Joining plywood to cast iron keel in an old Thunderbird

Discussion in 'Wooden Boat Building and Restoration' started by Hercules, Oct 31, 2007.

  1. Hercules
    Joined: Oct 2007
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    Hercules Junior Member

    I am repairing an old Thunderbird plywood yacht. The hull around the keel was rotted and needed replacement. As plywood is particularly vulnerable on the edges for water damage, I am trying to come up with a better way to join the ply and the keel. I will seal the edges of the ply with a special epoxy as best I could, but want to go one step further and would appreciate any comments on the following.

    I am thinking of inserting a hardwood strip between the ply and the keel and perhaps also in between the plywood sheets. I am thinking that should any water find its way in, the hardwood will expand and stop further water entry.

    The alternative is to use a special waterproof glue that bonds to steel and wood, and expands to fill cavities.
     
  2. Landlubber
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    Landlubber Senior Member

    Hercules,

    The flat sectioned T'Birds are very good attachments for the keel, why is it that you feel the need to alter the basic design?

    rRplace the damaged ply, and bolt the keel back to the hog as it was originally, using, say 3m 5200 sealant at the faces of attachment. It would be a good idea to sheath the bottom of the boat though as many Thunderbirds were just painted.
     
  3. PAR
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    PAR Yacht Designer/Builder

    I wouldn't alter the garboard attachment, but I also wouldn't use 3M 5200 either, preferring to use polysulfide (3M 101) in underwater applications, especially if the pieces may need removal at a later date.

    Planking is a consumable, just like the air filter on your car, it eventually wears out and needs to be replaced. If you're trailering the boat, then I wouldn't 'glass the bottom, just let her dry out in the car port. If she's moored, then 'glassing is an option.
     
  4. Hercules
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    Hercules Junior Member

    Error
     
  5. Hercules
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    Hercules Junior Member

    Landlubber,

    Thanks for your reply. I do not want to change the design. The keel is still attached to the boat. The ply that form the hull join the keel where both is screwed onto the "hog" (not sure if this is the right term). I believe that water found its way in there over time and caused the rot. The hog was only slightly damaged. The hull took the beating.

    I have read on a few forums that a lot of people believe that because epoxy crack if subjected to some movement, it is almost impossible to guarantee a long lasting water tight seal even when stitching in those areas.

    If leaking might occur, will it not be a good idea to have a strip of hardwood say 15mm between to plywood edges (provided that are sealed properly) that could expand if whet and so stop further leaking? Perhaps even more so between the keel and the ply.

    You've mentioned 3m 5200 as sealant between faces of attachment. I am new to boating and have very limited knowledge of potentially good products or solutions. Could I use this joining the ply and then stitch over it?

    As to the bottom of the boat, I have stripped it completely (of paint) and let it dry for a few months. I intend to give it a coat of epoxy, then fair it with epoxy filler, then cover with another coat of epoxy and a final boat coat anti foul coating.

    PAR
    Thanks for your comment on 3M 5200. Could you eloborate on the reason for your choice of polysulfide?

    The boat will be moored (difficult to trailer because of the keel). I have been told that if water gets in between a glass surface and the ply, that the play will rot a lot faster. Could you comment on this?

    I will get some pictures of the area over the weekend and post them to show the problems I need to solve.

    Regards
    Hercules
     
  6. Landlubber
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    Landlubber Senior Member

    No, do not use 3m 5200 as a glue, it is a sealant, and as PAR rightfully says, 101 is a better underwater (permanently immersed) sealant, again it is not a glue.

    "Stitch over it".....are you referring to stitch and glue construction here? If so this is not the right way to be repairing this section. The hog (yes you are right) is the place to bolt the keel to, it is further supported inside the boat and right through the keelson.

    Also when you say give it a coat of expoy, I assume you are also using reinforcing material?
     
  7. Hercules
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    Hercules Junior Member

    Landlubber,

    Thanks for the reply and correct terminology. I have a lot to learn.

    How does 101 and sikaflex compare?

    "Stitch over it"... I've cut out rectangular pieces of the hull around the keel that needs to be replaced. Once refitted I intend to place a 50mm cloth over the joint to seal it. On the inside I will fit hardwood strip over the joints for strength. I was thinking of putting a strip of fiberglass across the joint between the keel and hull, hence the reference to stitching.

    "Also when you say give it a coat of epoxy, I assume you are also using reinforcing material?" I did not intend to. Perhaps you could help me out.

