Hi Mark, I'm no expert on these old boats, but I have done a bunch of work on them, I'm not a perfectionist, and I tend to do things a little different than some may, but I do try to build one safe, decent looking, and will last at Least as long as I will. Oh ... and I like em to go Fast !
1 - Do you drive 1/4" dowels in the holes left from the foam cans before laying down the main mat, or what?
no .. but I have driven part of a pencil in a couple that were still expanding when I was ready to lay mat, and sometimes I just put a pice of masking tape over the holes, when you leave the cans in the hole overnite the hole seals itself, then the resin from the mat covers them up.
2 - How many layers of mat do you put down on the floor?
First, I will fill any gaps or voids on the edges with filler, then I will lay the woven roven mat on the seams, then one lay-up over the whole floor, overlapping the seams, then sand, or grind the entire surface to get rid of any hairs or bumps before the carpet is layed.
3 - I've read a lot about leaving drain passages under the floor along the main stringer. Any thoughts pro or con?
I choose not to put any drains under the floor when I am going to inject the foam, if I'm just going to lay foam logs or foam pieces under there, then I will put some drain holes. Personally, I'd rather fill the space up with foam, seal it up good so water has no way to get under there. Most foams, other than the Styrene, or Polystyrene closed cell type, will absorb water when in constant contact with it, even the foams most boat builders used for years for flotation, they will float, but in constant contact with water they will absorb it. The Great Stuff foam I use may eventually absorb water also, if it's subjected to it continually from a break in it, but when it remains sealed, by it's own expanded shell, it will not let water in, the stuff will actually expand and set up under water.