there's about 3/8" of saddle, followed by about 1/2" of metal, whose threads are almost completely stripped. I guess I'm glad I never looked closely before. I would have worried.
It LOOKS as if the metal's stainless. I can't seem to scratch it easily with a screwdriver, the way I could if it were aluminum.
There's also lots of space below the metal, so I think I can safely drill and tap it. Maybe I'll go with a 3/8-16 helicoil; maybe just with a 7/16 bolt.
Thanks for all the thoughts and suggestions, which got me to look more closely at what I've got. With luck, there'll be one more Tornado on the water this summer!
-John
