How can I fix this rotten mahogany cabin wall?

Discussion in 'Wooden Boat Building and Restoration' started by LandFish, Aug 17, 2021.

  1. LandFish
    Joined: Jun 2021
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    Location: Norway

    LandFish Junior Member

    Some of you might know I had plans of building my own liveaboard boat. However I stumbled upon this beautiful 38 ft wooden boat for 13 k usd. And I had to buy it. It has all the things I want from a liveaboard and has been used as one for at least 8 years. Though the people who lived on it prior haven't done much work on it.
    [​IMG]


    As far as I can tell from inside inspections the hull is in good condition.
    However it has some rot around the cabin, the worst spot is on the starboard side.
    [​IMG]
    A previous owner added a fiberglass layer on the roof. And water has crept under it and caused damage. Thankfully the teak deck underneath is in good shape. It is only dark because of some black sealant used to glue the rotten plywood to the deck.

    [​IMG]
    However the mahogany walls are quite damaged. I took out as much mulch as I could from the wood and I removed the old varnish layer on the cabin wall to help the wood dry better.


    I have tried to understand how the wall is built and I think this crude drawing I made is mostly correct as for how the pieces fit together.

    Outside:
    [​IMG]

    Inside:
    [​IMG]

    The blue pieces are solid 3 cm mahogany that have been routed so that the red pieces can fit inside them.
    Yellow is the deck and the green pieces are roof beams.

    The thing I don't know is how far the red pieces go under deck. I don't really know how to find out. I tried looking behind the wall panels inside the cabin, so that I could look up towards the deck, but there is no signs of mahogany there. Seems to be some other wood that is treated with lead oxide oil.

    Here is a pic of the inside wall. There is also a mystery rot spot (the black spot) under where the two windows meet.
    [​IMG]

    [​IMG]
    I've only taped to keep fiberglass dust out while I sawed off pieces of the roof.

    Now my question is how much should I replace? The rot is only on the top, except that one piece that is pretty far away from the problem area.

    Could scarf joints that are glued with epoxy work to repair these damages? Or do I need to replace the whole pieces completely?
    Any other advice for how to proceed?
     
  2. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    How can I fix it?
    Just make up a thick mix of epoxy and $100 bills, and slather the goop liberally over the entire boat, inside and out.
    Good luck!
     
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  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    You can scarf anything, but can you do it well?

    Anything that is not visible...remove all rot; none can remain. Use cpes on the wood that is left, then form and place thickened epoxy there. Use cabosil to thicken. It must be really stiff or it will sag. You are limited in thickness to avoid fire. Epoxy thicker than about 3/8" will get very hot. You can also use wood flour to thicken, but cabosil does not sag much, so I'd use some for sure.

    it is a wood boat and the best way to fix is new wood, but for a small bit of rot; create your own repair mix with say 50/50 cabosil and wood flour and epoxy...

    remove the inside trim and you'll likely see the cabin walls are simply set atop the deck
     
  4. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Any piece of wood that is more than about 5-10% gone, I would replace, and only save with patching pieces that are less damaged. Setting some standard will help you from going crazy. Or if a piece is fully rotten on a tenon; it must go.

    Say you have a board 12"x12" or 144 sqin,5 % std is replace if more than 7.2 sqin rot..
     
  5. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Replace any rot with wood not epoxy
     
  6. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    You can use scarfs to put in new wood, but you have to insure you bond to solid wood without any rotspores. For the area of the big hole, the entire piece has to go, you would have two scarfs at the ends, under the next plywood panels. The single black spot you repair by carving out a big hole with angled sides and glueing in a patch.
    The whole repair is more involved then you think and will involve some disassembly. First you take out the windows, then the trim battens top and bottom, top cover board and maybe the bottom waterway. Then the ply panels (red pieces) have to come out, maybe not all, but at least two (the one with the hole on top and the one with the "mystery spot", wich is no mystery at all, just water between plywood and solid wood). Then begins the fun of fitting new wood in, then everything goes back together.
    This is just scratching the surface, the real work comes when you have to make the deck watertight again, the previous owner did not put ply and fiberglass over it just for fun.

    When you remove the bottom trim batten (triangular piece between cabin side and deck) you will be able to see how the red pieces from your drawing are fitted and if you have to remove the waterway (first deck plank next to the cabin side) or not. It can be on top or inside that plank, no way to know right now.
     
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  7. LandFish
    Joined: Jun 2021
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    Location: Norway

    LandFish Junior Member

    Here are some pics of the area without the plastic covering:
    [​IMG]
    [​IMG]
    [​IMG]


    The big rot area is on average 4 cm in height. And 5 cm in height at the worst spot.
    Over this there should be about 0.5 cm mahogany ply wood, but it has also rotted away.
    [​IMG]


    Here is some mahogany wood I have for replacement, but it is only 2.5 cm thick which is not quite thick enough (walls are 3cm).
    However it is just 0.5 cm shy to be the same width as the outer roof deck plank. So it could maybe be used to replace that if I need to.
    [​IMG]

    I also have this mahogany table inside the cabin. It is almost 3 cm thick. 70 x 101 cm. It could sacrifice it to completely replace some of the pieces.
    [​IMG]


    The walls are 3 cm thick, so if filling with epoxy is limited to 1 cm its probably not a viable option for me. Or can I do it in several layers?

