Acacia laid onto existing plywood deck

Discussion in 'Wooden Boat Building and Restoration' started by avialae, Sep 21, 2011.

  1. avialae
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    avialae Junior Member

    Hi to all in the Forums- glad to join!

    I have a Thunderbird 26', wooden boat from 1971. She's in great shape, been repaired and restored once about 10 years ago.

    To add a tad for reference: the Tbird is a plywood boat. The cabin-top deck and bow deck are painted plywood.

    I have had the idea to lay down a wooden deck atop the existing (and sound) plywood. Recently I came across a good deal on 2'' width acacia, and now I am feeling insufficient to resist the notion of doing this project. I thought I would take to the internet to see what advice might be out there!

    I understand it is a bad idea to varnish deck wood (due to the slippery nature when finished). The 26' is sailed completely from the cockpit and in this way there is little travel on the areas I'd like to 'wood'. I would prefer to varnish the deck, simply because of the protection inherent with it.

    Anyways, I won't go on too long here at first. Thanks to anyone who can offer their advice.

    Cheers.
     
  2. masalai
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    masalai masalai

    Why the "artificial bling". additional weight and risk of a slip and fall when not-sailing, (anchoring, coming alongside, putting on the trailer or whatever) ??????????????
     
  3. Petros
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    Petros Senior Member

    I love the T-bird, crewed in one several times. I may buy one some day.

    Other than the extra weight I think it could be done if you install it with a lot of sealant under and around the edges to prevent trapped moisture. If you trap moisture it will allow the deck to rot even faster than no cover at all.

    I would rather see a textured finish, or even a paint and fabric finish on a plywood deck. light weight, lower maintenance and good slip resistance, though it would not be as attractive.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Acacia is a fairly large family of trees, I have over 30 in my data base. I'll assume it's Albizia procera or a close relative and this is an interesting decking choice, though wouldn't be my first.

    You'd need to quarter saw it to get some natural stabilization. I'd also not use very wide planks either. It does machine well and has moderately good rot resistance, but being a coarse grain wood, it'll likely collect moisture readily.

    You first have to decide on a veneer or solid decking. Veneer would be the choice to save weight, but it's difficult to "spring" a veneered deck so a straight laid pattern is the usual way to go. The thickest veneer you'd want to use would be 3/8". This would be set on a deck covered with at least 12 ounces of 'glass and fastened from below until the glue cured (3M 5200 or epoxy). The temporary fasteners are removed, then the seams filled.

    A real laid deck would add substantially to the weight of the boat. This wood is moderately heavy at 40+ pounds a cu. ft. so again keep the planks as thin as you can, while still having room to bury fasteners deep enough to accept bungs.

    You can add texture to your varnished deck by sprinkling salt or sugar over the wet varnish. When dry you wash out the salt or sugar and it leave a slight texture which can be enough to permit working on the fore deck with some security. Yep, it'll look like it has texture in it, but it's not as objectionable as it sounds and if done with water ways or just in required areas (around the mast, strips on the fore deck, etc.) it can be quite "artful".

    Personally, this is way more "treatment" for a Thunderbird. It's an attempt to make a silk purse from a sow's ear, but to each their own . . . The Thunderbird has enough burden without adding this contrivance to the boat. I do understand the desire for a pretty laid deck, but I wouldn't put a Rolls Royce grille on a VW either . . .
     
  5. avialae
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    avialae Junior Member

    Thanks for the input guys. I knew I wouldn't be able to avoid the jeers- I got a pretty good chuckle out of the "sow's ear" :p.

    It's good to mention that this Tbird had it's cockpit rebuilt out of mahogany board, 3'' width. The boat had its' original mahogany back rest boards when it was taken up by the fellow who rebuilt it out from rot. This mate chose to make the seating, and floor out of these boards, and trimwork nearby. All of this was then varnished, the floor glassed. The 'bird was also built different than the standard plan originally- the cabin width extends out to the edge on each side.

    In this way the bow deck doesn't have the 'continuity' that you would normally have with the running lanes along the sides.

    The wood has already been milled to 2'' width board. It's currently at 3/4'' thickness which I am dearly hoping can be milled in two. The wood was not quarter sawn.

