How to install a new Fiberglass deck over an old carvel planked deck

Discussion in 'Boatbuilding' started by rud, Dec 27, 2011.

  1. rud
    Joined: Dec 2011
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    rud New Member

    Hello - looking for some advise as I move forward in regards to laying a new plywood/fiberglass deck over the original southern-yellow pine carvel stripped deck. Any advise is greatly appreciated.

    What I'm improving:
    *50' commercial wood dead-rise
    *Deck gets heavy use.
    *600+/_ sq. feet of deck space
    *original deck (which I'm covering) is 3" wide southern-yellow pine in decent shape...not watertight but 90% of wood is solid and in good/level condition to adhere and fasten the plywood.

    Questions:
    1 - Type and thickness of ply?
    2 - Type and thickness of cloth?
    3 - Does a step-by-step "how to" guide for exactly this repair exist somewhere?

    Thanks again - Rud
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    If the boat is carvel planked, making a rigid deck may cause severe structural problems. Covering the deck with Dynel in epoxy may work better. It has more flexibility and abrasion resistance than fiberglass.
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I disagree Gonzo. Covering an old tongue and groove deck with plywood, then sheathing is a good thing for most carvels. The resulting deck will not leak, is much stronger and will add considerably to the athwart stiffness of the deck.

    The first thing you need to do is insure all the existing decking fasteners are in solid wood, no hole rot or elongation, etc. This means you'll have to remove the bad spots and pack in some pieces to bulk it up to surrounding areas (thickened epoxy). You'll also need to check each fastener to insure it's tight.

    Next mark off all the deck beams, so when you're cover in plywood, you can put fasteners into these. With the fasteners checked and repairs to the old deck made, you can now toss down the plywood. Two layers of 1/4" (6 mm) will be much easier to bend to the deck crown, but one layer of 1/2" is all you need. Put this down in a bedding compound, thickened epoxy or over felt. Once the plywood is down, has screws into each deck beam and also into the old planking, it's time to seal up the seams.

    Seal the seams by gouging out each seam with a 7" disk grinder and very coarse grit pad (16, 24 or 36 grit). Plow out a 7" wide shallow groove, the length of the seam, just barely through the first veneer of plywood. Coat this with straight epoxy, then apply two layers of 8 ounce cloth (not mat), into the shallow depression you've made. This seals the seams and now you have a single monocoque deck slab. Smooth off the seams with a sander, possably filling some of the depressions as you go.

    Now, you're ready for the actual sheathing. Coat the plywood with 2 coats of straight epoxy and let this get just past tacky, then apply your fabric. Xynole is the material of choice, because it's tough, very tough, far better then regular 'glass. I'll drink epoxy like I do beer on a Saturday night, but it's tough stuff. One layer will do, but if this is a serious work boat, then two will be better.

    This is the "Cliff's Notes" version and has glossed over the details a good bit. Log onto westsystem.com and systemthree.com and download their "user's guides" for more information about sheathing stuff. No, there isn't a 1, 2, 3 step guide for this sort of thing, as it entails several different things, but once you read the user's guides, you'll get a better idea of how things work.
     
  4. gonzo
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    gonzo Senior Member

    I know it adds stiffness. However, I wonder about the shelf and whether the hull/deck joint will be stressed. Maybe wrapping the laminate to the sheer strake would help.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I wouldn't wrap the sheer strake or tab to the shelf. In fact it's important to let the deck be just a deck in this case, so the fasteners sill bear the load on the clamp/shelf/beams, etc. Yes, you are "capping" the movement prone hull structure with a monocoque deck, but in all previous treatments like this (mine and others) the result is a more stable structure, particularly from sweet water infiltration, but on a engineering level a stiffer and more stable upper flange to the "hull shell girder".
     
  6. rud
    Joined: Dec 2011
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    rud New Member

    Thanks Par and Gonzo. Good stuff! If you guys don't mind a few new questions for ya...

