Floor of old Runaboat

[Wave1960]

I have a 1960 Runabout with very shallow hull. There is little space between floor and hull. Maybe 1" at deepest points. Would this space be hollow? Foam filled?

 

[Woodonglass]

Back in the day very few hulls were foam filled. You could use some of the blue or pink slab foam sold at the big box stores to lay under the deck.

 

[Wave1960]

So the hull may very well have originally been hollow?

 

[Ned L]

Yes, most likely just empty under there.
As WOG said you can put foam under there, but unless you can find room to add enough cubic feet of foam to keep the boat at the surface there isn't much point in it.
( The math isn't difficult to get an idea of how much you would need.)