Mine is thru' bolted at a point above where the under deck hollow stops, tween the toob and the hollow bit if that makes any sense. It's done with a U - bolt each side (and a matching set up forward) as the boat was originally an FRC (to use the modern terminology) and so was equipped for slinging in a previous life. I have about 3/4" of thread poking out the bottom of the boat in eight places!
The transom attatchements are similarly done, but with the "U" of the U bolts on the outside to give anchor points for towing bridle / trailer tie- down. Scream if you want some pics.
I have a plan..... soon I will get it out on the water and get someone to point the camera at it while it's on the plane. I reckon the forward bolts will be well clear of the water, however I'm not so convinced about the rear ones. The idea is that if they are "in" then I'll replace the U- bolts with Pan head screws with the head on the outside as suggested above, but use eye nuts - not because I want to sling it, but becase they make remarkably good fuel tank tie -down points! As it was slung the backing plates are both enormous and thick (5mm) so they will be replaced with something a little more appropriate and streamlined (i.e thin!). Alternatively if I find they are clear of the water, then the whole lot will stay put! (tho' I might cut the threads down a little)
Alystra, does your frame look like it's been welded up from mini scaffold pole? It sounds like we may have similar attatchments. I also have a lump of copex stuffed up the bottom of the upright with the cabling in........