My Avenger also has minimal foam under the deck, however, it has a chunk under the bow, and there is room for more under the splashwell. She does float when swamped, fortunately. Foam also is used to support the hull, so less stringers and frames are needed. Hopefully your hull didn't count on that.
What was the original max HP on that hull before you rebuilt her? Was her max power rating really capable of pushing her to 80MPH. I am not an expert, but doubt that.
She looks like a typical runabout hull with a fairly flat bottom in the rear. That flat bottom hull and her light weight and narrow beam give the good fuel economy you are seeing. In boats it costs fuel to carry weight.
Good luck with her. You cite yourself as an experienced driver. I would recommend you do not let any unexperienced drivers take the wheel as "fast boats get into trouble fast".
Don't exactly know the factory hp rating on it. I pulled it out of a farm field with no data plate on it. Not a good sign I know, but it was completely mush aside from the fiberglass and the guy said he had it for 20 years or so. I belive it had an old merc 650 on it originally tho. I'd figure it wasn't rated for more than 75 hp because the original transom and stringers would have not been to great, the transom wasn't even tabed in to the sides of the hull.
When I say they have been proven in to the 80s I mean after someone has gone in and done a bunch of custom work like I have. She would come apart for sure with the factory stuff, even in good shape. I just meant to say the basic hull design has been proven to not have any outstanding bad characteristics at speed like some hulls do. I can think of old sidewinder for sure, I've had some sketchy rides in those.
As far as strength lost to foam goes, well it really doesn't matter anymore. All the structure is of my own design and nothing like the original. Center stringer is a 2 inch laminate of marine ply, transom is the same. Both sides of the floor to hull are a 4x4 box of half inch marine ply, tabbed to the hull then the floor is tabbed in to them. And the hull is 1/2 inch balsa core from transom to near the dash. I'd hang a 175 on it as far as strength goes, but that would be too much for the style of boat.
As far as me saying I'm experienced driving boats, I mean I've always been around them and owned them. I've built 3 point hydro that are quicker than this. But I'm no professional or someone who could go out and enter in a race. I just mean ive had enough time driving them that I know when something bads about to happen. And of course I'm not going to let people drive it, I can tell you that it will act quite badly on turns at speed. Not a boat you can throw around, if you need to turn your probably not doing it over 30 in any stable way.
Anyway I've just been totally amazed by the power, weight, torqe, sound and most of all fuel economy on the old merc. I've been I die hard omc guy always. This blows any v4 I've driven out of the water. Definitely going to try more merc stuff. I have a nice 90 hp tower out back as parts, but it just might get a little 3 point hydro built for it before I savage it for parts.
