Core material again.

[Dan]

This has been rolling around in my head for a long time.
As some of you know, I need to repair the core in the walls around the windows in my Catalina Sailboat.
The existing core is 3ply plywood. The thickness of each ply is as follows, the two outer plys are .035? thick with the grain running fore and aft. The inner ply is .070? thick and the grain is running vertical. This yields a total plywood thickness of .140? - 3.5mm. The glue used in the original plywood has desolved and the wood de-laminated. It isn?t rotten, just un-glued.
This wood doesn?t but up against any other core material. It is just a core/filler around the windows. I can remove the center ply with my hand, just slips right. I then laid this center ply piece on the surface of the boat and can see where the exact ends of the core are, all the way around. This core piece is roughly 14? tall and 9 ? feet long or so.
I have some photo?s if needed to explain this better.
I am thinking of using 4mm Okume or Meranti to replace the original plywood. Is the original Doug Fir? I can sand the surface of the 4mm in the sanding machine or run it in the planer to match the existing plywood thickness of .140?.
Is it OK to use Okume or Meranti as a core material? I was under the impression that this is not the correct wood to be used as a core material but I?m not sure where I got that. Okume and Meranti are hard woods and Doug fir is softwood?
What should I be using here?
I can get Meranti in 10-foot piece from far away. I can get Okume in 8? locally. Is a scarf joint required or will butting the core be acceptable? Seems a scarf joint will create a stiff section but maybe not anything to worry about. Actually the scarfed section would end up on the same vertical plane as the mast and chain plates are. This may be a good thing.
Any thoughts?
Thanks in advance everyone.
Dan

[Tim]

Dan,

I'm not quite clear on how this coring works, so if you have those photos it would definitely help visualize it.

Either Okoume or Meranti would be premium choices for this application. Either will hold up far better than marine fir plywood. The construction of Okoume and Meranti is to a very high standard, and each contains multiple laminations of material--several more than typical marine grade fir ply. More laminations are more stable and stronger.

The application you're talking about doesn't sound highly structural in nature. You could probably get away with almost any plywood in the coring, if that's the case. That said, I personally would always choose the better product. High quality, multi-lam marine ply is always a better choice, I think.

I can also better address the questions about whether to scarf or butt joint the plywood if I see the photos of the area in question. My gut feeling is that a butt joint would probably be fine. Either way, I would avoid the high shipping charges for 10' plywood; there's no benefit in the application you're talking about.

[Dan]

Thanks for the reply Tim. I will definitely use okume or meranti for this repair.
I posted some pictures for you to better understand the chore ahead. Any comments will be greatly appreciated.
I am also thinking about cutting the core back from the edge of the window cutout about 1 1/2" and filling it with thickened epoxy. I am planning on installing the new looking polycarb or acrylic windows. My wife hates the old aluminum frame windows. This way they will be through bolted through the epoxy and not the wood.

Thanks in advance,
Dan
http://home.wmis.net/~dhapp/core/core.html

[Tim]

Here's a question:

How sound and well-adhered are the two outer layers of this core sandwich in the cabin sides? Since you can so easily remove the center lamination of the plywood, and it seems that it is still in sound condition, what about coating this center lam with epoxy adhesive and inserting it back into position? You'd have to come up with some way to clamp it in place while the epoxy (thickened with cabosil) cured, and perhaps the mechanics of the whole area wouldn't allow this idea to be a possibility.

Just a thought. Replacing all the old core in the cabin trunk sides is probably the "right" way to do the job, but it's a substantial amount of work for an area of the boat that is not particularly structural (i.e. not weight-bearing, like the horizontal areas of the deck), and that might be just as well served by regluing the existing plywood. This may or may not be viable, and there may well be reasons why it's not a good idea in your situation. This assumes that the two outer laminations of the old plywood are in good condition and well adhered to the fiberglass on each side, and that the center lamination, which you removed, is clean, sound, and dry.

If a good repair can be effected without violating the outer skin, it's often a better option. The key to this is qualifying what entails a "good" repair. Each situation requires careful consideration of all the options, but sometimes you can do just as well with a less invasive approach.

[James384]

I agree with Tim on this. As soon as I saw the photos of the cabin trunk sides I thought, Why bother to completely recore? By not cutting the outer skin you not only save time and materials, you retain the integrity of the outer skin. You can pull out whatever core is loose, using a bent coat hanger or something if you want to go back further. Then cut some narrow lengths of ply or closed cell urethane foam and push them in smothered in a thickened epoxy. Push them about a 1/2" beyond the edge and fill the edges of the window cutouts with thickened epoxy. Plenty strong for that boat IMO.

James #384

[Dan]

How sound and well-adhered are the two outer layers of this core sandwich in the cabin sides? Since you can so easily remove the center lamination of the plywood, and it seems that it is still in sound condition, what about coating this center lam with epoxy adhesive and inserting it back into position?

