# Proper Strength of Plywood

Discussion in 'Wood Construction' started by blane.c, Jun 7, 2019.

1. Jun 7, 2019

### blane.c

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Were aircraft plywood is specified in the plans there is no logical room for argument about what plywood to use and that plywood is readily available today. If however the plans specify a marine grade of plywood for a part then it is also logical to use the marine grade if it is available. I do not know if the marine grade of plywood specified in the plans is the same marine plywood being sold today (VP plans are circa 1960). Further there are at least 3 types of marine mahogany plywood (1) Okoume 12lbs 5/32"x4'x8' sheet,(2) Sapele 20lbs 5/32"x4'x8' sheet, and (3) Lauan also called Merranti 21lbs 5/32"x4'x8' sheet (I use 5/32" for example because they all come in that size). Aircraft mahogany plywood in 1/8" and 3/16" is 12lbs and 24lbs respectively and I surmise 5/32" would weigh 18lbs if it was made. The strength of sound wood is closely associated with it's weight and there is quite a difference in weight of the different plywoods for the same size so I imagine quite a range of strength as well.

What are other people doing to address this issue?

Also the plans call out for A-A grade marine plywood, I have not seen anything but A-B grade for any of the marine plywood available.

also how is this being addressed?

Thank you.

2. Jun 7, 2019

### narfi

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https://en.wikipedia.org/wiki/BS_1088
I've been using Okoome (Gaboon) BS1088 from BoatBuilderCenteral in florida on the canoe and boat ive been building.
I have never worked with aircraft plywood, but this is very nice to work with.

Knowledge gained from boating forums: You can purchase marine plywood from local lumberyards, but that just refers to the types of glue used and does not necessairly mean the cores meet the standards you require. If you purchase BS1088 stamped plywood, you know there will be no voids in the cores (even if the faces look good)

https://boatbuildercentral.com/wp/proddetail.php?prod=PLY-PL_Okoume_BS1088
They run a very friendly business and if you email or call them, they can work with you on best shipping and their prices + shipping are often better than local prices without shipping.

4mm BS1088 Okoome
BBC = $57.18 a sheet Aircraft Spruce =$94.75 a sheet

6mm BS1088 Okoome
BBC = $89.12 a sheet Aircraft Spruce =$114.85 a sheet

Not sure if it matters, but when purchasing metric plywood you get an extra couple of inches or so over an 8' sheet, that plays with your mind the first couple of times you lay some templates or measurements out trying to see what you can fit on a sheet.

Edit: I think I see what you mean by A/B, but the wording is not clear, it looks like it is saying the veneers are made from A/B, but I suspect the A is facing out on the faces....

Last edited: Jun 7, 2019
3. Jun 7, 2019

### blane.c

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4. Jun 7, 2019

### narfi

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I don't think the cores or back specified there meet what you said your plans would require.

5. Jun 7, 2019

### narfi

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What are the plans for?
Where is the plywood to be used and how?
Is there aircraft plywood specified in other areas of the same plans?

Example: Plans call for marine grade plywood for its structural parts.... you would want to make sure it meets all the minimum standards
Example 2: Plans call for aircraft plywood for its structural parts and marine grade for its floorboards and cosmetic pieces.... Here it would be much less critical what plywood you use.

6. Jun 7, 2019

### blane.c

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Exactly.

7. Jun 7, 2019

### blane.c

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Plans call for A-A marine grade Doug Fir in wing ribs, bulkheads and A-A marine grade Mahogany in some of the fuselage skins.

I have not seen a single piece of marine ply advertised A-A it is all A-B. There is at least one place that advertises that it will special order A-A, at the moment I am afraid to ask, it may cost more than aircraft ply my guess is it won't miss it much if it isn't.

But even if I wanted to just say the heck with the BS and just use aircraft grade there is no guarantee that a given aircraft grade is actually as strong as the original marine grade specified. The aircraft stamp isn't magic. The marine grade A-A Doug Fir and A-A Mahogany from the 1960's is strong very strong.

Everything regarding specifications that I have found is glue integrity and defects allowed. Which is all well and dandy but how much weight will it hold up. I would like to see strength tables comparing different types of plywood then it is easy to make a choice. So far I have found nothing but blah, blah, blah. Great they both use strong material and they both use strong glue and in the case of marine you can expect more defects in less than A grades but all of that information is just herding chickens when you want to know what is stronger than what.

8. Jun 7, 2019

### Nicholette

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You could probably gain a lot of information from looking up the plans of the DeHaviland Sea Venom also of course the DeHaviland Mosquito. Yes they were both made of plywood. Interestingly, the Sea Venom was also the first carrier borne jet to have a guided missile, the Firestreak. Another interesting fact from my website - http://nikkiwaif.tripod.com/carrier-born-aircraft.html

9. Jun 7, 2019

### Vigilant1

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Does the American Plywood Association or the American Wood Products Assn have technical info you could use? I've been impressed by some other studies and standards they issue for various materials.

10. Jun 8, 2019

### pictsidhe

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Interesting. Narfi is getting British Standard plywood shipped to him in Alaska from Florida...

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11. Jun 8, 2019

### blane.c

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You can get anything in Alaska it just costs more. Except Oratex, shipping is cheaper in state.

