I didn’t expect to find foam in there. The other boats from this vintage didn’t seem to have it from photos I’ve seen on this site but somebody will correct me if I’m wrong about that.
If it’s a 77 it should have a HIN on the starboard side of the transom. The last two digits of the number will be the date of manufacture. I’m perplexed about the foam as well. It does appear from the photos that that floor is original. I didn’t think Hourston started adding foam until the federal buoyancy requirements came into effect sometime in the 80’s. My 77 hull has a HIN. This was not a requirement in Canada until 1981, but in the States it was, so I believe HG originally started doing it for the American market. I mention this because your boat might not be a 77?
My second thought is at some point it’s had a repair and they filled her withbubbles, or someone else did?
The black stuff you are seeing is black mold.
The real question is…… is the wood mush?
The floor looks pooched?
I soaped and scrubbed and bleached the ever living crap out of my hull (mind you it was after the gut) and was surprised afterwards how good it looked.
Obviously, in the pics the inboard stringers have been extracted. If you want after photos they are on the forum somewhere.
Also as I’ve mentioned you can see the two 3” holesaw holes in the forward step down floor board which was full of water from a leaky bow hatch. There never was any drains in the structure and I siphoned out 10 gallons of water with a shop vac.
The other thing that you should keep in mind is that for less than 5k in materials you can have a boat that’ll be good for maybe a couple more decades.
I had some fun building my boat. I also learned a lot.
Also know that you can easily walk away and recoup your cost. It’s a very popular hull even as a project.