    PAR- many thanks for the writeup. Of course, veneer will be out of the question in this case. May I have your opinion on the possibility that the solid boards are simply fastened to the existing deck with a PL premium glue. Maybe a fresh layer of glass neath the board before they would be glued down? If, this could be a method worth attempting, would it limit the finishing options in the direction of varnish to steer away from rot issues? It would be great to install these boards in such a way that could allow a simple treatment with some linseed oil and regular maintenance.
     
  6. PAR
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    PAR Yacht Designer/Builder

    3/4" thick x 2" flat sawn boards will be problematic at best. You're going to get lots of movement from these boards as they wet/dry cycle, which will test each fastener's pull out ability, the caulk lines and any adhesive used to bed them down.

    Any plywood deck that receives a wooden overlay (of any type) needs to be stabilized with epoxy and 'glass. My usual recommendation is at least two layers of 6 ounce, better if it's 8. Once this is done, you can usually fasten the planks from below which prevents moisture from weeping past the screws into the sub deck. If the planks are bedded in 3M 5200, then you stand a good chance of no leaks and a long life. Lastly a deck fastened this way isn't likely to leak either, plus you don't have thousands of bungs to drill install, sand flush and not pop out.

    I wouldn't encapsulate the planking, just bed it down and fill the seams. This type of deck needs to wear uniformly and have a place for the oil to live. I think a 3/4" deck will push your bow down by an inch at least, which isn't good, but considering the preformance envelope of a Thunderbird, not so much a problem.

    I don't like PL premium adhesive, even though it is easy to apply. I wouldn't trust it on a weather deck that's going to be moving as much as this one. If the deck is fastened in the traditional fashion from above, I think it will be the death of the plywood sub deck in fairly short order. The combination of flat grain stock, none resinous or oily, coarse grained species and a traditional oil finish will be asking much of the decking choices made. Generally the decking is hand selected for grain run out, grain orientation, defects and color. Quite often traditional decking is some of the finest stock on the boat and it has to be or it'll leak, cup, pop fasteners, etc.

    Lastly when you do fasten this stuff down, orient the growth rings so the outside of the tree is facing up. Polysulfide is the seam compound normally employed. The two part stuff is best.
     
  7. Petros
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    Petros Senior Member

    PAR,

    I have never heard of anyone disparage the Tbird in this way. Yes it is an inexpensive boat, but everyone I know considers them excellent sailboats, especially in light air. Besides the non-traditional construction, what "burden" does the Tbird have to bear?
     
  8. avialae
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    avialae Junior Member

    Anyone able to confirm that 3/8'' X 2'' solidwood acaia boarding is preferable to 3/4'' ?
     
  9. alan white
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    alan white Senior Member

    Only if the planking is to be set in an adhesive that sets hard like epoxy. Then thinner is better. For traditional planking, you've got to have enough thickness to use screws effectively. 3/8" is really pushing it if its going to screwed down.
    Why not have the wood resawn/milled to 1/4" (thereby doubling its coverage) and epoxy it down?
     
  10. avialae
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    avialae Junior Member

    Good to know. I expect I will be using epoxy to fasten the board down. I'm waiting to hear from the mill as to whether they can third, or half the boards (1/4'' or 3/8'' final product).

    I've used fiberglass in the past, but not extensively. Can anyone confirm that to properly prepare the plywood deck for planking, you lay out the 'glass cloth on the deck, mix your 'glass and hardener, then apply it to the point of generally saturating the cloth.
    (Perhaps, any advice on how to keep a smooth finish at the edges of deck top- maybe a 'form' of some sort wrapped around to keep the liquid from dripping down)

    After the fiberglass is set, using epoxy to lay the board and sealant to finish in the spaces... (http://www.martin-yachts.com/yacht-building/building-phases/teak-beam-deck.htm)

    Given the consensus on Acacia (I will try to find out exactly which species) that it will have a good amount of movement, any advice on the spacing?

    Thanks all. Cheers.
     
