Just curious JG, how thin can the granit slab be to safely use over a plywood counter on a boat? Granit is very cheap in Brazil and I'm getting ready to replace my formica counters. I just don't want to add a ton of weight though.
I'm not sure, usually we didn't use it over plywood sheet though. Just mounted on the upper edges of the cheap wood the cabinets were made out of. Interestingly, the fragile granite I was talking about the back was usually fiberglassed for strength. Regular granite didn't need that. It was about a half inch thick as I recall, maybe a bit less in thin sections(such as in front of a sink) a metal rod was placed into a groove cut with an angle grinder, and epoxied in place. This was more so the sheet could be carried up stairs etc without breaking, than for strength after install. When the middle might only have a couple inches wide either side, and need to handle two guys hoisting and carrying it by hand things could get a little dicey.
To do the edges, we'd make a cut about an inch in from the edge, flip the cut off piece underneath and epoxy it in place, then grind the edge smooth and polish it. This gave you the thick edge feeling like a full thickness, a smooth shiny bottom edge, even nicer than a full thickness slab and some extra strength too.
Perhaps you could do the same(with the fiberglass on the back, and the laminated edge, skipping the metal rod if not needed.)
I was always amazed at the amount of abuse these slabs took being carried around.
The only thing more durable was the quartz contertops(basically stone+acrylic. They didn't need the re-finishing like granite, and we even dropped a slab without damage once.
Ours were usually a single layer of glass and resin, but I did a little googling, and came across this:
Lightweight Stone for Yachts Elevator Floors & Walls,Transparent Marble,Granite,Onyx
GRAMA BLEND LUMO
4mm of stone and 5mm of GRP backing, might be even better/lighter!
Looks like it's rated for countertops and floors, might be easier than making it yourself, though if it's economical to find granite there, I'd think going with the thinnest avail over 3/8" and then having the back done in fiberglass up to a couple inches off the edge, with the edge doubled over for visual/physical appeal would do the job quite well, and possibly be overkill :-)