Plywood specification questions | Boat Design Net

I’d like comments on the plywood selection I’ll have to make.

The most common advice regarding plywood selection is to play it safe and buy marine plywood confirming to BS , at least for any part of the hull that will be below the waterline. As I understand it, BS specifies the veneers quite well, but doesn't actually specify the material strength. Most important with BS is that there shall be almost no inner voids (small splits allowed) and that the glue shall be WBP (water boil proof). Does anyone have figures on the materiel properties for BS marine plywood, preferably hardwood?

There are no manufacturers in Sweden making BS plywood, and I haven’t been able to find any Finnish, Norwegian or Danish manufacturers on the internet either. (I want the factory within yelling distance.) Wood tradition is strong in Sweden and a major export business, as in Finland and Norway, so this came as a surprise.

I can find construction plywood “P30” made in Sweden that is far more thoroughly specified than any BS plywood I can find on the internet. The construction plywood is made from Scandinavian spruce and sometimes pine. Glue is always WBP. The construction plywood is used for floor (underlayment), structural walls, structural beams, simple walls etc. The construction plywood is manufactured under quality control according to norms for more or less anything, and “P30” specifies the materiel properties. The manufacturer provides load tables for dimensioning.

Many types of construction plywood are specified with C+/C surfaces, meaning that any voids in the surface veneers will have been repaired. I contacted an engineer at one of the major plywood producers to ask about the inner veneers. There may be inner voids with a diameter of max 40mm plus some splits. The thickness of the veneers is within the limits of BS .

From my point of view the P30 construction plywood is a safer choice than BS because of the guaranteed strength and the quality control to ensure it. The possiblity to discuss directly with the manufacturer's experts is also a factor. I haven’t been able to find any figures on the strength of BS hardwood plywood, but would expect that I will have to go up in thickness to match the strength using spruce construction plywood. The density of spruce plywood is lower than for hardwood plywood, so I expect the difference in weight to be acceptable. C+/C construction P30 plywood is cheap compared to BS hardwood plywood.

I will seal the ends and cover both surfaces with epoxy + glass and paint afterwards. Will the larger voids and splits in P30 construction plywood compared to BS plywood be any problem then? I don’t care about re-sale value and the opinions of insurance companies. The boat is for me. It will be 10 - 12m long and max 3 tons.

Erik

Erik,

The P-30 standard is really a certification standard requiring certain structual requirements of the finished sheet, having nothing to do with the actual piece of wood you have in front of you. So yes this means that the engineering numbers are definate, since any piece of ply that is stamped meets at least those minimum numbers. This stuff is great, but it is designed and intended for structual members for buildings, not for Marine use, so to that extent while it may be fine, it may also fail because you are asking it to do something it wasn't intended for. At least in the US there is not species requirement for P-30 type plywoods, where for Marine Grade it all the layers must be from a list of species known to be resistant to mold, rot, and water (note the US uses a different classification scheme than the EU where BS and P-30 come from)

BS- tells you a lot more about the manufacturing and quality control required than specify engineering details, so while it may be harder to find engineering data like tensil strength you know what you are getting. The question is what these requirements are compared to your needs...

BS- basically requires:

WBP Glue Line -- BS plywood must use an adhesive, which has been proven to be highly resistant to weather, micro-organisms, cold and boiling water, steam and dry heat. The product's bonding must pass a series of British Standard tests.

Face Veneers -- These must present a solid surface that is free from open defects. Face veneers must be free of knots other than "sound pin" knots, of which there shall be no more than six(6) in any area of one(1) square foot, and there can be no more than an average of two(2) such knots per square foot area over the entire surface of the plywood sheet. The veneers must be reasonably free from irregular grain. The use of edge joints is limited, and end joints are not allowed.

Core Veneers -- Core veneers have the same basic requirements as face veneers, except that small splits are allowed, and there is no limit on the number of pin knots or edge joints. However, end joints are not permitted.

Limits of Manufacturing Defects -- Defective bonds, pleats and overlaps, and gaps in faces are not permitted. Occasional gaps may be repaired using veneer inserts bonded with the proper adhesive.

Moisture Content -- BS plywood must have a moisture content between 6% and 14% when it leaves the factory.

Finishing -- Boards will be sanded on both sides equally.

Distilled what this means is that while specific engineering data may be difficul to find, BS- is specifically designed for the marine environments ( the anti fungal properties specifically), and because it is substantially free of defects when it is bent or shaped there are no/fewer week spots which can substantially degrade local strength.

The reality is that is the best quality plywood you can find for Marine applications, but not necessarily for all applications. The P-30 standard is great if you need very specific and predictable characteristics, while building load bearing building tresses but isn't realy designed to act as the skin of a vessel.

It sucks, but when looking at products made to a specific standard like plywood you can almost always look at the price for a sheet and almost universally be correct in stating that the one with the highest price is the one made to the tougher standard and is of higher quality (with the exception of plywood made to non-standard specifications like fireproof plywood).


One thing to be careful about is that Okoume plywood simply means that that is the venier fact of it, it has nothing to do with the standard the sheet meets. This is rarely a problem, but some suppliers carry exterior or interior grade Okoume plywood that is a far cry from the Marine Grade Okoume most people think they are getting.

