Re-decking 40ft commercial fishing boat...with what materials?

Discussion in 'Fiberglass and Composite Boat Building' started by sandusk, Jun 18, 2011.

  1. sandusk
    Joined: Jun 2011
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    Location: Juneau, Alaska

    sandusk Junior Member

    This winter, I am planning to re-deck and re-frame my fiberglass hull 1978 40' Beck Seiner. The main objectives are good functionality, long life, and low cost.
    The back deck is currently framed on non-treated 4x4s and 4x6s on 24in centers. There is one area centered on the beam, right behind the wheelhouse that supports a mast and boom. the framing under this area is a non-treated 4x10 layed flat on its weak axis (I am assuming due to clearance for the engine mounted below).
    The decks have a camber of maybe 4in vertical rise and fall over the 13ft beam. The current decks are built of 3/4 plywood, glassed on the top, bare on the bottom. At this point, the deck has got several soft spots, several leaks, and quite a bit of rotten framing. The fish hold bounds the sides and bottom of the boat, so is completely fiberglass with the exception of the ceiling, (the rotten plywood) and fore/aft bulkheads, which are glass over plywood and in good shape.

    Through the process of rebuilding my decks, I would like to divide my single fish hold into 8 smaller insulated individual holds.

    For deck framing, I have been thinking of laminating some kind of structural foam together, then glassing the whole thing. As mentioned though, this boat has a 13ft beam, and in some cases the longitudinal frame members will have to span the entire beam without any kind of intermediate support.

    For the decking itself, I have been looking at Nida Core. Several internet articles have indicated that Nida-Core is not so great for impact resistance, and this being a commercial boat is likely to see plenty of impact on the deck.

    Whatever I chose to use for the deck core, it will have to be glassed from both the top and the bottom. Being that I have zero clearance in the lazarette between the fuel tanks and the decks, I am going to have to glass before installing. The problem then becomes glassing/bonding to the sides of the boat, because there is still no clearance between the fuel tanks and the side of the boat in the lazarette.

    Any general thoughts on my tentative plan?

    A couple prelim questions are:

    With enough glass, will Nida Core be strong, rigid, and impact resistant enough to last on the deck?

    What do you think of using glassed in foam for the deck frame core material? Any suggestions as to what type and density of foam would be appropriate for use as a core material?


    Thanks in advance!
     
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  2. keysdisease
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    keysdisease Senior Member

    You got 30 years out of glass over ply in commercial conditions. I would consider replacing with same but use epoxy resin instead of polyester. On the undersides of the deck plywood just epoxy resin, no glass. Epoxy resin coat the lumber too and your grandchildren will still have a strong deck to work on.

    Harder, more flexible, much stronger, much better adhesion, all worth the added expense.

    I think Nida Core is great stuff, and foam core framing is good when done right. But I believe just switching to epoxy will result in a strong, durable deck made to a known and proven design and against the additional time and expense of the proceses you mention perhaps a wash in cost. Epoxy is more expensive than polyester, but the entire project in wood epoxy is probably about the same cost as NidaCore and foam framing not counting the learning curve and waste.

    $.02 Steve
     
  3. gonzo
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    gonzo Senior Member

    Foam core panels are less puncture resistant than plywood. The thin skin will break through with the typical use on a commercial boat. A deck that lasted 33 years was well built. Most probably, the damage is from badly bedded deck fittings or small holes in the laminate that didn't get fixed.
     
  4. Saildude
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    Saildude Junior Member

    You want to make sure that when not if the bedding compound fails that the water can't get into the core. Many ways to do this, you want to do more than just coat the holes your mounting bolts go through with resin. Some people use solid fiberglass rods potted in the core, you can also make a plug out of resin with the proper high strength additives, depending on the loads you might think about insetting solid pieces if fiberglass sheets under some of the deck fittings also to spread out the load of there is any evidence of core crushing.

    I just finished a major project on my boat due to water in the core and all of the penetrations are now properly done so water should not ever be able to get into the core.
     
