Marine ply has water resistant glue and no voids between or in the ply, or no knot holes if you like.
Standard ply may not have waterproof glue and may have voids. However you can use a form of construction ply in some areas, it has water resistant glue and a few voids but costs about a third less than marine ply.
I've always understood that there's three grades of Plywood. Marine, WBP and Interior.
Marine is the best grade. Good grade of veneers and each filler layer with tight grain and all stuck together with waterproof glue. The right stuff for exterior of boats.
WBP. Water Boil Point or something similar. Sometimes also called Exterior ply. Good grade of veneer available, though also a rougher (cheaper) grade can be used for areas that are hidden or not important. Filler layers not as high quality. Glue is water resistant. Ok for interiors of boats or areas that are not subject to constant or frequent wetting. Cheap structural exterior ply is the sort of stuff you could build a shed from, not an excellent grain on the faces, but OK with a couple of coasts of creosote.
Interior ply. Again can be high grade veneeers for appearance on outside or cheap rough finish if not going to be seen. The filler layers are generally softer and wider grains and the glue is generally water based, so totally uselss for boat use, even in the cabin where some dampness will inevitably occur.
Of course it's always best to only use marine ply if you can afford it, but it's horrendously expensive and in a lot of cases overkill and not necessary. WBP is good enough for all interior work and I'd have no hesitation in using it for that application, but anything outside then it's marine ply. There's no place on a boat for interior ply.
No matter which ply you use it's always a good idea to varnish any hidden edges before fitting.
If you've found a ply that you fancy using but not sure of it's grade (though it should be stamped along the edges of the 8x4 sheet), get a small offcut and just leave it outside in the garden or in a bowl of water for a few days untreated and see what happens to it
Marine ply is good quality, durable hardwood veneers glued together with a waterproof glue. It should be manufactred to BS 1088 and is termed "water and boil proof," this presumably reflecting the test methods.
Exterior ply is glued in exactly the same way but is composed from lesser quality veneers and may not be so durable in the rotting sense. I don't know about voids and knots.
Interior ply is poor quality veneers bonded with a non-waterproof material.
I would be careful of 9mm marine ply at £35 per sheet - there are different grades (even though they are marked up as to BS). I've used some of the cheaper boards and they seemed little better than exterior in terms of voids and filler. I think they're fine if you epoxy all the edges but not otherwise. If you want the best , you could try Robbins Timber (Bristol) or K J Howell in Poole, who both have quite a good range. Teak faced ply is in the £90 region though...
Yes but he's only using it in the interior and at £35 for an 8x4, that's going to be good enough quality for that. As you say top grade marine is around £90 and for hidden interior joinery, well if you're building an Oyster - maybe, but otherwise perhaps overkill.
'Nuther problem with wpb and exterior grade plys is that the internal laminates are normally softwood veneers. Also, quite often there is one single thick interior veneer, with 2 very thin outer veneers of hard(ish) wood. BS1088 SHOULD - but doesnt always - have veneers of equal thickness all the way through, and all should be of hardwood, with voids not more than a certain width where the veneers join upo internally.
Result? a) wbp, and external plys are much weaker for a given thickness, so will bend or fail quite quickly, and b) if water gets to the interior laminate(s) being soft, they will rot away very quickly. I recently came across a laminated transom built up of 3 layers of 9mm wpb. All the 3 internal veneers had failed, and it was possible to poke a spike right through the transom - top to bottom between the veneers! It looked pefectly OK from outside, but far from being a healthy 27mm of solid timber and glue, all that was left was the thin outside veneers, largely unsupported.
However GOOD quality wpb should normally not present a problem for interior joinery, but check all round the edges of each sheet for possible quite large voids where internal veneers have been badly butt joined. But seal those edges!
Roger, I'm in the process of "doing up" some of our club launches and sort the same info that you require re plywood. Luckily we have a member in the trade and this is what he said, Marine ply is bonded with waterproof glue and has ALL laminates made of hardwood, Exterior, has waterproof glues but MAY contain some softwood laminates although not necessarily. He suggested (and we are doing) using Marine ply because it will last much longer than exterior,but we are using GRP resin to bond/seal the edges first before we seal them down with mastic or glassing them in position, this will ensure a watertight edge bond. He did say that if you treat exterior ply very well it could be used for decks etc providing water drained off well. Hope this helps?
Roger I think Coliholic was about spot on with his suggestions......all hardwoods are not always durable and indeed some softwoods are so the fact that one ply may or may not be formed of hardwood laminates , that does not prove its durability. I think the definition of a hard and a soft wood has something to do with its bark and or if its evergreen or not. Anyway the major difference I think between 1088 and good quality Ex wbp is in the quality of the laminations and the number and size of allowable faults and joints. 1088 being the better as a rule. Also 1088 lays down a max and a min size for lamination thickness, think its about 1.2 to 3.5 mm and it lays down a requirement or total number of laminations depending on overall thickness. It is possible to get Ex wbp of a quality thats as good as some and better then the worst of 1088 . If you really want the best then go for that which is certified like Brunzeel etc and this is made of very durable woods and is of the very best quality and of course it comes at the very best of prices....if its for outside or is to be wet then this is the kind you want, its what is really meant by the term Marine ply and if its inside then you will probably be ok with good quality Exterior at between 10 and 24 pound per sheet.