Ply boat - filling gaps between hull and bulkheads?

Discussion in 'Wooden Boat Building and Restoration' started by boony, May 21, 2022.

  1. boony
    Joined: May 2006
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    Location: Sydney, Australia

    boony Junior Member

    24' cold-mould moulded plywood boat with many years. Needs general TLC but generally in good condition.

    For aesthetic reasons and for long term rot prevention I was wanting to saturate with epoxy the ply under the bulkhead and then completely fill the gap between the bottom of the bulkhead, frame, and hull with timber strips and thickened epoxy. The ply is the inside part of the hull and is 9mm thick. Attached are a couple of photos showing the area.

    The bulkhead is attached to the frame which itself is attached to fully bonded stringers. The frame also has a gap between the hull and their base. The shroud chain plates are attached to the bulkhead.

    In the bow area there is a bulkhead forming a watertight crash box when closed. There are no gaps between the hull, frame, and the bulkhead here.

    I've always understood that the more bonding of different parts of a boat to the hull the better as it will make it more stiff and stronger. It seems however what I want to do is not necessarily a good idea. There are discussions on some forums advising that this creates a hard spot.

    There are probably pros and cons with what I want to do but being a non-professional I want to make sure I'm not making a major mistake (as opposed to just a con)?

    Thank you

    20220517_172319.jpg 20220517_172324.jpg
     
  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Do you have rot now? It mustn't be epoxy coated if so; needs removal.

    I see no gap. Is the photo just not good?

    If there is a gap and water reaches it now and then; it would be better to tab the bulkhead to the hull with fiberglass tapes, but it must be done perfectly or a pinhole leak will not allow the boat to dry out and rot will be faster than nothing at all. And this means both sides.

    And to do it would mean removing all the paint.

    Do not booger that area up with wood and epoxy strips...

    Another easier method may be to just caulk the area with some good quality marine caulk. Probably not 5200..
     
    Last edited: May 21, 2022
  3. rangebowdrie
    Joined: Nov 2009
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    Location: Oregon

    rangebowdrie Senior Member

    Been a fan and follower of cold molding and its techniques for, oh, I guess some 40 years.
    Several times I've come across writings that say, (paraphrasing,) "Proper construction does not fit bulkheads directly
    to the inside hull surface, this creates a continuous "hard spot" that encircles the hull at that point.
    And: "Good practice is fastening a bulkhead to a frame, in which the frame is fastened to longitudinal stringers".
    I don't pretend to be an engineer, but this seems similar to the practice of using tapered foam strips inside a fiberglass hull that are wider than the bulkhead thickness, fitting the bulkhead within the foam, and then glassing the bulkhead in place.
     
  4. AlanX
    Joined: Mar 2022
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    Location: Perth, Western Australia

    AlanX Senior Member

    My understanding is that "hard spots" are more of a problem for fiberglass, rather than for plywood (or aluminium).
    I have not had problem with my plywood boat (7 m) bulkheads using "stitch and glue" construction.
    So if you don't have a problem at the moment, I would not worry about it.

    Regards AlanX
     
  5. boony
    Joined: May 2006
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    Location: Sydney, Australia

    boony Junior Member

    Thanks all for your replies. Yes its a bad photo. There's about a 10 - 15mm gap. There's no rot there at the moment though my shipwright treated some dry rot elsewhere per the photo below. The photo actually shows the other side of the bulkhead in the photos above.

    rangebowdrie, this is the exact description of the construction method of my boat "Proper construction does not fit bulkheads directly
    to the inside hull surface, this creates a continuous "hard spot" that encircles the hull at that point. "Good practice is fastening a bulkhead to a frame, in which the frame is fastened to longitudinal stringers".

    fallguy, to me tabbing the bulkhead to the hull with fiberglass tapes would more likely cause rot as it would leave a void behind (rather than timber strips with thickened epoxy) but maybe I wasn't clear. The timber strips would just be a filler material and would be epoxy coated for their own protection against rot. As for the thickened epoxy this is to deal with any irregularities as the timber strips are pushed into the space so that there are no voids or pinholes.

    Hopefully it doesn't sound like I'm arguing. I'm just trying to inform myself. I have a West System manual for repairing timber boats but I couldn't locate information specifically in regard to my query.

    Kind regards
    20220517_172406.jpg
     
    Last edited: May 23, 2022
  6. fallguy
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    fallguy Senior Member

    Generally, a 'hard spot' creates a deformity in the hull surface that is visible and near impossible to repair because the hull is larger than it ought to be.

    For the OP, if he sees the BH has transferred to the hull exterior; it is far too late to change it.

    For cold molded, single skin hulls, I have seen older boats transfer BH to the hull exterior, but the hulls were failing. Overtime, weight of the hull, say on a poorly bunked boat, etc, can also transfer some BH to hull exterior.

    The time to worry about that BH creating a hardspot has indeed well passed.

    The best way to forestall any rot here is glass tapes on fumed silica and epoxy fillets minimum 1/4" radius fillet. Use a ratio of about 2.1 fumed silica to epoxy, adjust for type of epoxy. All paint must be removed prior to any work. Tapes should be minimum 400g or 12 oz biaxial to 1708 and for this job a tape of 4" wide is best. The fillets go up and over the stringers and the tapes are darted at the midpoint of each stringer. After it cures, look for any laminating defects and repair with same epoxy putty next day.

    The reason tapes are better than wood is because they offer a home for the epoxy which is waterproof. And you can see the seam. Wood, on the other hand does not allow you to see the repair once jammed in place and, despite the plan to seal it; wood is hygroscopic, epoxy, fumed silica, glass are not. If any rot occurs between a poor putty job with the wood, water would pool there. And this is why I will repeat, don't booger that area up with wood. Doing nothing at all is better.
     
  7. boony
    Joined: May 2006
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    boony Junior Member

    Thank you for this information. Most of it I understand (or believe I understand) but in any case it seems like I need to hit a few more manuals or hire a professional for an hour or so for some pointers. Cheers
     
  8. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Your frame is already glued to the skin, the "hard spot" is a moot point. If you like you can simply mush in some thickened epoxy and form a fillet, or use some battens to save some goop.
    The only problem is that everything is painted, removing the paint is going to be a hard job with all those little corners. Unfortunately, you can't epoxy over the paint, so my advice is leaving it alone and clean regularly to insure no moist debris accumulates there.
     
    fallguy likes this.

  9. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    In my opinion, you are fixing something that ain't broken. If it has lasted all these years with no problem, that is strong evidence that no problems exist. Go sailing and enjoy the boat.
     
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