Sorry about the delayed response, I was out of town again.
The outer two layers are very sound and well-adheared. A large percentage of the center lam is long gone, so the void ratio is pretty high. I don't feel that re-inserting the original material would be possible.

I was actually thinking about removing and replacing the center ply with something, but felt it may not be the best thing to do. Kind of a crazy idea but it is certainly doable.

I can remove the center ply easily and fill the gap with something using thickened epoxy. I will have to fabricate a tool to mop the existing laminates with un-thickened epoxy to saturate the surface, then mop it with thickened epoxy and slide the new gap filling material in. Easy stuff here.

I feel it would be difficult to insert a piece of wood that thin. I would have to make my own thin laminate plywood to have something strong enough to insert in that deep. Labor intense to say the least. Then the epoxy to wood ratio would get pretty high. Why not use epoxy sheet if the resin to wood ratio will be high?

So I am now asking your opinion about using Acculam G10 sheet.
It is stiff enough that I feel I could insert it deep into the void. I will be able to grind a course tooth into the sheet surface and I can also grind/sand the sheet to the required thickness.

I could make my own epoxy/fiberglass sheets but again, too labor intensive. I think G10 is probably made in a vacuum press. I don't have that, yet.

Have any of you used Garolite or Acculam G10? Do you think this would work? I can get a sample sheet and play with gluing it up and run some adhesion tests. I heard about it from West Marine. I was told they use it for testing of some sort. I will call and inquire further.



http://k-mac-plastics.net/acculam_therm ... anates.htm

Acculam? Epoxyglas G10/FR4 Glass Reinforced Epoxy - natural (yellowish to light green) Technical Data Sheet Chemical Resistance Chart G-10/FR4 Sizes/Prices
The most versatile all-around laminate, this grade is a continuous glass woven fabric base impregnated with an epoxy resin binder. It has extremely high mechanical strength, good dielectric loss properties, and good electric strength properties, both wet and dry.
Certifies to Mil-I-24768/27 GEE-F

AS always, Thank for your help.

[Dan]

I heard about it from West Marine.

Wrong here. I heard about this from West Systems Epoxy, ya know, the Gougeon Brothers, not West Marine. Duh!
Dan

[Tim]

Not being there to see the situation closely at hand, it's hard to judge what the best repair option truly is.

That said, I would still vote for first trying to find a workable solution that involves reinserting something into the existing void, rather than destroying the inner or outer skin and starting over. Perhaps this will end up being the only way, but I would use it as a last resort.

I don't think it matters much what material you use to reinsert inside. You're talking about some pretty thin stuff, for sure, and understandably whatever you use needs to be stiff enough to insert into the opening. Wood, foam, or fiberglass sheet would all work fine. The fiberglass product you mention would surely be fine. Given the cost quoted on the website you posted, it would get expensive quickly. I assumed the 0.062" product, which is quoted at $5.25/foot (I assume it's 12" wide, though I didn't see this spec). What is more precious: your time, or your dollars?

Laminating an effective sheet of fiberglass of appropriate dimensions would not be difficult, and you don't need a vacuum system for this application. There's nothing wrong with hand-rolled fiberglass in the slightest. Even a sloppy job here would end up being stronger than the lousy plywood you pulled out of there, which apparently served well enough for all these years.

You could do just fine rolling out onto a smooth panel (or even a sheet of plastic on a flat floor), and all at the cost only of some fiberglass and a small amount of resin. In this application, I'd even say that you could go ahead and use el-cheapo polyester (gasp! He said the "P" word!), if you truly wanted to save a buck. I'd use epoxy mostly because I have it in stock and ready to go at all times, and we're not talking about a ton of resin or glass required either. You could certainly build the pieces you need for far less money. From your earlier descriptions, you mention that these pieces are nearly 10' long on each side, so I assumed 20' of product would have to be purchased. Perhaps this is not the case, in which event the economics of the situation might change.

Also check out the McMaster-Carr site (www.mcmaster.com). Go to pages 3344 and 3345 for a variety of premanufactured fiberglass products, including sheets. You can find the various specs for each product on the site as well for comparison. They have electrical grade fiberglass sheets in appropriate thicknesses for your application; I haven't read all the specifications to see what the true difference between electrical grade and standard fiberglass, but both are built with polyester resin. They also have a premium sheet made with vinylester resin, but I'm not sure if there is anything thin enough for your needs.

My experience is that it is all too easy to overthink a given project when it's on your mind, but you cannot run right out and dive in. This often happens in winter, for obvious reasons. In almost all cases, repair projects end up requiring less engineering and forethought and often work best just diving in with your chosen technique, and modifying as you go. I have fretted over many a project in the past, only to find that the actual project ended up being less of a problem than all those sleepless nights warranted.

In any event, those are my latest thoughts...for what they're worth.