Differences in marine ply?

[CCrew]

Is there really that much of a structural difference between A/A Douglas Fir marine ply and 1088 Okoume that the Okoume is twice the price? Getting ready to order a bunch, and just wondered. <br /><br />1/2" best for decking, or should I go 5/8" or more?? I'm also going to ply composite the stringers as Bondo suggested, so I want something good, but darned.. double the price hurts!<br /><br />-Roger

 

[JasonJ]

Re: Differences in marine ply?

I used 1/2 inch for my floor, but I have six stringers that are less than a foot apart. Also I used my foam as part of my support system, and it is rock solid. Your thickness will depend on how far apart and how many stringers you had, and whether you are foaming or not. How thick was the original floor?

 

[Bondo]

Re: Differences in marine ply?

CC, <br />Before I go spouting off about all the money some of you guys Blow on Fancy Plywood, You'll have to answer a Question.....Or two.....OK, three...<br />Are you using Epoxy, or Polyester ?????<br />Are you going to Encapsulate the wood in Resin,+ glass ?????<br />Are you rebuilding a Boat, or building a copy of the Spruce Goose ??????<br /><br />I like CDX..... The roughnest,+ Voids leave More room for the Epoxy.....

 

[CCrew]

Re: Differences in marine ply?

Ok, let me try to explain my train of thought here...<br /><br />Hull only has 2 stringers. 24" apart, 12" off the keel centerline. Original stringers were 1x oak. Outer edge of the deck is another 18" from the stringer at the widest point, and only rests against the hull. 1/2" Ply deck. No indentation in the hull to support it, so it's just bottom edge of the ply against the hull, with it glassed over. Bottom of the ply unglassed, top chopper glassed. Two bulkheads, one fore and aft of the gas tank. Outer side of the stringers foamed, inner gas tank and ski locker. One support where the ply seams meet at the front of the ski locker, everywhere else just butt joint. That's the factory config. <br /><br />I was thinking 3 ply laminate stringers in the same position, with splice pads everywhere plywood butts, and a better fit against the hull. All wood epoxy encapsulated (epoxy), with or without mat would be a question. Epoxy mat and filler where deck edge meets hull. Decent foam job to match factory placement. Questions would be whether to mat the ply one side, both sides, or not at all, and whether staying with 1/2" would be enough. <br /><br />Bondo, you can have your CDX. I'll differ from you here. All CDX I've seen is rife with voids, and no amount of epoxy is going to penetrate that ply enough to guarantee fill on that void. I only intend to ever do this once, and if it costs $200 more now to have to do it so be it. Crappy CDX is $18 a sheet here, A/A Marine Fir is $46, and Okoume is $96. At the LEAST it'll be A/A, I just wanted to know if there is a difference enough to justify the Okoume, and if so, what the difference is. <br /><br />Thanks!<br />Roger

 

[crab bait]

Re: Differences in marine ply?

this is where i totally disagree.. cdx imho,, is crap, garbage, junk not suited for a doghouse let alone a craft.. that's just me..<br /><br />but you surely don't need aa marine.. an okume is not a good structure wood.. <br /><br />cdx is full of big voids.. an voids are bad for the obvious reason ..even tho laminating.. but voids are trapped air pockets that condensation forms in as the difference in ambient ( air ) an water temps collide.. i.e. ROT.. <br /><br />matter of fact,, i really can't condone the use of ply as stringers,, at all in boats .. as ply,, as good as it maybe can't take the rigors of stringers job an should be from soild proven boatbuildin' wood such as white oak ,mohogany, spruce, locust , such... <br /><br />no wooden boat i know of ,,uses ply as stringers... but am not sayin' not to in your case.. i'm just stating how i feel.. <br /><br />back to plys... ab exterior doug fir is a good ply.. may have or may not have any voids.. if so ,, very small... <br /><br />homecenters don't carry it .. but ANY lumberyard can order it an have in less that a week... <br /><br />plus ,, it's a lot cheaper tham aa or okume..

 

[G DANE]

Re: Differences in marine ply?

I dont know by you, but here 1" mahogony ply is available at 35$. I used it for my stringers. Its not hard wood, but it feels strong. I think it comes from Thailand or so. I always heard mahogony was good for boatbuilding.

