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Thread: Reparing rot on a plywood dinghy

  1. #1
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    Default Reparing rot on a plywood dinghy

    I've got a good repair project that looks doable.

    I built this stitch-and-glue Weekend Dinghy III designed by Karl Stambaugh. Here's a picture of it when it was launched in 2007:

    launch_boat2.jpg

    I kept it upside down with a tarp on it. The tarp disintegrated, and water collected on the rubrails, and that's how the rot started. The rubrails were made of ash. I don't know why I chose that wood. I think I knew at the time that it wasn't so rot resistant, but it was probably what was available instead of what I wanted, and I figured I'd keep it varnished.

    Well, 15 years later, this is what the boat looks like:

    IMG_9317.jpg

    A few years ago I noticed it but I just couldn't deal with it. Then the plywood was rotted only about 3" from the sheer, and only in the forward part of the boat. It would have been an easier repair. I'm an idiot, but what's done is done. And now I want to repair it. Since I have other things I have to do around the house, this will take a while, but I can at least start by stopping the rot.

    A few good things: The solid mahogany breast hook and quarter knees are fine. The transom seems fine. All the structural elements are fine, too. That is, the large glassed epoxy fillets. The "stem" is solid as a rock. (Of course, an epoxied fillet won't rot, but it's nice to know these parts held up over the years.) The plywood frame piece running athwart seems mostly fine. I see a little yellowing, but it might just be damaged varnish. I'll check it out more closely. The seats are fine, too.

    So it's really a matter of cutting out and replacing the rotted plywood on the sides and then replacing rubrails. There are some places where the rot seems spotty. However, I think I'll replace long sections, as if replacing the "sheer planks" (if you will). That would help keep a fair curve. I might take off the same amount on each side, and then a little more where it's necessary.

    It would be hard to scarf the plywood in. I think it might work if I use a butt joint with fiberglass on the outside. Also, the rubrail will help support the repair and keep the lines fair.

    Another good thing is that I still have a mostly whole sheer line on one side. I thought I was going to have to redraw it from the plans. So I can copy this sheerline and transfer it to the other side. I'll have to trace it onto flexible plywood before I start cutting out rot.

    A big challenge is that I currently want to keep the inside varnished and not painted. So my repair has to look OK. I could slop on some veneer to cover up any lines, but then that won't look right. I think seeing some epoxied lines won't be a problem, since there's already fillets everywhere.

    OK, so here's the question: Once I cut out the rotted wood, do I need to treat the remaining wood where it's cut? I don't really know how fungus works, but I assume it's still there. And I don't trust it. Should I use Git Rot or a fungicide? I'm not trying to restore rotted wood. I'll cut out all rot that I see. But I'm concerned that rot spores might still be around and then start on the "fresh meat" when I cut the wood. Any ideas?

    Charles

  2. #2
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    Default Re: Reparing rot on a plywood dinghy

    Charles, I don't have a lot to suggest, but wonder if you have used an awl or similar sharp implement to check the extent of the rot / damage to the plywood sides - Did you note any damage / rot on the bottom of the dinghy ?



    Rick
    Charter Member - - Professional Procrastinators Association of America - - putting things off since 1965 " I'll get around to it tomorrow, .... maybe "

  3. #3
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    Default Re: Reparing rot on a plywood dinghy

    I'll check more carefully, but from knocking and feeling, it seems like the whole bottom is OK. It's coated in epoxy and fiberglass on the outside. Nothing's getting in there.
    The boat was upside down, and the rot started from the rubrails, then got into the plywood sides and slowly worked upward. I can feel exactly where the rot is. The marine plywood certainly slowed it down. But good idea to check with an awl or something.

  4. #4
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    Default Re: Reparing rot on a plywood dinghy

    You might look at some of the threads on repairing Mirror dinghies. The kits didn't include the best materials and they used polyester instead of epoxy, so there was a slot of rot. Fortunately, they have a huge and dedicated following who have been pretty good about sharing their rebuild/restoration efforts.
    http://forum.woodenboat.com/showthre...-mirror-dinghy

  5. #5
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    Default Re: Reparing rot on a plywood dinghy

    Anything is repairable, but it would be easier, simpler and more enjoyable to build a new one.

  6. #6
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    Default Re: Reparing rot on a plywood dinghy

    Replace the sides of boat...not really that hard. You never know how far rot has traveled in voids in the plywood.

  7. #7
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    Default Re: Reparing rot on a plywood dinghy

    Replacing the sides would mean completely dismantling the boat.
    Build a new boat.
    Itís an opportunity to try a different design or a different building method.

  8. #8
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    Default Re: Reparing rot on a plywood dinghy

    Is the ply occoume?
    If so , forget it , she is toast

  9. #9
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    Default Re: Reparing rot on a plywood dinghy

    It depends on how far down the rot extends and where the sound wood begins.
    If the rot does not extend too far down, fit new sheer strakes, glued lap clinker to the existing ply.
    Otherwise, as Bruce says, us it for a garden planter until it returns to the loam from which the wood grew.
    It really is quite difficult to build an ugly wooden boat.

