Plywood Choices

[Tom Lathrop]

My comments on checking in meranti plywood was intended to be just a warning in that meranti has not been around in the plywood market very long and many have not ever seen or used it. It may be that it is unusual or common as in fir. Some lumber woods are prone to check and some are are not, whether soft or hard, from temperate or tropic sources.

One thing that sets most of the plywood that we use in small boats apart is that it is all rotary cut from green or wet logs. This operation is very rough on the wood as there is a high pressure shoe just ahead of the slicing knife which curls the veneer off the log much like a hand plane makes curled shavings. The result is that much longitudinal cracking takes place in the veneer as it curls off the log. Some woods survive the slicing operation much better than others and do not show significant cracks (checking) later on. Some like fir are more prone to checking and so we downgrade it for boatbuilding for that reason. Since we use the veneer flat and it was originally wrapped around the tree, that also has some effect on it checking more than sawn timbers.

I simply am warning that meranti, in addition to being heavier, may also, like fir, need to be looked at more carefuly as a boat building plywood.

If others notice the same issue with meranti, we need to let it be known.

[Ken_Potts]

One other thing about checking. On the 1088 okume I'm using for my boat the outside veneers are about half the thickness of the inner ones. It is my thinking that whether rotary-cut or linear a thinner veneer will have less of a tendency to check than a thicker one.

I didn't know that 1088 okume was sliced linear - I thought that all ply these days came from rotary cuts. Thanks for the info.

[Pipefitter]

That's always good to know,Tom. I will be aware to look for it now anyways and atleast know why. Knowing that will also tell one how it may need to be addressed initially. I did buy the meranti with caution, knowing I may have to glass it at some point or live with the checking.I'm hoping for the best and I'm hoping I dont care by the time I get to finally use this dang thing.Worst case scenario is I may end up with something looking like a wood boat.

A used wood boat looks better than a new plastic one.

[Tom Lathrop]

I didn't know that 1088 okume was sliced linear - I thought that all ply these days came from rotary cuts. Thanks for the info.

The only plywood that is sliced flat (flitch cut) is furniture grade faces. All the rest is rotary cut and includes 1088 okoume. BS1088 ply should have even thickness plies. BS6566 sometimes has even thickness but often not. Face plies that are much thinner than cores should be avoided. If it's to be sheathed, then I guess it's acceptable. Quality, price and availability of marine plywood is a moving target.

[Pipefitter]

Has anyone noticed like a fine string webbing in between the plys? While cutting the meranti I would get thread looking fiber that wasn't wood and it wasn't the circular saw taking my jeans with it or anything. I noticed it throughout the stack of sheets. Is this maybe to allow for a slightly thicker glue lines resulting in thinner veneers to cheat a way to the same panel thickness with less wood? Was I imagining this?

[Ken_Potts]

Tom,

It was my thought (through reading more than experience) that the Lloyd's certified BS1088 okume was pretty good stuff. I thought that the 1/2 thickness face veneers would be less likely to check than a full thickness veneer because the stresses through the layer (from the inner to the outer surface) would be lower on a rotary cut (which I thought all ply was these days).

What is the danger of a 1/2 thickness face ply (what have I missed)? I just headed out into the dark and cold to check for sure and my 1/4 BS 1088 okume has 5 layers with the outside 2 being approx 1/2 thickness of the inner 3 and the 3/8 has 7 layers in the same layout (1/2 thickness face veneers).

The reason I ask is that I'm planning to leave the topsides unsheathed - I've already sheathed the bottom in 10oz cloth for abrasion resistance but I had planned to leave the sides, deck and interior unsheathed (just epoxy coated and painted).

[Barry Pyeatt]

The outer plys start the same thickness as the inner plys on most all plywood. After it goes thru the press and heat to set the glues, itis run thru a pair of parallel saws that cut it to final width and then it is trimmed to length as well. Those sides are in almost all caes exactly parallel and at 90 degree angles to each other. The sheets then go thru a huge drum sander top and bottom at the same time which evens out the surfaces. The material lhat is removed is the difference in ply thickness. On lesser grades of plywood (generally graded for construction), even more is removed to present a smooth face on both sides. That is why most construction grades are now about 1/64 under thickess. You don't notice it on Metric measurments but the constructions grades are under also.

