Marine Grade Plywood Debate Thread...

[Yacht Dr.]

This issue has been debated on so many occasions that sometimes the OPs' thread gets lost..so I made this one to debate pros and cons. Hope this works out the way Im hoping

MY Ideas on MG Vs others.. I Use Doug Fur MG btw.

1. Its More expensive ( pro or con..lets find out from our collective input ).
2. It takes less resin to Prep for fiberglass lamination ( pro ) .
3. It does not Warp when cut ( pro considering some other plys will warp in extream when cut ).
4. Same water resistant glue compaired to EX ( wash on pro/con ).
5. Resale value ( Pro when telling prospective buyer you used MG compaired to " well I used EX .. but prepped it right ).
6. MG is Void free ( pro considering you dont want voids in your ply directly in the path of Motor Mounts,Outdrive bolts or OB clamps ).

Ok Iboaters..lets go for pro/con # 7 .. 8 .. 9 ..... etc Keepem' coming..

OH..there is one product I need to mention is MDO ( courtesy of Woodonglass ). I Have worked with this product making Custom water proof signs .. but never in a boat application so I cannot input anything on this stuff ( one way or another ). Im sure Wood will chime in..im hoping so .

Lets roll Iboaters .

YD.

 

[Peteco]

Re: Marine Grade Plywood Debate Thread...

I'll chime in with a modification of my text from the other thread


"Keep in mind that, in theory, the plywood acts as a core and the fiber-resin matrix provides the strength. The core is there to act only as a shear web.

In this case, the plywood is GROSSLY over-strength, and some small voids have absolutely no bearing.

Some structures are actually built this way - a true composite sandwich. Bear in mind that in those cases, the laminate bond to the core is critical.

In practice, many parts of our small production boats are almost like "stitch and glue" structures, where the plywood provides some percentage of stiffness and strength, with chopped strand mat primarily attaching pieces together but also providing some percentage of the strength of the overall pseudo-sandwich. The CSM also adds considerable stiffness due to its bulk (not to be confused with strength) and of course waterproofing. In this case, any suspect underlying material could be reinforced by throwing one more layer of CSM on top of it, if in doubt, or you could fill the void with any structural filler.

This method has the disadvantage - well not really in our boats - of being resin rich and heavy. Epoxy and cloth are stronger and lighter, and use less resin.

From a practical standpoint, these structures aren't even remotely stressed to the point that you would experience a failure due to an interlaminar void in the plywood.

The ONLY application relevant to us is where marine ply (voidless ply?) would mean squat would be in the case of compression forces on the sandwich that would somehow occur over the void. An example could be through-bolts with a large amount of tension on them. In that case, normal sandwich hard point techniques could be used and all would be well. Those techniques would probably be a better choice than simply drilling the transom, exposing the core should the 5200 on the bolts leak.

In fact, it wouldn't be difficult to make a transom out of a fiberglass-styrofoam sandwich; the hard points would have to be placed appropriately, and the stresses calculated in order to calculate an appropriate layup schedule. Aircraft wings capable of withstanding 10+ G's are made this way, so a boat transom is well within the capability of these materials."

 

[sasto]

Re: Marine Grade Plywood Debate Thread...

"Keep in mind that, in theory, the plywood acts as a core and the fiber-resin matrix provides the strength. The core is there to act only as a shear web.

In this case, the plywood is GROSSLY over-strength, and some small voids have absolutely no bearing.

."

Being a boatbuilder and having worked with naval architects, I am in total agreement. We have used composites for years that have no initial strength, whatsoever. I've been in 10 foot plus seas in a 65footer with 2 engines that weigh 4,320lbs each sitting on foam stringers.

 

[Cadwelder]

Re: Marine Grade Plywood Debate Thread...

YD are you just trying to stir up some....well stuff?

Ok #5.....that's iffy. As you know I restore and resale a boat every year and I do use MG, but I've told customers and most haven't a clue what the difference is. So for the most part I don't think it helps much for resale value....not for me anyway.

Now to me #3 alone makes it worth every dime of the costs.

I'm on your side with the MG, but I do have a con

#7 Not readily available in most towns. (Con)

 

[Yacht Dr.]

Re: Marine Grade Plywood Debate Thread...

Nope..this thread wasnt started to start a Forum War LOL..

Just for Informational and Learning .

Nor was it intended for Posting copies from other thread arguments .. just a place for Iboats to suggests Pros and Cons of MG or other materials on there project ( hopefully from members that Actually used X product and not just looking on the web ) .

So we have #7 ( con not readily available ) .

Anyone want to try #8 ?

YD.

