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plywood thickness question. 14 Jun 2009 04:02 #1

  • sillyocis
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i have an 87 capri 1700 that i am restoring. i am replacing the deck with marine grade plywood 1/2 inch thickness and cant help but think that it's "a little flimsy" . The original deck was 1/2" thick and the only other choice in marine grade is 3/4 inch which seemed like overkill and unneccesarrily heavy. i have just finished fitting the two pieces inside the hull and they are not fiberglassed in/ resined or even screwed at all so i was wondering if things will really stiffen up that much after that process. just making sure im not missing something before i make a huge mistake. or should i go thicker before i glass it in.,.....once that happens theres no going back. any thoughts? thanks. also in a related topic...the old deck was attached to the stringers with resin but i was going to use exterior grade construction adhesive on the new deck since it's easier to deal with than resin. any thoughts on that? thanks for any input...this site is a valuable resource i couldn't function without.

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plywood thickness question. 14 Jun 2009 12:29 #2

  • Merlin
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Hi Sillyocis,

Was the original ply deck glassed on both sides?

A good layup of CSM and woven roving laminations on both sides of the ply will stiffen it up as much as using 3/4" ply only glassed on one side. Prime both sides of the ply with a thinned solution of resin or purpose made primer, Polyester resins don't stick to wood too well without a little help! All structural wood in a hull should be sealed with a resin for protection against moisture.

Fix the deck to stringers etc. with Epoxy thickened with microfibres. This'll give you a good, strong, long lasting bond that won't be affected by salt or fresh water and adheres to the polyester no problem. Make sure the joints are well keyed and degreased with acetone though. Check the fit of the joints for hardpoints too where stress fractures can occur. I dunno how well the construction adhesive would hold up to the movement and vibration in the hull over time and if it breaks away could effect the hulls structural integrity! There are some good books out there with lots of good information on the kind of restoration you're doing.

It's good that you want to get this right first time!

Hope this helps you along a bit and you dont think I'm trying to teach my Gran how to suck eggs?

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plywood thickness question. 14 Jun 2009 13:02 #3

  • 88fourwinns
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I'll bet this boat had foam in it...if so the foam is not only for floatation but also for structural reasons....when I did the deck in my 88 FW, I had removed the foam to repair the stringers....then fitted the new deck panels without the foam...and thought the same thing....not all that solid. I e-mailed FW and they told me the foam was necessary for both floatation and structural stiffness. I put in the 2 part 2 lb foam....let it expand...cut it even with the tops of the stringers...and then test fitted the deck again...much more solid....I wound up installing it with 3M 4200 and stainless screws...and had my 'glass shop glass it in and then they coated the whole thing with a non skid gelcoat...no carpet to hold moisture...and all screw holes for seat mounting were also filled with thinned epoxy and sealed with 4200...So re-install the foam and you should be good to go...

Attached files [img]/media/kunena/attachments/vb/309361=6690-ATT00016 (2).jpg [img]/media/kunena/attachments/vb/309361=6691-Boat 1a.jpg [img]/media/kunena/attachments/vb/309361=6689-foam in.jpg

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plywood thickness question. 14 Jun 2009 16:02 #4

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i appreciate the advice from the reples. i'm going to have to postpone glassing in the floor until i can come up with some foam to reinstall. im glad you reminded me about the foam to begin with because i removed so much wet foam so long ago i forgot to even consider re-installing it. this restoration has me spinning in alot of differnet directions and its easy to forget the details which i have found can make or break any project. anyways...i distinckly remember the foam being suprisingly stiff so it makes sense that it would help with support. thanks for the speedy replies. when i am done hopefully i will still have the energy to post my restoration. i have documented it pretty well so far just for my own memory.

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plywood thickness question. 14 Jun 2009 16:50 #5

  • boomer
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interesting, mine didnt have foam...
i used 5/8 sanded flooring plywood and glassed all sides. I then laid strips of 50" bi-axial cloth over the top for the deck surface. the floor is solid as a rock and total weight of materials wasn't that high. I feel that marine plywood is great but sanded flooring plywood, if properly sealed, is a economical alternative.

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plywood thickness question. 14 Jun 2009 17:12 #6

  • 88fourwinns
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boomer;309451 wrote: interesting, mine didnt have foam...
i used 5/8 sanded flooring plywood and glassed all sides. I then laid strips of 50" bi-axial cloth over the top for the deck surface. the floor is solid as a rock and total weight of materials wasn't that high. I feel that marine plywood is great but sanded flooring plywood, if properly sealed, is a economical alternative.


The foam was required in boats 20' and under as per CG regs....if you have a 22'er its not...so that's why it may not have been installed...some builders do put it in...and if the boat is foamed up to the gunnels it really does add safety but many feel that with glass and wood construction the foam contributes to rot because it holds moisture...

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plywood thickness question. 15 Jun 2009 00:15 #7

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88fourwinns;309460 wrote: The foam was required in boats 20' and under as per CG regs....if you have a 22'er its not...so that's why it may not have been installed...some builders do put it in...and if the boat is foamed up to the gunnels it really does add safety but many feel that with glass and wood construction the foam contributes to rot because it holds moisture...


yea if it holds moisture then thats obviously a con, but nothing is free in this world so its a trade off.
If my boat sinks the only remnants in 100 years will be the new floor i just installed :)
its bulletproof....ill add a thread in the project section soon enough.

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plywood thickness question. 15 Jun 2009 02:33 #8

  • jamsac
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I'm in the process of floor, stringer, bulkhead replacement also, and the floor, the old floor, was all 1/2", double and triple in specific load areas like around the seat pedestal bases. The old foam was just chunks, around 3-4" squares, triangles tossed into the voids with some sprayed in chopped glass/resin over it to hold it down. I like the idea of the 2 part expanding stuff. I had considered the high expansion spray in type, but I'm sure it's not water proof, floatation worthy. I also have been documenting quite well and will post upon completion, hopefully sooner than later. Quite the ordeal.

James

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plywood thickness question. 22 Jun 2009 04:35 #9

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well. i just ive got the deck in now. ordered 2 part foam from a place called u.s. composites. good stuff and less than half the cost of tap-30 foam. i went with the four lb. foam for more strength. holy cow what a difference. florr is solid as a rock now. that foam was huge! really impressed with the difference. screwed down the deck with some stainless screws after laying down a bead of bondo microfiber resin/fill. used hair resin for big gaps and sanded smooth., then laid two layers of glass mat around the edges. should be bomb=proof now. im going to paint the ski locker with rustoleum 1 part epoxy used for garage and basement floor. i am gong to contact the company first to make sure that its safe for use on fiberglass. wish me luck.

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