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  10. #10
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    Default Re: Reparing rot on a plywood dinghy

    Quote Originally Posted by Dave Hadfield View Post
    Anything is repairable, but it would be easier, simpler and more enjoyable to build a new one.
    Yes. Very simple build. Salvage what you can, if anything, and start fresh. Or pick a new design you might enjoy building, and using, even more.
    David G
    Harbor Woodworks
    https://www.facebook.com/HarborWoodworks/

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  11. #11
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    Default Re: Reparing rot on a plywood dinghy

    Quote Originally Posted by David G View Post
    Yes. Very simple build. Salvage what you can, if anything, and start fresh. Or pick a new design you might enjoy building, and using, even more.
    My tuppence worth is likewise. I'd cut the sides up with a view to salvaging the mahogany and anything genuinely undamaged. If lucky you'd get the breast hook and knees, transom, the bottom and seats leaving you two new sides to mark out and cut then reassemble like the first time around. There isn't much to it shape-wise so I personally can't see much benefit nor great chance of varnish-able success of scarphing in 3D. I think it'll ultimately be faster to just cut the dinghy up into kit form again and start over with new sides.

  12. #12
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    Default Re: Reparing rot on a plywood dinghy

    Quote Originally Posted by Peerie Maa View Post
    It depends on how far down the rot extends and where the sound wood begins.
    If the rot does not extend too far down, fit new sheer strakes, glued lap clinker to the existing ply.
    Otherwise, as Bruce says, us it for a garden planter until it returns to the loam from which the wood grew.
    I'm more or less with Nick on this.I might vary it by cutting the sides part way down and attaching a longitudinal strip of 1/2 inch ply to the inboard face of the panel and then butting a new section against it.I'm guessing that about 6 inches down would be a good place for a first cut.

  13. #13
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    Default Re: Reparing rot on a plywood dinghy

    PIECE-O-CAKE

    'twon't meet "expurt's approval butt for the sake of sentiment and ease of repair she'd look fine from 10'

    you built her so you must still have the plans

    turn her downside upwards and level her out to the best of your abiity

    w/ the above mentioned ice pick method determine how far down the sides the rot extends and mark it so you can keep track of it

    measure down from the chine and make note of that distance then make equal marks along both sides at regular intervals(12")

    take a fair batten and line up the marks screwing it to the sides(these holes can be erased w/ filled epoxy)

    check the fairness of the batten's curve and insure that both sides are evenly spaced below the chine

    turn her back right side up

    carefully cut the center framing(vertical/shear high elements of the center thwart) free from the sides above the rot line leaving them in their original shape to the best of your ability

    cut the line you created

    this step could easily be dunn w/ a router and a flush cutter following the batten to end up w/ a symmetrical cut on both sides

    now just recreate the top portion of the sides frum the original plans to extend about 3/8" below the cut

    just overlay the new uppers to the original hull w/ epoxy and clamped gently so as to not starve the joints

    the slight irregularity between the framing and the new side will be hidden w/ new filets and fiberglass tape and new wales of a more appropriate material can now be installed

    finish to taste

    ONLY YOU WILL KNOW unless you tell or they read this post ;-)

    just an old cheapsteak's 2$ worth

    sw
    "we are the people, our parents warned us about" (jb)

    steve

  14. #14
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    Default Re: Reparing rot on a plywood dinghy

    Add a lapped sheer plank to replace the rotted ply and paint it a contrasting color. Now it's a feature, not a defect!
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  15. #15
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    Default Re: Reparing rot on a plywood dinghy

    If even Wizbang thinks she's toast then there ya are. And he'll fix most anything.
    Quote Originally Posted by James McMullen View Post
    Yeadon is right, of course.

  16. #16
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    Default Re: Reparing rot on a plywood dinghy

    Lots of great responses so far. Thanks. I never thought about the idea that a boat has a lifetime. Here's some perspective, though. Here's the boat in 2009:
    lucy_boat_2201.jpg
    That was when my dog first swam. A great day. That dog died this past year at the age of 13. Plus, I'm middle aged, and I'm starting to feel like the rotted version of the boat.
    I'm fixing the dang boat.

    I'm pretty sure the rot is localized. After I cut it away, I think I'll treat the newly cut edge. I spoke to SystemThree about their borate powder product that kills the fungus which causes rot. I can make a solution and let the edge soak it up. Then when it fully dries out, I'd seal it up with penetrating epoxy before doing the actual repair. That will seal in the borate, so it may help prevent the rot from spreading. Plus, there won't be any more moisture to feed it.

    I like the ideas for replacing the sides, creating a faired upper part of the sides, or adding a sheerstrake. Lots to think about. And the link to the mirror dinghy repair was helpful. Seems like it's possible to scarf a panel into a boat, even though it's a 3D surface. Might not be pretty, though (the belt sander caught my attention).

    Charles

  17. #17
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    Default Re: Reparing rot on a plywood dinghy

    I like the idea of cutting the topsides down and adding new plywood sheer strakes. That way you retain an existing sound structure on which to build.

  18. #18
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    Default Re: Reparing rot on a plywood dinghy

    Have yet to build my first boat, but have been reading this forum for about 11 years. On at least one thread asking about rot repairs, one reply was that the spores could be in solid wood up to two feet away from soft wood. If that is so, your boat could have spores all the way down to the chines or even into the bottom if there is wood to wood contact between the sides and the bottom. Don't know how far a borate solution would penetrate if only applied to the edges. A better chance of success with the the borate would be to wood the interior and exterior then saturate all surfaces, then let if dry for a month or two.

    It looks like the center seat/rib and rear thwart might be the only things salvageable, and maybe the breast hook and knees would be ok after borate treatment. Looks like this simple design would be much easier to build rather than rebuild. It looks like a new build would only take 1 more sheet of plywood than a repair would require. Your choice.

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