PS. this is not a test LOL ..

 

[j_k_bisson]

Re: Marine Grade Plywood Debate Thread...

From the engineering side of things. Its not the marine grade or outside grade plywood that makes the difference. It is the raw marerials that go into make the ply and the glue used to laminated it. I must admit though it's less the glue whan the wood. The reason being that more failures when testing pywood is not the plys delaminating but the wood breaking/failing. Voids weaken the strutural integirty of the plywood reguardless of the material wood spieces.

Also there is another side to the foam core construction that you are not addressing. If you go to foam core you must make the fiberglass the structural element in the boat design. Make is thicker to compensate the foams brittleness. I wood design the wood is the structure and the fiberglass just glues it together in a sense.

I will admit there are foam out there that are stuctural elements. But most people can not afford to play with them in their garages.

 

[Woodonglass]

Re: Marine Grade Plywood Debate Thread...

As YD said I am a strong proponent of the MDO/HDO product. One of the things I like the best is you do NOT have to pre soak the panel with resin. ONLY the edges. Save a LOT on resin costs. Also it does not WARP EVER!!! It has very few voids and is strong as heck. I really have not found a negative for this product when it comes to boat building especially for boats under 20 ft. I would be glad to hear anyone's thoughts on negatives for it's use. Here's an article about the product.

http://www.pacificwoodlaminates.com/img/PDFs/PlywoodGuide.pdf

 

[ezmobee]

Re: Marine Grade Plywood Debate Thread...

2. It takes less resin to Prep for fiberglass lamination ( pro ) .

I can attest to this one. It took me easily 3 times as much epoxy to properly seal my exterior ply deck as it did North Beach who used marine grade.

The Arauco ply from Lowes is a VERY nice product. No voids from what I've seen. I used it for my transom and it was MUCH nicer than the regular stuff I used for my deck which was void-city.

 

[Peteco]

Re: Marine Grade Plywood Debate Thread...

I will admit there are foam out there that are stuctural elements. But most people can not afford to play with them in their garages.

Aww, sure you can. The only difference between the pink/blue sheets in Home Depot and the 2 pound per cubic foot aerospace foams is uniformity of density, and in some cases fuel resistance and absence of voids.

If I design and build a custom rear seat for my boat, I'll do it with styro, epoxy and my hotwire saw. Sure it will cost a little more, but I am able to hotwire cut foam and shape it much easier than my meager woodworking skills allow.:redface:

 

[Peteco]

Re: Marine Grade Plywood Debate Thread...

Nor was it intended for Posting copies from other thread arguments .. just a place for Iboats to suggests Pros and Cons of MG or other materials on there project ( hopefully from members that Actually used X product and not just looking on the web )

My copied post was more germane to this thread than to the other thread, so I copied it over with a few changes.

Just trying to bring an alternative viewpoint to the forum, since my experience has been with airplane parts, and while I'm hardly suggesting that the whole forum move to foam, nomex honeycomb, or carbon fiber, I do think that sometimes plywood isn't a good fit and can make things harder. It also seems that people think that one small void in a sheet of ply will sink their boat, but structurally it ain't happenin'.

Pete

 

[reelfishin]

Re: Marine Grade Plywood Debate Thread...

I think often its a matter of what is needed as much as what is better.
For many of us, simply doing a good job using Ext. ply will no doubt outlive the owner.
For me the way I see it is that most of the boats I own lasted often with minimal maintenance or care for over 40 years. Most were stored outdoors over that period. I own several aluminum boats with bare wood transoms which have passed 40 years with original wood that is still solid and doing it's job. For me I just don't see the need to spend often more than double for Marine Ply when good ext. grade ply will certainly outlive me by a long shot, not to mention the fact that I resin and glass coat it as well, which will further protect the wood from water and rot.

 

[Cadwelder]

Re: Marine Grade Plywood Debate Thread...

So far these are mostly pros and cons of plywood period or plywood vs other materials. I think this thread is looking for #8, #9 etc. in the pro's and con's of Marine Grade vs Exterior grade plywood.

 

[Yacht Dr.]

Re: Marine Grade Plywood Debate Thread...

So far these are mostly pros and cons of plywood period or plywood vs other materials. I think this thread is looking for #8, #9 etc. in the pro's and con's of Marine Grade vs Exterior grade plywood.

Not really..it was ment for a MG vs. other products ( some of which is MDO, Nidacore, honeycomb, foam etc. )

MG was just a Benchmark for other materials for comparison in application.

I was trying to keep the format Clean and streamlined .. kinda like this..

