Is it plausible that a 1,500 lb boat with over 500 lbs of ballast would have flotation foam? That never occurred to me, but I'm coming from a 9,000 lb boat where positive flotation was not in the design brief.
I do go on the Montgomery Trailer Sailers forum, but it is pretty dead. I posted to the Montgomery Owner's List Serve but got no response. Maybe they figured only an idiot would not know that was flotation foam.
By the way, I lived in central New Jersey in my youth and have actually sailed on Lake Nockamixon.
Yes. It doesn't take all that much foam. There are lots of factors to consider, especially the density of the boat's construction material, i.e., how much heavier is it than water. To keep the boat afloat you only need to support difference between the weight of the construction materials and the weight of an equal volume of water.
Here's an example. Aluminum weighs about 169 pounds per cubic foot, water weighs about 64 pounds per cubic foot. If we take that aluminum block and make a boat, it will float because when you consider the volume of the boat (beam x length x depth) the actual weight per cubic of the boat is much less than the same volume of water. A 10x6x2 row boat would have a volume of 120 cubic feet and only weigh 169 pounds, whereas that same volume of water would weigh about 7,680 lbs. The boat floats because it is lighter than the water it displaces.
If we fill the boat with water and sink it, the boat no longer displaces 120 cubic feet of water, it only displaces 1 cubic foot of water, the original volume of aluminum that the boat was built from. To make the aluminum float, we only have to add enough floatation to get the weight per cubic foot of the boat below 64 pounds. If we put in 2 cubic feet of foam weighing 4 pounds each we now have a boat with 3 cubic feet of stuff that weighs in at 177 lbs while 3 cubic feet of water weighs 192 lbs, so the boat will float at its gunwales because is it lighter than the amount of water that it displaces.
In the real world on a boat, lots of other factors are at play. Wood coring serves a floatation function as well as a structural one,reducing the volume of floatation that is necessary. Location of the foam also plays a role. Flotation placed higher in the hull has to support less weight than floatation at the bottom of the hull, so you need less of it.
A Flying Scot at about 950 lbs will float at its gunwales with 2 foam billets of about 6"x8"x 8' tucked under the deck at the rails. That's a little over 5 cubic feet of foam. Assuming the construction of your boat about the same, a 1500 lb boat would require about half again as much or 7.5-8' cf of foam, an amount that can easily be put in a boat.
Not too confusing I hope.
