Re: Marine plywood vs regular plywood question
Marine plywood has a layer of mohagany on the surface, is usually thicker, has a smoother surface, and a different glue/bonding agent. It is usually heavier 5/8" vs 5/8". Wood is porous and will need to be sealed completely or will rot from the inside out, marine plywood or not, I'm in the process and am staining it, varathaning and recovering. In 20 years it might rot out again and need rebuilt, but by that time, everything else will need rebuilt too.
Good luck.
Not that I'm trying to rip into anyone here, but I felt like I needed to post a correction or two.
First, Marine ply can be any species of wood, not just mahogany. Usually it's made from a good grade of one of the more rot resistant species, like mahogany, okoume, meranti, or others. Note that the rot resistance of marine ply comes from the species of wood it's made from, not from any special "anti-rot" treatment.
It's usually somewhat lighter than a corresponding piece of plywood made from pine, fir, etc but that varies by species. Some are heavier than others.
Mostly marine grade ply is specified as marine grade by the manufacturer.. that means it's sanded both sides, has no voids, and meets a number of other standards specified by Lloyds of London for boat construction (not kidding). Real marine ply can be used to build boats that also can get Lloyds certified.
The glue used for marine ply is usually very similar if not identical to exterior plywood glue... a phenolic resin that's cured under pressure and heat, and has some similarities to epoxy.
Marine ply tends to not warp in any dimension when compared to exterior grade for a number of reasons, most of them related to the number and orientation of plies. Being a composite wood product, plywood's strength is directly related to the number of layers in it and the orientation of fibers in the layers. Marine ply has more, sometimes twice as many plies, as the same thickness of exterior grade.
Last, the marine ply has equal thickness plies. Cheaper plywood sometimes uses a "clear" or knot free veneer for the outside ply for looks. This is acceptable for mechanically fastened wood, but when you glue to the surface with epoxy or other resin the thinner veneer layer means that joint is more likely to tear free of the rest of the wood.
Obviously marine ply is a good, high quality choice for boat building, and if the marine architect for your boat has taken its strengths into account and specified its use, your boat can be lighter and stronger than a boat using exterior grade or cheaper.
Most mass-produced boats with plywood structure use exterior grade, and due to the age of most of the boats repaired by folks here, the original plywood used was not very high quality, so generally replacement with exterior grade is not only acceptable but improves the boat over its factory new condition.
Marine ply is always a good choice for materials if money is no object.. but then, there aren't too many people rebuilding old boats here for whom money is no object, right?
If you're going to look at using a given type of plywood, here's what to look for:
* Exterior grade glue (may not be exterior grade plywood - that's a trade organization standard.. but it must have waterproof glue). Do a google search on "wood boil test" to find out how to check a piece of ply.
* Lots of plies - the more layers, the stronger the wood
* Minimal knots - each knot forms a weak spot because the fibers in the wood go "around" the center of the knot. Usually there's also a bubble of some kind in the knot which can permit water to collect. If it freezes, the water can delaminate the wood and whatever is around it.
* No voids if possible - related to knots.. gaps in one of the veneers can form a one veneer thick hole in the wood, as seen from the edge. This is weak and provides a spot for water to gather.
* Full thickness surface plies - as mentioned above, a paper thin veneer looks good, but it isn't strong to glue to
My personal favorite for boat work is Arauco ply, which is an exterior grade plantation grown plywood from South America. It's about $25 a sheet for 1/2" and has almost no voids. It's also AC or AB sanded.
Erik