 

[Columbia]

Re: Differences in marine ply?

For stringers the fir is plenty good IMHO. Okume is more, smaller plies and a pretty face suitable for varnish but not for stringers IMHO because of bendability and expense. 'World Woods' by Lincoln says Okoume ply is made from a wood properly called Gaboon from Africa. Mechanical qualities: "it is a weak, light density timber with poor wood bending classification, low bending strength, very low stiffness...." Of course, laminating several lams help to stiffen it a lot. I've seen fir ply used as stringer in the "on edge" just like a manufactured beam in a boats, though I must say I don't see that many boat stringers. It's stiff and easily bonded. Rot is the main consideration for fir ply and if you properly epoxy treat and bond it should last. Once again, IMHO. The mahogany that G Dane refers to is most likely not mahogany at all. True mahogany is from Africa and 'American' Mahogany is from Central America. Most of what is sold as Mahogany from Asia is a Luan or Meranti from white to red. These vary in durability according to "World Woods" by William A. Lincoln but tend to be durable in use. World Wood in Color is one of my favorite references. Hey, you gotta believe in something.

 

[BillP]

Re: Differences in marine ply?

My 02:<br /><br />Here is a link to find spacing and other info for ply http://www.apawood.org/ . I don't know about okume but any marine ply is plenty strong. It is stiffer than other plys because the plys are laid at 45 degrees instead of 90. AC exterior will work too. If you want to do 5/8" it will be fine and just give more strength. But you have to live with the extra weight (which really will be minimal in most floor jobs and I probably would go that route personally just for a stronger boat).<br /><br />My personal opinion is you only need to coat the underside of the ply with epoxy to have a lasting seal...but glassing is always better. Either one will last your lifetime if done right.<br /><br />I have to disagree with crabbait about plywood not being able to take the rigors of a stringer. It has been and still is used widely today in expensive ocean going wave jumpers...Pursuit Boats has been using ply as stringers and bulkheads for years (I visited the factory and know this as a product of my own eyes). Same as many other boat builders from puddle jumpers up. Some research will find plywood is proven beyond a doubt to work as a successful stringer material. <br /><br />By the way, Pursuit places a thin high density foam strip under the ply stringer to keep if off the glass hull...no the wood does NOT touch the hull. They do this to absorb compression and prevent stress points because the ply is so thin (as told to me by their production mgr). If memory serves me right they used a single layer of 1/2" ply for box stringers on their 24'CC. The top of the "box" is what the deck rests on with adhesive.<br /><br />Just to make sure you know it...polyester resin over epoxy is a no no. Epoxy resin over poly is ok.

 

[Whit]

Re: Differences in marine ply?

Not a huge difference structurally but I prefer the BS standard wood in general. It sounds like you are looking at all above water line applications and not bright finishing so you'd be fine with the fir. Use epoxy and glass both sides as the Fir can check if only coated with epoxy.

 

[crab bait]

Re: Differences in marine ply?

no offence takin' ... this is what this board is all about... <br /><br />if everone agreed that vannilla ice cream is the best... gee, how borin'..<br /><br />i tried to 'thin' my epoxy with denatured alcohol.. just a small amount... it never did get hard.. always soft & gummy...<br /><br />also,, it's great you get to the manufactorers to see boats beein' built.. <br /><br />but as history dictates,, their ways are usually,,far far from beein' the right way...

 

[Boomyal]

Re: Differences in marine ply?

Crab bait: did you try increasing the catalyst when you thinned the resin. And by what % did you thin it. I've been thinking about trying that as an alternative to CPES.<br /><br />Did you percieve good penetration even tho it did not cure? <br /><br />Also, what would happen if you applied a hot batch of resin over the gum or just went ahead and laid you glass over it?

 

[1965MT]

Re: Differences in marine ply?

crab bait your right on the money about the manufacturers. They are going to use the cheapest materials that they can, welcome to the USA. (I thought you were supposed to thin with Acetone?)<br />On the other hand I liked bondo's question regarding the spruce goose.<br />If boats were supposed to last a lifetime then we should be looking at Titanium or something.<br />I plan on rebuilding similar to what was there before with some changes. ACX on the transom and floor, store bought PT lumber for the stringers, cover it all in fiberglass and epoxy. <br />If some knucklehead had not put screws in the wrong place/fashion on the transom I would not be doing this job now. The lumber looked great where the water did not soak on this 38 year old boat.