    1 - Knowing that the deck crown is not an issue, I plan on using 3/8 A/B marine doug-fir. Any concerns? How about using marine treated (CCA .40) ply which has been dried to less then 16% moister from Homestead Marine Ply in Ohio?
    2 - When fasting the new ply to old deck you want me to go all the way through deck planks and into the frame??? if so, I'm using 3" screws to fasten 3/8 ply...overkill?
    3 - Any specific fasters? Will 316-SS deck screws work?
    4 - Does Xynole come in different oz. or is it more of a "one size fits all"? If different oz. what weight do I want?

    FYI - due to the deck construction I have to tab the deck into the bulwark

    thanks again gents - rid
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You'll use 4 ounce Xynole and be grateful it's a light weight cloth for all the resin it'll absorb.

    If you can actually find CCA treated marine grade (APA 1-95, nothing less) then you're okay, but it's moisture content should be very dry, say in the 12% range, not 16%. This is difficult to do in most areas of the country. Here in Florida, I can use a "self storage" warehouse and sticker up PT stock with a box fan. In just a couple of months, the metal building and Florida sun will bake it down to 12%.

    The real problem is the new PT chemical mixture is CA and doesn't stick to epoxy well at all. The old stuff - CCA is still available, but not at the retail level, so make sure of what you're getting. Epoxy does stick to CCA.

    When fastening the new ply to the old deck, you'll attach to both the old deck (short screws) and also into the beams (long screws). This will do two things: first it'll stabilize the old deck, locking it down and preventing movement between the planks and second, marry both old and new to the hull's structural system (frames, floors, deck beams, etc.). This will make the boat very stiff, which is a welcome change to a carvel built boat.

    Most SS deck screws are really cheap 303 series and in a salt water environment, likely to rust badly. This said, if each is coated with epoxy and puttied over with thickened epoxy, they'll work fine, but you have to be religious about coating and covering the heads. Log onto "McFeely's" fasteners and look up 316 stainless, square drive (or star drive) screws. They cost more then the 303 - 304 stuff, but they hold up in salt water, just in case you're not the perfect laminator.

    Tabbing into the bulwark is troublesome. Can you remove the bulwark, 'glass under it, then reinstall the bulwark? This is the preferred method, with lots of scuppers to keep water from pooling of course.
     
  8. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    I recently watched a 12 meter gill net boat sheath its old decks with ply.

    The old deck was sanded with a 36 grit grinder to expose clean wood.

    Many old fasteners were removed and new galvanized nails were used to secure the old timber deck to frames. hollows in the deck were epoxy compound filled to have a fair surface..


    The deck was primed with gray primer

    The ply was epoxy precoated both sides. fit into place then Biax fabric butt spliced together.

    These big panels were then troweled with construction adhesive ,laid down and screw washer clamped .

    When the adhesive cured, the screws were removed and the deck was sheathed with fabric set in epoxy.

    The sheer clamp edge and cabin house joint was covered with a mechanically fastened timber covering board to protect the new ply deck edges

    The job looked clean and well executed. Two men, two weeks

    It looked like 9mm marine ply and 10 oz glass cloth ?
     
  9. rud
    Joined: Dec 2011
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    rud New Member

    4 oz. Xynole - got it!

    I'm going to stick with A/B marine doug-fir and make sure I seal everything well...PT just doesn't feel right.

    I could remove the bulwark but I like the idea of tabbing and wrapping it in efforts to keep my bilge as dry as possible.

    thanks again. If you have some or think of any other useful tips don't be shy.
     

  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    If you want a failure down the road, use Michael's advice and glue your deck down with construction adhesive. Construction adhesives don't have the modulus of elasticity, to preform well in a dynamic loading situation. I have pictures of a bulkhead I just replaced, that was assembled with construction adhesive and the glue suffered flexural failure, in less then 200 hours of service.

    PT will work, but again make sure you get the old style CCA treated stuff, not the new CA stock. Also it has to be dry. Personally, I don't use the stuff, but some like it.

    If you elect to not epoxy the deck down, then bed it in polysulfide or polyurethane. This will make replacement easier, when the time comes too. Use a notched trowel to apply it, just like tiling your bathroom.

    I understand the desire to tab in the bulwarks, but you want the deck to act as a fully water tight "cap" over the full width of the hull structure, to which you can attach things later, like the bulwarks and hardware. Also read up on bonding fasteners over at the West System site. It'll keep sweet water from sneaking down below through fastener holes.
     
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