I prefer local wood for mostly sentimental reasons, and rationalize it by believing I’m making a smaller footprint on the environment. What I would have liked is a standard that ensures that plywood from our local wood is suitable for a boat hull, provided the plywood is sealed with epoxy. It would have been great if the plywood manufacturers had sorted out the dos and don’ts when using (Scandinavian) spruce, pine or birch plywood for hulls. I guess that the amateur boat builders who want to use local plywood don’t constitute a large enough market to be worth addressing.

Our local spruce and pine are reasonably rot-resistant in our climate if used correctly. Boats and ships have been built from Norwegian, Finnish and Swedish wood for more than a thousand years with good results. Pine is the traditional material for hulls. Properly maintained those boats can last a century, but the usual life expectancy is 30 – 50 years. Pine doesn’t have a problem with rot below the waterline where it is soaked. To my experience (which is limited), the problems occur above the waterline when there is a certain combination of moist and air. I only have personal experience of one small boat made from spruce, and I don’t remember any special problems with that boat. Plywood made from pine or spruce should be possible to use for hulls, if the plywood is correctly manufactured and correctly used.

I’ve understood that construction plywood is allowed to have some inner voids because it only has a very marginal effect on the strength. Typically, construction plywood is intended for flat surfaces but I want to use it for curved surfaces, so my load case is different. Is there anyone with experience on construction plywood for hulls? I’ve used “WBP glued furniture grade” pine plywood myself in a small experimental boat. So far there are no problems with the plywood, but one year is too short to tell.

Stumble,
I agree that it’s generally true that we get what we pay for. When buying BS plywood we get a beautiful tropical hardwood surface, more rot-resistant than spruce or pine. The log for the plywood has been transported from the tropics, instead of harvested from the woods on my doorstep. Every veneer is checked to be without voids. Of course such plywood is expensive.

Because I totally seal the plywood with glass and epoxy, and then paint it, the beauty of the surface is wasted on me and the rot-resistant qualities probably not needed. I ended up paying for qualities I didn’t need.

Erik

I used door skin ply, luan, nominally 1/8" or 3 mm, for some of my early boats, which were not intended to last. At first it was quite good but either I was lucky or the quality went down because the last few sheets were useless. Splits in the outer veneer were the least of the faults. There were large areas with no glue that resulted in bubbles when painted. There were often large voids in the center veneer that would cause it to collapse when bend. even the thickness was incorrect, more than 10% below nominal.

I would prefer to use locally made ply but the house grade ply I get here is also poor with knot holes in the outer veneers and voids in the inner veneers visible at the edge. I suppose for a house floor or roof that sort of thing is acceptable but useless in a boat. Even good lumber can be hard to find: I suspect the good stuff is only for export. I can get nice stuff in all the species needed for a good boat but it is a long trip and I would need to rent a truck or trailer to bring it back.

I wouldn't think of using anything but marine ply now. Of course, it depends on what quality ply is available in your area. I would use the kind of "house ply" I could get 30 years ago, judging from what I see in my house which is that old. And as noted before, the "Baltic Birch" ply I can get locally is lovely stuff: I think it comes from Russia, because the thicker sheets are made from 2 thinner sheets glued together, which I understand is a common Russian practice.

I got lucky and picked up a lot of water-damaged marine ply sheets, enough for several years of boat building at my rate of work - I am just an amateur, one or two boats a year. It is perfectly OK structurally and most of the sheets turned out to have just stains at the corners. So far I have a canoe and a small sailboat out of this batch and should get several more over the next couple of years.

Erik, have you looked at plywood made of European pine (furuplywood), it's also called in Swedish snickeriplywood, used for furniture and stuff. I believe the correct english name of this wood is Scotch pine. The construction plywood you are talking about is made of spruce (gran).

This pine plywood is more expensive than the construction grade, the double price, but it is still far cheaper than marine plywood. For a ½ inch sheet its around 50 US $, marine plywood anywhere I tried to find was three to four times as much, starting there and rising to ridiculous amounts for the fancy woods.

Well, what about the quality? You still have voids in this plywood, if you cut through a whole sheet you will find one, two or three voids, 5-10 mm in size. But i was surprized when I counted the number of plys. For a 3/4" sheet, 18 mm, there are 13 plys. For 7 mm, there are 5 plys. That's not far from the marine standard.

After some thinking and counting kronor, and searching in this forum and others, I decided to go for this plywood for my project, a 28' chine plywood sailboat. Except for the economic part, there is also a certain feeling building a boat with local woods. The rest of the boat is built with swedish pine and some elm. The boat will be glassed inside and out.

C-A,
I've used the pine plywood you're thinking of, from Ljungbergs, in a small boat and what little experience I have is only positive. I've also used ordinary construction plywood and the diffrence is like night and day. I think that the thin veneers are part of the reason why the furniture plywood is good and fo course it's WBP glued. I still would like specifications on the inner veneers; it's probably available from Ljungbergs and I'll ask them eventually.

Right now I'm trying to make sense of the data on the various types of plywood in the Handbook on Finnish plywood. It's freely available on the internet if you want it, just Google it. I think I have very successfully suppressed any knowledge of the plate strength calculations my colleges assure me I must have been taught at KTH so I'm struggling with the data. The difference between plywood made of similar wood but with different veneer thicknesses indicate that thin veneers is the key. If I manage to clear my mind of the accumulated fog I'll be back with a better founded opinion.

Erik