floppy phantom cure

Cut a few access holes and pour in some 2 pound foam. Let it ooze out of the access holes, saw the ooze out flush and 'glass patch the holes. You will not pick up as much weight as wooden stringers, the hull will become unsinkable and she'll stiffen up considerably. Lastly, it's important you have sufficient access holes or the hull could bulge in places. 3" holes with a hole saw and save the cut out portions to use as patches, when it's time to seal her back up.

 

With all due respect to PAR, before you go pouring in foam, try to make a calculation of the weight of the foam you would use. Most times you will be sadly surprised at the weight gain.
Pour in place foam typically has interconnected bubbles - If you break the outside skin anywhere then any water that comes in contact with the foam will just be sucked in. The weight gain is enormous.
If you don't leave enough ventilation holes in the upper deck (with a hole saw) you will develop pressure in the hull distorting it in an uncontrolled manner. You have already seen how easy the bottom is to deflect - are you sure it will still be in the same place when you finish the foam cure?

Fill it with foam can be done well. If you do it, make sure you seal the cut surfaces well, epoxy, etc., as PAR said.

If you don't get crazy with the stiffeners. I'll bet on a lighter repair with stiffeners and glass.

Good luck

 

I'd be willing to guess that this boat, if very similar to a Sunfish, would hold all of just a few cubic feet of foam, once all is said and done. This amounts to a 6 pack of beer in weight, which isn't enough to get worried about.

I do agree that foam can be heavy, if used in bulk, but in this case, there really isn't a lot of room between the liner and hull shell.

 

What is your estimate on stiffeners?
1" square cedar x 10' x 2 each, +glass and epoxy?
I bet it is right at 1#.

"which isn't enough to get worried about" especially with the potential weight gain cause by water absorption - which is not a guaranteed problem I agree.