  5. sandusk
    Joined: Jun 2011
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    sandusk Junior Member


    I think the main problem was that the plywood only had glass on the top side. The bottom side (where it bounds the fish hold) was exposed to constant moisture. I am starting to come around to the idea of just using marine plywood for the decks, and some kind of foam to insulate/divide up the fish hold.
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Now you're on the right track. You can divide up you hold with boards and/or foam.

    Your decking system sounds fairly well designed. Had they employed epoxy in it's construction, you'd be looking a much less damage and repair.

    Build it as you see it now, but embalm each wooden element in 3 coats of epoxy, before installation. Also use a heavy 'glass sheath in the areas of high traffic. Consider Xynole fabric as the deck surface instead of regular 'glass, it's 6 times more abrasion resistant.

    If you elect to go the foam or other "cored" route, you should consult if not hire a NA to go over the deck framing and related areas, as it's application is very different then the wooden structure you currently have. These types of structures need to have a pretty high level of engineering employed, if you want them to work right and preform up to your expectations.

    If you redo your wooden deck and support system, by well epoxied, it'll probably out live you.
     
  7. sandusk
    Joined: Jun 2011
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    sandusk Junior Member

    PAR,

    I like the idea of coating both sides of the new plywood with epoxy prior to installation, but am unsure of a few things:

    My deck has got quite a bit of camber built in. I am positive that bare (no epoxy coats) 3/4in plywood could be screwed down to the framing and achieve the desired camber. After 3 coats of epoxy prior to install, it seems like the plywood would not be capable of bending enough to get the camber right. What about building a jig that holds the plywood with the correct camber while I epoxy and glass both sides, then screw it down to the deck framing? After screwing it down to the framing, I could apply one more layer of epoxy and glass to seal up the fastener holes?

    What type of 3/4in plywood would you reccomend using? We have 3/4in Fir marine plywood locally for around $100/sheet. It seems like a lot of people are happy with Meranti 6565 marine plywood, and I have seen it online for as little as $80/sheet. I have no problem ordering plywood, especially if the local stuff is too expensive and/or not good enough.
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The plywood will easily conform to the camber if coated with epoxy prior to installation. You could build a jig and precoat and 'glass everything, but now your into a lot of setup. I wouldn't bother and would seal the plywood with 3 coats, then install, then seal each fastener hole, then sheath with fabric.

    If you can get the BS-6566, then use it instead of the Douglas fir marine stuff. It'll be a better material to use and much easier to finish.

    Once the plywood is screwed down to the beams, go back, removing one screw at a time and run some epoxy down into the holes, then reset the screw. This will help tremendously, keeping the fasteners holes from rotting.

    Try not to over think this or over engineer it, as it's probably way over built as it is. The epoxy alone, will not add any appreciable stiffness to the plywood, you need fabrics for this and a fair bit of fabric at that.
     
  9. sdowney717
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    sdowney717 Senior Member

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

    And you opinion is based on what research, testing and long term trials? YouTube? Oh please . . .
     

  11. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    just my own experience and opinion.
    Epoxy sometimes cracks lets in water, then the wood rots.
    Frankly the wood still moisture cycles under the epoxy and it eventually lets go. I think the effect is cumulative, takes years to have an issue. I have read the water actually moves threw epoxy coatings.
    The other poly's give and move with the wood.
    I dont exactly trust it anymore, for me the new wonder goos are polyurethanes and polyureas. I can look right now where a piece of epoxy covered wood has let go the epoxy, the edge has cracked along the top edge and I am going to have to redo it.

    You do what you want, I just dont use epoxy much any more. I just repaired a rotten piece of boat decking where water wicked between the engine room vent got under the glassed cover and rotted out a 2 foot area. Rot ran under the toe rail and was starting into the rim wood.
    I repaired that using PL Premium Polyurethane Construction Adhesive and also mixed it with some wood sawdust to bulk it up as a filler. Infact, I did not even replace the glass covering but simply used the poly to build up a protective layer on the plywood surface.
    Great stuff, it expands as it sets. Sand it out with 40 grit random orbit. Keep it pressed down using a HDPE sheet such as cereal box bags. Sealed perfect to the existing glass covering which whole thing really ought to be replaced as it has DEBONDED over the 30 years where the epoxy and wood interface meet.
     
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