12. Jun 8, 2019

### lakeracer69

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Meranti plywood is not luan in my experience. Luan is crap you buy at Home Depot or the like. The Meranti I've used is Llyods BS1088 and I've built a few boats with it. Nice stuff, so is Okoume or Sapele. You may want to look at Baltic birch too. A-A faces may not exist anymore in manufacturing, A-B may be the best there is nowadays. I have bought 45 degree layed up plywood too for a box spar, before as well.

People call a number of different species "mahogany" too. There is an old Mil-spec circular that talks about different woods, I don't remember if it talks about plywood, but it has data on a bunch of relevant information on wood.

I've bought from Harbor Sales in the past. Have a look there. I'm also lucky that I can get some of the above at a local specialty wood store (they have lifts of BS1088 marine plywood) and lumber mill (quartersawn material).

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13. Jun 8, 2019

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Our old friend ANC-18 (there is a copy somewhere here on HBA) has tables in section 2.5 that list out the strength values of plywoods that came to be covered under MIL-P-6070. BS1088 (and similar) allows for additional species, like okoume, that aren't in the MIL spec, so you'd have to find those values elsewhere.

Plywood made to different recognized standards will not vary in strength in any meaningful way. In other words, mahogany plywood made to the MIL spec won't be any stronger or weaker than mahogany plywood made to BS1088. Both types of standards require high quality veneers and durable glues. There are subtle differences in allowable defects and testing procedures, but they are close enough that differences in strength will come down to the wood species used.

It sounds like you're getting a sense of this already, but be aware of terminology. "Aircraft" and "marine" plywood designations, by themselves, don't really mean anything. I would look for plywood manufactured according to a recognized standard, so you have some assurance of what you're getting. (e.g. MIL-P-6070 for aircraft, BS1088 or BS6566 for marine, etc.) These standards control species, glue types, tolerances, permissible defects, and provide standards for testing validation. If a panel doesn't conform to one of these standards, you don't have any assurance of how it was made. It may be fine for your purpose, or it may not. You assume the risk. Additionally, terms like A or B are grading standards that are set by regional wood products trade associations, and their definitions can vary.

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14. Jun 8, 2019

### FritzW

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Don't overthink it. If you use A-B Marine from A.S. for the ribs you'll be fine. If you see any voids in the core after you cut the ribs out, just whittle a little stick and glue it in the void. If there's an ugly spot on the B side just steer around it or patch it. Put the B side facing down on the forward cockpit floor and put the B sides together when you glue the seat together. Save the best pieces for the bulkheads or use some some 1/4" Finnish Birch ply. Get some 3/4" exterior for the firewall and use domestic Birch or Mahogany for everything else.

The spars and the DF in the spar and strut carry thoughs are what you really need to sort out

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15. Jun 8, 2019

### TFF

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When I was a kid looking at airplane books at the library, it was either a Don Dwiggins or Pete Bowers homebuilt book that had the decision choice of marine ply. Highly technical. Essentially it was figure out what you needed in the good stuff and go up one size for the marine.
What has happened since the good days is advanced materials, that don’t eat up all the good supplies like putting high quality wood in the core of ply, and the loss of old growth wood as the common wood. They can’t make it like they use to. Everyone has engineered past it except for anyone needing the pure quality that we need. Heck a couple of years ago a ripped some 1x6 into 1x2 for a shelf at home because the 1x2s were just crappy at the store. If I’m not willing to use it in a storage room shelf, it’s bad.

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16. Jun 8, 2019

### blane.c

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Harbor Sales has the 1/4" Doug Fir in A-A Marine for less than other places have the A-B, thanks.

Were I get my information on Lauan/Meranti; https://www.wood-database.com/lauan/

and Lauan is specified for one plywood sheet in the VP-2 plans.

Thanks again.

17. Jun 8, 2019

### blane.c

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Finding Doug Fir in aircraft quality may take some searching, can find lesser grades easy. Same with Birch. Was wondering about substituting both for aircraft quality Ash?

18. Jun 8, 2019

### blane.c

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Yes I started pounding boards when 1 5/8" x 3 5/8" was the nominal dimension for a 2x4. Lucky to get a full 1 1/2" x 3 1/2" now.

19. Jun 8, 2019

### pictsidhe

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You must be younger than I am. Or British 4x2s shrank faster.

20. Jul 18, 2019

### tdm3

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On the Cygnet I am building Sisler calls for 1/4" marine plywood, no face designations, but no core voids larger than 1/8" and no patches within some distance (don't recollect the number at the moment) of the main spars. I purchased Roseburg marine ply from Menards in Owensboro, KY. I've cut all my ribs, haven't cut spar doubler plates yet. Bought 2 4x8 sheets. There were just a couple of voids in the two sheets. One less than 1/8" and one slightly more. I had 1 rib that ended up with that larger (3/16") void and plan to do what was said above and slide in a sliver with T88 epoxy around it.

In addition to the marine ply Sisler also indicated that clear spruce or pine was acceptable for the caps on the false front spar and the rear spar. The front caps are 1/2"x1/2"x156" and the rear caps are 1/2"x5/8"x156" on 3/32" mahogany aircraft ply webs. The lattice on the wings is also called out as clear spruce or pine and it is 5/32"x3/4" material cut to length (about 72"). Lots of room for material interpretation/substitution here. The main spars are called out for 3/4"x5 7/8"x156" spruce. Mine are ash of the same dimensions. The wings are over designed/overbuilt for the airplane if anything and very interesting.