For building out internal parts in a boat like rod trays and a rowers bench, is it necessary to use marine grade plywood or would it be ok to use nice 1/4 inch plywood that has been fibergalssed and varnished.

Don Tyson recently built a boat with construction ywood and it's a fine boat.

Marine has nothing to do with glue or water resistace. Marine plywood is just better plywood--with no internal voids, so it's easier to bend without breaking.

Learn something new every day! Thank you Sandy

Sandy, I thought that marine-grade plywood used a more water-resistant glue, however, almighty Google suggested that the British Standard 1088 does use waterproof glue.

I quote from "Boat Renovation People" website “BS1088 is the British Standard specification for marine plywood that applies to plywood produced with untreated tropical hardwood veneers that have a set level of resistance to fungal attack. The plies are bonded with Weather Boil-Proof (WBP) glue.”

Obviously, information gathered from the web may or may not be true. Do your own research to confirm my findings. As you know, the issue with plywood is one of water intrusion into the plies. If the plywood is completely sealed and kept sealed then water will not enter and will not cause delamination and eventual destruction.

As you and others have used foam board of the correct construction can also be used though often it requires a fiberglass lamination or three to provide sufficient structural strength. You take your chances and pay the cost one way or another.

RickN