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Old 30-03-2009, 13:49   #1
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How much fiberglass over plywood?

I'm replacing the plywood decks, cabin top and sides on the boat I'm restoring. How many layers and what weight of glass cloth should I use? I was planning to use System Three on it but read about US Composites epoxy on the Boat Design forum and it's about half the cost. Anyone have any experience with it?
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Old 30-03-2009, 16:45   #2
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Originally Posted by San Juan Sailor View Post
I'm replacing the plywood decks, cabin top and sides on the boat I'm restoring. How many layers and what weight of glass cloth should I use? I was planning to use System Three on it but read about US Composites epoxy on the Boat Design forum and it's about half the cost. Anyone have any experience with it?
Dan

Let me caveat this by saying:
  1. Try to get the plans if possible. It will be worth the money and they are useful for repairs in general. Besides the designer designed it (and spec'ed out the materials) that way for a reason. Presumable.
  2. Remember, the wood is carrying the load here, not the fiberglass. So use marine grade plywood. The glass is mostly for abrasion resistance.
  3. Up to a point, you can over do it bit and be just fine. Another good example of the adage: “You never want to blow a bridge twice”. However, you’ll be paying extra money for extra weight.
So, with that in mind. 6 oz cloth should do you. But I would definitely check the plans. Then again, I have them for my boat. Make sure you epoxy coat the edges. The whole thing should be done but the edges tend to get overlooked.


System Three: Good Stuff. West: Good Stuff. US Composites: I haven’t used it.
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Old 31-03-2009, 08:25   #3
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Different amounts differnt places.

First... everything Marin said
  • One layer of glass will keep the minor cracks that open in the surface of the ply over time from breaking the epoxy seal. Although you will coat the ply first, you always need one layer of glass for crack resistance. Lap the sheets, and lap them around corners ~1-2" whenever possible.
  • Joints. As Marin said this is design dependent. It depends on what is holding the joint together. However, any joint fill tend to be a flexation point, so adding several layers of tape over the joint is fairly norlmal.
  • Fairing. Glass does not go around sharp corners. A 1/2" radius is about the limit, so after epoxy saturating the wood, take thickend epoxy and smooth all sharp inside corners. Hopefully there is enough wood to sand the outside corners.
Saturate, fair, tape corners, one layer over-all. Then sand, fair, and coat once more. Scrub off amine blush before painting (soap and water). There are other levels of dilegence - it all depends on your goals - but the above is middle-of-the-road.
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Old 31-03-2009, 14:36   #4
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First of all, everything Thinwater said

If you can't get plans, I would replace with like with like. If possible though, I would scarph the edges. Ideal is 12:1 but that may not be practical. The reason is you end up with two identically long surfaces to epoxy together. You end up with a greater mechanical joint than you would if you just but two pieces together.

Second which, goes off thinwater's third point, a longneck bottle will give you an even radius. You can mix the epoxy with woodflour or microbubbles in a large-ish ziplock bag then cut a corner and pipe it out like a pastry bag. Of course the contents of the bottle will have to be emptied first
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Old 31-03-2009, 14:53   #5
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Aloha Dan,
Your Mariner was built using polyester resins which was less waterproof than epoxy. Your goal is to build using marine grade plywood (which is better now than when your boat was produced) and using the same joint and construction that your Mariner was done. Then, when you are ready to put the fiberglass and epoxy waterproof sheath on the outside use a light saturated mat layer on the ply with cloth over the top. I use 10oz cloth. The mat will help the cloth adhere to the plywood much better than just plain cloth. Sand your plywood first to remove any residue/treatment/oils and wipe it down with acetone. I'm a firm believer in using epoxy for all forms of boat construction.
My old Mariner was not made of the very best marine grade plywood so whatever marine grade you use probably will be better than production.
Good luck in your project. I really like the Mariners except for that flaw that you are now dealing with.
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Old 31-03-2009, 14:55   #6
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Oh, I might get shot for saying this but I think all epoxies are pretty much equal. I use Fiberglass Hawaii's 4 to 1 resin to hardener.
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Old 31-03-2009, 16:39   #7
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Originally Posted by thinwater View Post
First... everything Marin said
  • One layer of glass will keep the minor cracks that open in the surface of the ply over time from breaking the epoxy seal. Although you will coat the ply first, you always need one layer of glass for crack resistance. Lap the sheets, and lap them around corners ~1-2" whenever possible.
  • Joints. As Marin said this is design dependent. It depends on what is holding the joint together. However, any joint fill tend to be a flexation point, so adding several layers of tape over the joint is fairly norlmal.
  • Fairing. Glass does not go around sharp corners. A 1/2" radius is about the limit, so after epoxy saturating the wood, take thickend epoxy and smooth all sharp inside corners. Hopefully there is enough wood to sand the outside corners.
Saturate, fair, tape corners, one layer over-all. Then sand, fair, and coat once more. Scrub off amine blush before painting (soap and water). There are other levels of dilegence - it all depends on your goals - but the above is middle-of-the-road.

Just out of curiosity - are these suggestions for external construction only - i.e. cockpit seats/lazarettes etc... or does this apply internally as well - just furniture etc?

For example - in my boat, whenever I have a piece of furniture that touches the hull, it is filleted and heavily glassed in with 2 layers of 750gsm triaxial glass (this would be approx ~23 ounce cloth in imperial) to the hull. However, where the furniture is bonded to another piece of furniture (like a galley benchtop bonded to the face) - there are fillets on the inside corners but no glass (except at one or two places where it is obviously structural).

