Nobody didn't use wood in many ways in 80's boats. Balsa core, stringers, bulkheads, stiffeners, transoms... and unless your just looking at a foam filled boat (whaler etc) your not going to find many with any floatation foam. It takes alot of foam to float a big boat.
Somewhere down the line BIA certification required flotation material sufficient to float the boat upright. I remember when that was incorporated because bass boats in particular....like the Ranger I was running, all of a sudden lost a lot of storage space near the transom converted to foam to support the 200 hp engine, 3ea 27 size batteries, a 30 gallon livewell, 30 gallons of gas and all the rest of things that added weight. That was in the 1989 time frame as that was the model year of that particular boat.
Back in the day, Forrest Wood, owner of Ranger Boats had a commercial where he cut huge blocks of the boat away, sides, hull, interior and all, on one of his bass boats to show you how much foam he puts in his boats and even with chunks cut out of the sides and all it floated high in the water.
The commercial was informative, but with the sides cut away, you couldn't fill the boat with water which would have made a difference in it wanting to flip, sink or not. The only way something will float is if it's volume vs weight is lighter than the same volume of water that it displaces. The higher the volume vs weight, the higher it will sit in the water....aka a cork on a fishing line.
If equal it will remain suspended (jerk baits that were popular a dozen years or so ago) and if heavier it will sink (your fishing sinker). That's how you determine the weight of large objects. Put them in a tub rim full of water, and measure the weight of the water that comes out. The purpose of the foam is to substitute foam for the water that would otherwise occupy the interior of the boat and tilt the equation in favor of floating.
Course the definition of floating is not necessarily having the freeboard you had prior to the incident. The gunwale could be even with the surface of the body of water and still be considered floating.
I think I remembered all that correctly. Been awhile!
My 2c,
Mark