    Which piece has to go? The outer wood that is part of the roof deck?

    [​IMG]

    Yeah I thought I might have to disassemble the whole wall. The windows need to be resealed anyway, so its probably a good idea.

    The red pieces are also solid mahogany that are routed to fit with the blue pieces.

    Yep! I think I'll just patch the roof back up when I have repaired the side wall for now. And rather remove the whole glassfiber roof after winter.
     
  8. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    You need to understand rot.

    Any wee bit of rot is like an insidious cancer. You can scarf to a bit of rot and the rot will just start up again.

    The first step is no rot can remain.

    And, you need to budget for some proper mahogany. No shortcuts. Cutting up a table is nonsense.

    The only reason I suggest epoxy pb is because you cpuld have a dime sized section of rot that can be hogged out and just filled and there is no reason to replace an entire large board for it. Or you can whittle out a piece of mahogany and epoxy that tiny piece in.

    But first rule. Zero rot can remain.

    I once had a crown put on a back molar in my mouth. About 6 weeks later, I started getting a foul taste in the area. Went back and the dentist who owned the shop looked at the crown which had been done by one of his staff. He removed the crown and was looking in my mouth. He turned a little red. Then he asked the assistant for the xrays from 6 weeks prior. He turned really red and was visibly upset. He said I am sorry to tell you this, but we put a crown on a tooth with decay. About two crowns and 3 grand later, I had the issue resolved. He paid for the 2nd and 3rd crowns because the 2nd was made too small. No rot can remain. If you see a wee bit of it, hog it out.
     
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  9. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    The picture with the ruler still shows a lot of rot!
     
  10. LandFish
    Joined: Jun 2021
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    LandFish Junior Member

    I've started sawing away the rot with my oscillating saw.
    [​IMG]
    [​IMG]

    The way I check if I need to saw more, is I take a small screwdriver and tap along the wood. The places where it makes marks I cut away.
    [​IMG]
     
  11. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    This is looking better. I really like to use cpes on areas after they are cut. Any rot still there will be modified into a solid epoxy. I don't know if it is ideal, but I generally have soaked all areas in cpes once I cut away the old.

    These projects can take on a life of their own. This is why you need to consider the scope. The project here is pretty big. Did you pull trims off like Rumars and I suggested?
     
  12. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    The part you circled red is the top coverboard, you need o remove it to be able to access the top of the cabin sidewall (the one with the rot that you cut away). Then you take out the windows, and the front panels (red in your drawing), then you can see the entire top part (the top blue part). From this one (the top blue) you cut out the part with the big hole (where you are now cutting away rot) and replace it. It is theoretically possible to scarf in new wood without cutting everything away, but it won't look the same, and you still have to take out the red panels beforehand.
     
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  13. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Another thought I have to offer is you could cut out a straight line and epoxy bond a full length straight piece in. It needn't be a scarf. To template all those curves is pretty difficult to make nice vs a flat cut. Your goal is a 1 mm glue seam and it can be a butt seam here. Don't overtrust it to stay in place; some sort of blocking or mechanical hold is needed overnite. The cut itself will be tricky to make flat and you may need to sand with 40 grit to make the pieces fit. 40 grit is actually good. Prewet the wood with clear epoxy before the pb bond. Old wood tends to drink resin.

    It may seem silly, but I might be inclined to make a template from cheaper material first. That way, the mahogany would not have a oops unless cut poorly. At least a cardboard template may reduce errors.
     
  14. LandFish
    Joined: Jun 2021
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    LandFish Junior Member

    That is true, it is a big project! Thankfully I am not alone, I'm doing this together with my girlfriend who is a hard worker.
    We have already cancelled the lease on the apartment and intend on moving into the boat in november.

    Winter is approaching rapidly so time is of the essence right now and I need to figure out how to prioritize.

    I might have to postpone fixing the hole properly until spring because a lot of other things need to be done as well.
    Most of the caulking has to be redone, especially around the transom cabin because it leaks a lot.
    Many of the windows leak a little bit so they should be resealed, but they dont leak that much so I guess it could wait.
    We have stripped most of the old varnish off, but the whole cabin still needs to be sanded and oiled.
    And the fiberglass roof needs to be resealed with new plywood.
    The caulking on the deck needs to be redone as well, but I am not sure if it is a good idea right now. The previous owner decided to oil the deck with 6 layers right before we surveyed the boat. But he didn't redo the caulking first. I've heard that sikaflex doesnt bond well with oiled wood. Do you guys know anything about this?

    Is there a temporary fix I can do with the cabin wall so that we still can comfortably live aboard in the winter?
    Like temporarily filling the hole with expanding foam?
     

  15. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    You are gonna wanna fix that now.
     
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