Is this going to be Ok or will my furniture split when I sail?
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Old 31-03-2009, 17:30   #8
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J
For example - in my boat, whenever I have a piece of furniture that touches the hull, it is filleted and heavily glassed in with 2 layers of 750gsm triaxial glass (this would be approx ~23 ounce cloth in imperial) to the hull. However, where the furniture is bonded to another piece of furniture (like a galley benchtop bonded to the face) - there are fillets on the inside corners but no glass (except at one or two places where it is obviously structural).

Is this going to be Ok or will my furniture split when I sail?
Wow, In my last cat B/H were glassed in with a single layer of 440gsm db, floors and furniture got 200gsm plain weave at best, some shelves just got coved with no glass at all. No problems whatsoever

New 50 fter has BH done with single layer of 600gsm db, everything else will be single 440gsm db. Again, some shelves will just be coved with no glass at all.

I would have thought that a multi would have greater loads except for where the keel is trying to tear the bottom out

Dave
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Old 31-03-2009, 17:53   #9
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Wow, In my last cat B/H were glassed in with a single layer of 440gsm db, floors and furniture got 200gsm plain weave at best, some shelves just got coved with no glass at all. No problems whatsoever

New 50 fter has BH done with single layer of 600gsm db, everything else will be single 440gsm db. Again, some shelves will just be coved with no glass at all.

I would have thought that a multi would have greater loads except for where the keel is trying to tear the bottom out

Dave
OK, so it looks like I've overdone the bulkheads then (40ft mono, and I didn't do the hull or anything to do with the keel!).. good to hear.

From what I understand though - you've got at least some glass over almost all of your furniture too right? This is what I don't have at all. All bare plywood apart from where the furniture etc is bonded to the hull - is this needed?
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Old 31-03-2009, 19:07   #10
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Oh, I might get shot for saying this but I think all epoxies are pretty much equal. I use Fiberglass Hawaii's 4 to 1 resin to hardener.
Kind regards,
JohnL
BANG!!!! Dont believe it! I know for a fact that MAS epoxy remains more flexible and would be my first choice for rebuilding a deck. Try this: mix up small batches of your fav brands in plastic cups. Let them go off COMPLETELY. Crack them out of the cups and play with them. You'll see what I mean...
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Old 31-03-2009, 19:24   #11
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I think the gass is most needed where the furnature is bonded to something that...

Quote:
Originally Posted by akio.kanemoto View Post
Just out of curiosity - are these suggestions for external construction only - i.e. cockpit seats/lazarettes etc... or does this apply internally as well - just furniture etc?

For example - in my boat, whenever I have a piece of furniture that touches the hull, it is filleted and heavily glassed in with 2 layers of 750gsm triaxial glass (this would be approx ~23 ounce cloth in imperial) to the hull. However, where the furniture is bonded to another piece of furniture (like a galley benchtop bonded to the face) - there are fillets on the inside corners but no glass (except at one or two places where it is obviously structural).

Is this going to be Ok or will my furniture split when I sail?
flexes, like the hull sides. The benchtops sound fine, unless you find evidence contrary.
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Old 31-03-2009, 19:32   #12
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flexes, like the hull sides. The benchtops sound fine, unless you find evidence contrary.
Thanks, actually, in the benchtop example, this bench runs fore/aft and the outboard edge is glassed with the 750gsm stuff to the hull itself, so I imagine that that end won't flex/crack - but the inboard edge is just bonded to the vertical face with just a fillet..

Hmm... maybe I can just use a small roll of some glass tape and put a layer or two over the edges? (more fairing though.. aaargh)..

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Old 01-04-2009, 00:23   #13
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From what I understand though - you've got at least some glass over almost all of your furniture too right? This is what I don't have at all. All bare plywood apart from where the furniture etc is bonded to the hull - is this needed?
External furniture I had a 200gsm plain weave on the tops and outsides (weather/traffic) faces, and had a 3 or 4 coat (shiny) resin coat for the underside/inside faces.

On the new one I will use a 220gsm DB as it is a fair bit cheaper than the plain weave.

Internal furniture I had no glass, just shiny resin and paint over the top finish
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Old 01-04-2009, 00:27   #14
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BANG!!!! Dont believe it! I know for a fact that MAS epoxy remains more flexible and would be my first choice for rebuilding a deck. Try this: mix up small batches of your fav brands in plastic cups. Let them go off COMPLETELY. Crack them out of the cups and play with them. You'll see what I mean...
System 3 was a flexible resin compared to some as well.
I didnt like it as I saw creep on some high load areas on some boats that didnt get seem to get the same problem when built with other resins.

Anecdotal evidence only, but I have never seen a so called stiff epoxy creep or fail.

Dave
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Old 01-04-2009, 00:32   #15
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External furniture I had a 200gsm plain weave on the tops and outsides (weather/traffic) faces, and had a 3 or 4 coat (shiny) resin coat for the underside/inside faces.

On the new one I will use a 220gsm DB as it is a fair bit cheaper than the plain weave.

Internal furniture I had no glass, just shiny resin and paint over the top finish

Thanks for that - I don't suppose you have any handy hints on fairing concave areas inside (like against the hull between shelves - kinda lumpy at the moment..) where you can't get a sander or a torture board in?

Cheers..
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