8. MG vs. MDO ( MDO uses less resin then MG saving $. MDO does not warp when cut and Completely compatible with boatbuilding ) Possible insert pics of usage or links to product...Testimonials etc..

( insert Personal use )

Woodonglass said "One of the things I like the best is you do NOT have to pre soak the panel with resin. ONLY the edges. Save a LOT on resin costs. Also it does not WARP EVER!!! It has very few voids and is strong as heck. I really have not found a negative for this product when it comes to boat building especially for boats under 20 ft."

If you agree/disagree with #x .. then format and referance like EZ's Post " I can attest to this one. It took me easily 3 times as much epoxy to properly seal my exterior ply deck as it did North Beach who used marine grade. "

Or..

9. CDX ( this is hypothetical and not inteded to reflect Any other posts from members here ) Is less expensive and will out-live your boat when glassed correctly. ( This may be true but what additional prep was done should be explained for the readers ).

MG vs. x .. or X vs. MG ..

Why I started this thread.. MG Comparison to other products..pros and cons, What and Where..Hows and Why..

YD.

 

[Rellik546]

Re: Marine Grade Plywood Debate Thread...

woodonglass: question about the MDO and HDO. are the outside veneers the same on both products: as in both dont need a resin coat? in terms of strength which do you actually use? HDO on transom, and MDO for stringers?

 

[Yacht Dr.]

Re: Marine Grade Plywood Debate Thread...

woodonglass: question about the MDO and HDO. are the outside veneers the same on both products: as in both dont need a resin coat? in terms of strength which do you actually use? HDO on transom, and MDO for stringers?

PM's are there for a reason....

YD.

 

[fishrdan]

Re: Marine Grade Plywood Debate Thread...

I used MG doug fir when I resto'd my (aluminum) boat and while it was pretty much void free, there were voids. Not many, but I did hit a couple voids screwing down the deck, kind of disappointing at $60/sheet... Could be that the lumber yard got a bad batch or their supplier isn't top-notch.

MG is the only ply (correct me if there are others) that has the plies laid at 45*, superior rigidity and strength.

On the HDO and MDO, exactly what is the "H" and "M" (medium/high density overlay)?

Here's something that would be interesting to find out, how much does ______ weigh per 4x8 sheet of 3/4 material?
- MG
- exterior plywood
- Arauco (seems pretty light)
- MDO
- HDO

 

[Cadwelder]

Re: Marine Grade Plywood Debate Thread...

3/4" marine grade and 3/4" AC fir both weigh in between 72 - 75 pounds, (4 x 8 sheet) about the same. Someone else please chime in on pressure treated and the MDO as I'm not as familiar with those.

 

[Woodonglass]

Re: Marine Grade Plywood Debate Thread...

MDO & HDO both weigh approx 2.3 lbs psf for a 3/4" panel. This would mean a 4x8 ft panel would weigh about 75 lbs. The main difference is that the HDO final overlay is produce with a High pressure High density resin overlay. In the construction business it is used for concrete forming and can be used multiple times. Glues used are 100% water proof and wood laminations are Douglas fir with minimal voids.

 

[fishrdan]

Re: Marine Grade Plywood Debate Thread...

MDO & HDO both weigh approx 2.3 lbs psf for a 3/4" panel.

In the construction business it is used for concrete forming and can be used multiple times.

I have a neighbor who's a concrete guy and he had a couple ("extra") sheets of that (MDO/HDO?). It was heavier than *ell, certainly a lot heavier than extreior or MG ply. Both of us were commenting on how heavy it was, compared to normal ply. Not sure if this is the stuff you are talking about, but I wouldn't use it in a boat,,, just for the weight factor.

 

[archbuilder]

Re: Marine Grade Plywood Debate Thread...

I used the MG for ease of use; It is relatively light, (epically compared it to some of the yellow pine exterior grade plywoods), machines nicely, doesn't warp much. Those qualities alone make it worth the price to me.

That being said we are really talking about it in generic terms here. The forces put on a stringer, transom, and deck are all different.... then you can throw in using it for seat frames ect. I like the idea of non wood composite construction (doesn't rot), but I have yet to see a good replacement for plywood for decking (marine or otherwise) or seat frames (custom, not off the shelf plastic units). I still think that is the Achilles heal of my restore. I would love to hear some alternatives.

Wood, the OSB products are great, I just wonder about the bonding characteristics of the poly or epoxy with the phenolic glues the OSB's are saturated with. Any one look into that? They are already full of glue, therefore don't suck up resin....just wondering what that does to the bonding characteristics? Most of their typical uses in construction rely on mechanical fasteners, not glue. They are glued in I-joist webs, but I have no idea what the glue is.