  11. alan white
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    alan white Senior Member

    You'll get two 1/4" planks from one 3/4" plank once the pieces are dressed, maybe slightly more if you resaw on a skinny bandsaw blade.
    One issue I see with acacia is softness, though density can vary between heartwood and sapwood found in the same tree, the quarter-sawn heartwood being the most desirable. Frankly, it's good that you're willing to epoxy the deck down. Prep involves the original deck surface being dry, clean, and flat. Bare wood is ideal, though if the deck has already been skinned with cloth, leave that and just rough sand and coat with epoxy (three average coats). If bare wood, just coat with three coats of epoxy.
    Putty all holes and cracks and gouges.
    Spacing can be milled into the planking. One eighth inch is a good spacing. Actually, the pieces touch but because the left or right top edge of each piece is rabbeted out (1/8" x 1/8"). Polysulphide or similar caulk is then paid into the seams. The seam is first masked to each side with blue tape and a putty knife is used to scrape excess off. Tedious but worth the results.
    sand and seal with penetrating sealant or varnish with something that's not too slippery.
     
  12. PAR
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    PAR Yacht Designer/Builder

    I've never seen a laid or veneered deck last very long, over a plywood substrate, without a moderately heavy 'glass sheath between the two. This isn't an off the cuff common, it is the general recommendation from manufactures and prolific book writing experts as well as most engineers and designers that understand the issues involved.

    1/4" decking would be an ideal thickness for veneer, but it doesn't offer a lot of wear room, before replacement. This assumes the deck will be used and not just a varnished place to mount a set of twin chrome plate horns. If the deck is left natural, oiled regularly, scrubbed and walked on, you'll want the 3/8".

    You can epoxy this type of deck, but it would be better if it was bedded in polyurethane (3M 5200) and fastened from below. This will permit the planks to move, while the sealant bedding keeps things dry. If it's epoxied, you will freeze any movement, but because the stock isn't encapsulated, you'll be trapping moisture in a U shaped container (each plank), which will lead to no good fairly quickly. Although I have seen this done with success, the decks where also well maintained, which is key, because if the laid portion goes dry (loses it's oil) it suck up moisture like a sponge.

    Spacing should be a 1/4" and tile spacers are great for this. Fasten the laid portion down with fender washers and screws in the seams until the goo cures (at least a week, preferably longer), then remove and fill with polysulfide.
     
  13. alan white
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    alan white Senior Member

    I agree that those unencapsulated planks need to be kept well sealed. There's a reason few boats not belonging to millionaires have fancy decks to begin with. They are a lot of work to do well, and more work goes into maintaining them properly. This is true especially where bare wood is desired (where the sealant is the penetrating kind rather than on the outer surface like varnish).
    The low-maintainance route would require you to glass over the deck with epoxy (after bedding planks in epoxy) with maybe 6 oz cloth, several coats of epoxy, and a good UV resistant varnish (there are two-part formulations that last several years before recoating but regular varnish would be my choice).
    This is similar to strip canoe construction. The method works fine just as long as the varnish is religiously kept up but it can be disasterous if left un-recoated for more than a couple of seasons (regular varnish) as the sun burns through the comprimised varnish and destroys the underlying epoxy.
     
  14. PAR
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    PAR Yacht Designer/Builder

    On a hardwood like Acacia, I don't think a 'glass sheath over it is really necessary. This assumes the deck is a decorative element and will receive little foot traffic. If it will receive a fair bit of foot traffic, then an oiled finish will offer much better traction, if at the cost of regular up keep and wear.

    Alan is correct in that a sheathed surface will need religious care or you'll have both damaged wood and epoxy, neither of which are easy to repair. This is why there are the two types of decking approaches, a veneer for looks only, often varnished and a natural with oil finishes. I've had a number of natural decks and it's not so bad if you keep up with them, but hell if you don't.
     

  15. avialae
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    avialae Junior Member

    The attachment here shows the two areas I would like to lay the wood down. Note again how this Tbird has a cabin that extends the width of the boat.

    The main reason for the desire of a nice deck, if for the use of enjoyment sitting in the bow area, etc. The traffic along these areas is minimal, and is decreased also further by the fact that it is quite the step up on to the roof of the cabin from inside the cockpit, in order to get to the bow.

    I'm currently living aboard the Tbird, and she gets alot of sailing in; but still, minimal foot traffic up front.


    Is the sealant application a real tricky business?
     

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