    A friend repaired a small ply dingy and reinfroced it with woven matting. The ply completely disintegrated within six months after the repair. Asking questions about this he was told that if you put fiberglass over plywood and the ply somehow did get wet, that decay will happen at a much faster rate. It apparently had something to do with the absence of oxygen, that started an anaerobic reaction or process??

    I have been unable to confirm this although a fair number of people seems to agree. There might have been other reasons for the sudden decay but I have not discovered them yet. Because of this I am cautious about the fiberglass although painting the hull with epoxy is very much the same thing?

    Regards

    Hercules
     
  8. Landlubber
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    Landlubber Senior Member

    Emmmmm, if the ply was damp, then yes it would promote rot, but if the ply is "normal' ie., about 12-15% moisture content, they sheathing the boat is only a good thing, although I know some that disagree.......

    My neighbour in Sydney in 1972 built a ply boat, I helped on that project, it was built with dynel sheathing, the boat was epoxy washed inside too whilst still raw, and today it is as good as new. Tap it and it rings.

    If your boat is soggy, you are just delaying the inevitable, if it is sound and dry, then by all means sheath it. Just 300gsm cloth is plenty to do your job.

    Plywood boats are very much underated, mostly because, like ferro, they are often, almost always, amateur built, and to various standards, if you call them that. Don't get me wrong about amateur building either, the best boats built are done by private individuals that have a passion and skill to create, but generally, that is only in their minds, the end results speak for themselves.

    Personally, if the old girl is getting on and feeling soggy, send her to heaven and start again. There is no good spending money on a useless project. You could, if you really like the design, strip it of all the fittings and make a new hull for her, but usually the fittings are in the same state as the rest of the boat. They are an easy design to build, they sail well, but from memory my Primaat was a better sailer than the T'Bird, at least I could beat them usually in racing.
     
  9. PAR
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    PAR Yacht Designer/Builder

    3M 5200 is more adhesive then sealant, and has difficulty holding onto to soaked wood, unless it was under considerable pressure during the cure. 3M 101 is a polysulfide and works much better in underwater applications. It is more sealant then adhesive, though it does have some adhesive qualities. Sikaflex 291 and 292 are commonly used in the industry. 291 and 292 are polyurethanes with 291 being the weaker brother to 292. 292 is almost as tough as 3M 5200, 291 has more adhesive qualities the 3M 101.

    When 'glassing old plywood, the surface needs to be very clean, well roughed up and the moisture content below 15%. A common problem with putting 'glass on old wood is surface contaminates, which limit the bond and eventually the plastic sheathing sheers from the wood. Old paint, sealers, oils, marine growth and other assorted things can get into wood, especially if it's be on a mooring. Getting this stuff out can be difficult and drying it can take quite a while, but it has to be done or the bond will suffer.

    Your repair area should have the new piece scarfed in, meaning the edges should have a taper (8:1 ratio of the plywood thickness or better). For any plastic resin (epoxy or other wise) to effectively seal wood, the entire exterior surface must be coated. This includes screw holes, cutouts, notches, end grain everything. If this isn't done then the wood isn't stable and will seek a natural balance with the moisture content in its local environment, which on a boat can be quite high. Of course this "moisture content" changes, so the wood swells and contracts with the amount of moisture available. This movement of the wood will quickly shed off a plastic coating and is the reason why every square inch of the wooden piece needs to be coated. Even a screw hole will permit moisture to get into the wood, which will eventually lead to coating failure and likely rot too.

    The piece (if I understand you correctly) you are replacing is a highly stressed part of the hull. Pictures would be helpful. Your repair needs to be able to tolerate the repeated, cyclic loading this location of the hull will have to endure.
     
  10. Gypsie
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    Gypsie Randall Future by Design

    Use Everdure to treat the ply, forget about 5200. The next poor bugger who wants to remove the keel will wreck the boat trying to get it off. Sika products are expensive, they need primers and they go off in the tube after use. Use Fixtech MSP 190, a flexable polymer based high strength marine adhesive.
    Check this site: http://www.gypsiemarine.com/convert_and_invert1.htm
     
  11. Hercules
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    Hercules Junior Member

    Gypsie,
    Thanks for the info. The ply around the keel that form part of the frame is not badly damaged, so your advice makes a lot of sense. I've had a look at the site you've referred. Do you guy's have brochures that I can view online?
     
  12. Gypsie
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    Gypsie Randall Future by Design


  13. Hercules
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    Hercules Junior Member

    Thanks
     
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