On expensive hardwood veneers, the face plys are a little thinner in cutting than construction materials. That is pure and simple to get more flitches out of the log.

[Pipefitter]

Maybe thats why these marine plywoods bend oddly. In construction you read the face plies which was a deciding factor on which direction you place your plywood in relation to framing. Maybe if the face plies were thicker,this would make a difference. I bet this might be a good reason as to why marine plywoods can cause that starved horse look because it doesn't as efficiently bridge the gaps between frames,etc.

[Tom Lathrop]

Ray,

The main (maybe the only reasons) for rotary cutting plywood veneers are: It's much cheaper to produce, the veneers can make a full panel width without joins and there is less waste.

The higher quality marine panels start out with equal thickness veneers of the same specie as Barry says and final sanding takes some away from the outer layers. This is less noticeable on high quality stuff. In lower quality plywood, the manufacturer skimps by using low quality stuff for the interior plies and a thinner outer ply where they have to use higher quality to fool the buyer. The worse will have one thick inner layer and two very thin outer plies. A lot of the common 3 ply lauan is like this. If you make a knife score lightly across grain on one outer layer, the whole thing breakes easily over your knee.

The main dangers of the 1/2 thick outer layer is that it's easier to rupture. If you need to do a sanding repair, it's also easy to sand through the surface. Any one ever sand some teak or other expensive veneer?

It's worth saying again that the BS plywood standards are no longer supported by the Brits. The "Standard" still exists on paper but we have to depend on the honesty of the supplier whether it's followed or not. Some of them hedge a lot but still might put the stamp on there even though there has been no inspection.

I have never experienced checking on okoume and have no hesitation to leave it unsheathed unless there is a good reason for a sheath.

[wkisting]

A little off topic, but I'm wondering why I never hear anyone talk of sheathing in 3 oz. silk weave instead of just epoxy coating? I know a lot of kayak and canoe builders who use the silk weave for sheathing when structural strength is not an issue. It goes on almost as easily and quickly as the standard 3 coats of epoxy most people apply when using epoxy alone, but yields noticeably better abrasion resistance (still not a lot, but better than epoxy alone, by far) for a fairly negligible increase in weight. Ensures you have a consistent-thickness barrier of protection, and one which will take far longer to wear away--if at all.

Anyway, just curious... in the ply boat-building world, I usually hear people talking about either sheathing in heavier glasses 6 oz. - 10 oz. or not sheathing at all. Obviously, the heavier glasses may be necessary below the waterline and for high-abrasion areas, but those folks who forego 'glass in some areas to save weight may still want to consider the silk weave option.

FWIW, I've heard of Okoume & Meranti checking several times, but in every case, it seems to be when the wood is damaged and not repaired promptly, or not allowed to dry out thoroughly before the repair. Again, never seen it myself, just heard it over the years while perusing forums like The WoodenBoat Forum. If you search on Okoume on that forum in particular (I'm sure I've seen the issue mentioned at least twice there in the past few years), you might be able to turn up the posts describing the problem, though I cringe to think just how many posts a search on "Okoume" would turn up! In any case, didn't mean to suggest in my previous post that Okoume and Meranti are necessarily prone to checking. If sealed properly, they aren't. And even unsealed, it sounds like it takes a much longer time than Fir to check... folks on the Internet have done the usual dishwasher & weather tests and found that it takes (sometimes) years of abuse inside a dishwasher before a chunk of Okoume will begin to check and/or delaminate. Other folks insist it never will. I have a dishwasher, but far too many dishes to be wasting the space on wood scraps.

[Pipefitter]

I used 4oz. satin weave on the DF ply on the sheer deck on my boat to stop the checking. You are right about it not taking much time and it holds more of the epoxy on the initial wetting out to maybe even save a coat or 2. I would treat any DF plywood this way. If the meranti checks and since it was primed with epoxy resin,I will sand the paint off to the epoxy and glass it in this way. That is what is called a veil when the glass isn't needed for structure. I think it's a great system.