Plywood specification questions

Discussion in 'Wooden Boat Building and Restoration' started by erik818, Apr 8, 2010.

  1. erik818
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    erik818 Senior Member

    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 1088, at least for any part of the hull that will be below the waterline. As I understand it, BS 1088 specifies the veneers quite well, but doesn't actually specify the material strength. Most important with BS 1088 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 1088 marine plywood, preferably hardwood?

    There are no manufacturers in Sweden making BS 1088 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 1088 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 1088.

    From my point of view the P30 construction plywood is a safer choice than BS 1088 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 1088 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 1088 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 1088 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
     
  2. rasorinc
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    rasorinc Senior Member

    You can get BS 1088 in England. I would stick with this for Ocean, salt water use. It is the highest standard. Whatever you do don't buy ANY Chinese made plywood.
     
  3. hoytedow
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    hoytedow Wood Butcher

    Quite right.
     
  4. magwas
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    magwas Senior Member

    I would like to as why. I don't see anything in the BS 1088 standard which would make that more suitable to marine use than P30. Of course I can be wrong.
     
  5. Stumble
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    Stumble Senior Member

    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-1088 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-1088 basically requires:

    WBP Glue Line -- BS 1088 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 1088 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-1088 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 1088 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.
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    I think BS1088 with Lloyds certification is the highest marine plywood grade. BS1088 standard plywood is made with a variety of woods. Depends how much you are prepared to pay.

    My local hardware store has a nice hardwood ply (Baltic Birch), not BS1088 but very strong and nice to use. Usually any mention of Birch will result in disparagement of its rot-resistance, but wood only rots if it gets wet ... much the same can be said about delamination. If the ply is sound, thoroughly painted and well maintained, it will work.
     
  7. Autodafe
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    Autodafe Senior Member

    I find it curious that BS1088 has no strength specification.

    The local (Australian) standard AS2272 has minimum strength requirements (F14) as well as specifying glue type, veneer quality, timber species, voids etc. I you haven't already done it, perhaps you could get a copy of BS1088 and check if it also has reference to structural properties.

    I personally feel that if you can find out what wood is used for the veneers, and the glue bond type then P30 structural ply should be fine with epoxy encapsulation techniques.

    However, I would also say that any genuine BS1088 is unlikely to have significant structural deficiency, so is probably just as strong (for a given density) as the structural grade.
     
  8. erik818
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    erik818 Senior Member

    I finally found a Danish manufacturer of BS 1088 plywood. They make marine plywood from imported tropical wood. This company also dislayed more of the standard (BS 1088) than I had found before. Quote:

    "Scope
    This British Standard covers plywood for general purposes manufactured from tropical hardwoods......."

    This explains why I haven't been able to find any BS 1088 marine plywood made from Scandinavian wood. The standard doesn't allow it, no matter how good the plywood might be for the purpose.

    I don't have my own copy of BS 1088. It cost around GBP100 unless I download an illegal copy. This seems to be a typical price when purchasing a copy of a standard, and BS 1088 isn't the only standard that is relevant for boat building.

    The standard BS 1088 has been withdrawn for some reason, and it makes even less sense to spend money on a standard that isn't valid anymore.

    I would like to use a plywood made from local wood. Unfortunately BS 1088 isn't applicable.

    Here's a link to the English verison of a Finnish handbook on plywood. It contains some interesting material that will keep me occupied for a while.
    http://www.wisaplywood.com/upm/internet/wisaplywood.nsf/images/Handbook_EN.pdf/$FILE/Handbook_EN.pdf 

    Erik
     
  9. magwas
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    magwas Senior Member

    Thats it. I neither feel good about importing wood from rain forests.
     
  10. hoytedow
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    hoytedow Wood Butcher

    They can grow more.
     
  11. erik818
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    erik818 Senior Member

    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 1088 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
     
  12. ancient kayaker
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    ancient kayaker aka Terry Haines

    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.
     
    Last edited: Apr 14, 2010
  13. C-A
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    C-A New Member

    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.
     
  14. erik818
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    erik818 Senior Member

    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
     

  15. erik818
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    erik818 Senior Member

    I’ve compared two kinds of Finnish spruce/pine plywood with Finnish birch plywood. Data is from the Handbook of Finnish Plywood. I have compared data for characteristic strength, which means that 95% of the plywood is better than this. I have no data on BS 1088 marine plywood and have assumed that BS 1088 plywood of high quality is similar to Finnish birch plywood regarding strength. I’m also assuming that the plywood is covered with epoxy/glass so the soft surface of pine or spruce is not a problem. Rot is assumed not be a problem either because the epoxy stops air from getting to the wood.

    The glue is according to EN 314-2 class 3 exterior. This is the same as the older standard BS 6566: WBP. There are voids in the inner veneers but any voids in the surface veneers have been repaired. The birch plywood and the conifer (pine or spruce) plywood with thin veneers have a veneer thickness of 1.4 mm. The conifer plywood with thick veneers has veneer thickness approximately 3 mm. The conifer plywood with thick veneers has one stronger direction, given by the face grains. I’ve used the average of the two directions, at 18 mm thickness for the thick-veneered conifer plywood. The two other plywood types have equal strength in both direction.

    Density [kg/liter]:
    Birch plywood: 0.68
    Conifer plywood thin veneers: 0.52
    Conifer plywood thick veneers: 0.46

    Relative bending strength:
    Birch plywood: 1
    Conifer plywood thin veneers: 0.57
    Conifer plywood thick veneers: 0.40

    Relative modulus of bending elasticity:
    Birch plywood: 1
    Conifer plywood thin veneers: 0.75
    Conifer plywood thick veneers: 0.66


    According to examples the limiting factor for failure ought to be bending strength for typical boat load cases. Bending strength is proportional to thickness squared and the stiffness to thickness cubed. Because the relative difference is also larger for bending strength than for bending elasticity, up-dimensioning for equal strength will increase the stiffness.

    For equal bending strength the following scantlings could be used:
    - Birch plywood 15 mm, resulting in a surface weight of 10.2 kg/sqm.
    - Conifer plywood, thin veneers, 15 mm /(0.57)^0.5= 19.9 mm, resulting in a surface weight of 10.3 kg/sqm.
    - Conifer plywood, thick veneers, 15 mm /(0.40)^0.5= 23.7 mm, resulting in a surface weight of 10.9 kg/sqm.

    Personally I'm leaning towards the conifer plywood with thin veneers (sold as “furniture pine plywood” in Sweden). What I’ve bought has so far has been of good quality. I don’t have the same positive experience of the quality of construction plywood, but I haven’t bought any P30 plywood yet.

    Quality is the key. It’s important that the quality is good from batch to batch so I get the product I expect.

    I’m not used to panel strength calculation. I hope to be corrected if my reasoning in this post is wrong.

    Erik
     
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