 

[Columbia]

Re: Differences in marine ply?

Hey Bondo, I'll go out to the museum with the Spruce Goose tomorrow and let you know what the workmanship is like. Hell, maybe it's CDX...

 

[G DANE]

Re: Differences in marine ply?

Old Sailor - I think your are perfectly right - the Thai "Mahogony" i bought was Red Meranti.<br /><br />Now that I used it - is it good or bad ?. It seemed a nice quality, but will it rot easy. It seems strong enough, especially after it got a couple of layers of glass.

 

[BillP]

Re: Differences in marine ply?

crabbait<br />Chances are you used rubbing alcohol? The key to thinning with alcohol is to use the type that doesn't have h2o in it. That's the reason denatured is used instead of other types for thinning. Everyone should visit boat factories, it would surprise lots of people and put many myths to rest. If you crunch the math on ply's structural engineering properties it will show why the boat guys are using it. It is better than regular lumber and less cost than composites. The only problem I have with the stuff for stringers is it is difficult to center screws in when doing a floor over them. <br /><br />1965MT,<br />Polyester resin is supposed to be thinned with styrene. It is also what you should wipe the surface down with to prep before glassing. I'm not a chemist type but poly and styrene bond together chemically and acetone just mixes with it. <br /><br />Here's a way to see the difference between acetone and styrene...make four 1" sq cubes of cured poly resin and divide them into two pairs. Take one pair, wipe down with acetone and "glue" the two cubes together using only plain polyester resin. Take the other pair and do the same but use styrene. Come back later when fully cured with a hammer or something to pry them apart. The styrene pair will be 10x harder to pull apart.

 

[JasonJ]

Re: Differences in marine ply?

I used 3/4 ACX for my stringers (Original stringers were 3/4 dimensional, didn't know what kind of wood), as two of them were almost 15 inches tall and I couldn't find dimensional lumber in that width. I also used a layer of 3/4 ACX and a layer of 5/8 ACX for the transom. I put that boat through some serious pounding this summer and there was no creaking or discernable flex, which is what I was looking for. I also ended up sitting a good six inches higher in the water than before due to weight savings (I guess removing all that waterlogged foam helped as well).

 

[1965MT]

Re: Differences in marine ply?

BillpP, I thought he said epoxy not polyester...

 

[BillP]

Re: Differences in marine ply?

1965MT,<br />Sorry for the confusion...I thought you were asking about styrene vs acetone with the question.<br /><br />I see you are using PT. I'm doing one now with 3/4" pt ply for transom, floor and side stringers. The center stringer is a 2"x12"x20' pt plank cut to fit. I also cored (original construction)the bottom with one sheet of 1/2" pt ply. All the wood was air dried for months and I'm using epoxy.

 

[Columbia]

Re: Differences in marine ply?

Dark Red Meranti is the best of the Meranti that is listed in the book. Lighter color can indicate less rot resistance but not necessarily, from my reading. It varies a bit and not likely worth concern as it is glassed. The strength and working characteristics seem quite decent. I'd sure not worry about it. It sounds pretty good from here.

 

[Columbia]

Re: Differences in marine ply?

I'd add a couple other remarks to using ply for stringers. One of the worst..IMHO..things is to run fasteners into the edge grain of ply. Ply is most vulnerable to rot in the edge grain and has the least fastener strength. If you do it, I'd say caulk the hole and fastener with a good adhesive caulk at the very least. On the occasions I've used ply as a vertical or to hold fasteners in a marine or potentially wet application, I've tried to cut it a mite short and glue and cap it with horizontal grain ply for protection or to accept the fasteners. I like to edge treat the cap ply to further protect against rot. The other is to use solid wood in a similar manner, much like a manufactured beam. I prefer to rabbet a shallow slot in the cap to more completely cover the edge grain. Maybe that's overkill, I think not, but I like to build for the long haul. I'm too much of a worrier, I guess, to do less.