Stiffen Dinghy Floor

Discussion in 'Materials' started by bcripps, Jun 8, 2024.

  1. bcripps
    Joined: May 2011
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    bcripps Junior Member

    I’ve got an old 12-foot Carolina Skiff I’m using for a sailboat dinghy. The inner hull (the floor), is badly cracked exposing the foam. It noticeably undulates under minimum speed. I am planning to repair this by laminating in a couple of 3/8-inch sheets of ply (thin enough to aid in bending to the contour of the floor), with layers of glass roving and mat. The boat is polyester but I’ll use epoxy for the repair.

    I would appreciate ideas on a laminating schedule or any other helpful ideas. I’m in the DR so resources are limited.

    Thanks everyone,

    Bry
     
  2. Milehog
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    Milehog Clever Quip

    Keep in mind you need to taper the ends-edges of stiffeners to keep from creating hard spots in the existing structure.
     
  3. kapnD
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    kapnD Senior Member

    I’d be saving my epoxy buck$ for more technical work, using polyester — keep it apples to apples.
    I believe the Carolina skiffs bottoms are built of transverse blocks of foam glassed in series with a Z shape.
    The plywood probably won’t contribute much strength to the floor, I’d just put a layer of biaxial over existing.
    Of course it must be dried out and properly prepped first.
     
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  4. bcripps
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    bcripps Junior Member

    Thanks guys. Happy to have some discussion on my problem.

    The idea of using plywood is to stiffen the floor. I can’t imagine that putting a layer of biaxial would correct the undulating movement, nor would the repair last with the joint between new and old constantly flexing. And this would be, of course, a mechanical bond, not a chemical one. The ply will be butted up against the chine along the sides and the transom across the back, but to Milehog’s point, I would need to feather-in the leading edge... I guess by grinding a bevel in the leading edge of each sheet of ply and then over lapping the lower sheet with the upper. Then fill and tape.

    I e-mailed Caroline Skiff for more information about the construction lay-up but they couldn’t be bothered to respond. As far as prepping, I’ve arranged storage for two months before I start grinding the glass down with a forty-grit floppy disc in my angle grinder. The rougher the better, right?

    And I was surprised to hear you favor polyester over epoxy for the repair, kapnD. My understanding has always been epoxy has superior bonding capabilities and structural flexibility and is to be favored for any repair work. But if I had all the answers, I won’t have needed to ask :)

    Again, I really appreciate help with this. I have no idea as to how many layers of glass I need. I know I can get mat, cloth and roving locally. I’d have to look into having biaxial flown in from the States.

    Thanks so much…

    Bry
     
  5. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    The undulation is likely caused by the design combined with some failure mode.

    There have been forum posts showing the design of these uses transverse members, foam and that the transverse members may not be structurally attached to the floor.

    I may be inclined to repair differently if I understand the failure mode.

    If you lay under the boat and the outside skin is detached from the transverse members; hydraulics will destroy the hull. So, first, try to understand the failure mode; if one exists.

    My canoe is very thin and waves undulate the bottom.

    If the exterior skin or the sole is detached changes the approach.

    For the sole detached, I’d probably take out a section of inner hull laminate and add longitudinals down onto transverse, pour some 6 pound foam and skin over it all.

    For the hull detached, remove the entire sole and all the foam and reattach the exterior hull to transverse members, then follow back same as prior method.
     
  6. kapnD
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    kapnD Senior Member

    Drill some test holes, then, if the foam is wet, all bets are off, and you’ll probably be wi$er to look for a replacement skiff that’s structurally sound.
     
  7. bcripps
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    bcripps Junior Member

    Hey kapnD, I don’t need to drill holes to see if the foam is saturated. After a storm, water squirts up through the cracks in the floor for several days after bailing. But buying a new skiff here in the boonies is impossible. I’d have to build my own. And yeah, fallguy, I’m sure this was a crappy design from the start and lots of YouTuber’s agree with me. As you say, the larger Carolina Skiffs have transverse frames with no fore-and-aft stringers. Don’t know if this applies to my smaller boat (and the folks at CS aren’t talking), but the lack stringers would seem to contribute to the undulating floor and subsequent cracking. But I’m basically working on the beach here so major re-construction isn’t an option.

    The best I can come up with is a layer of cloth over the existing floor, two layers of 3/8 plywood, a layer of roving followed by a layer of mat. But all that is just a guess on my part. Might be a case of try it and see… and be prepared to add fore and aft stringers if necessary. Unfortunately they would be inside the boat and something else to stub a toe on. The other option is to beef-up the seats, attach permanently and cross brace the floor to the underside of the seats.

    Thanks again,

    Bry Skiff Floor Cracks .jpg Skiff Floor Cracks .jpg
     
  8. gonzo
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    gonzo Senior Member

    That is a boat that can be repaired, but not worth it. Epoxy is definitely a waste of money. I would lay 1/2" PT plywood bedded on liquid nails and screwed to the existing fiberglass.
     
  9. fallguy
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    fallguy Senior Member

    You cannot fix a soft bottom with a hard floor. The bottom will continue to be affected by hydraulics.

    Remove the sole liner to within 2” of the edge. Scrape out the wet foam. Make sure the hull bottom and transverse members are not detached.

    Add back foam and then deck.
     
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  10. gonzo
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    gonzo Senior Member

    I have seen many boats, particularly work boats with a plywood layer over the deck. The boat is not worth rebuilding. It will cost more than a used boat in good condition.
     
  11. bcripps
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    bcripps Junior Member

    Thanks gonzo. If I could buy a replacement dinghy here, I would. I’ve been looking for a couple of years and found nothing. The only option would be to ship from the States and the cost of shipping and import duty is enough to make rebuilding the only course of action at the present time. And, fallguy, the only foam available is the stuff you use for cushions. I don’t think that’s what you are referring to :)

    Anyway, thanks guys... I’ll have to struggle along with what I got.

    Bry
     
  12. bcripps
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    bcripps Junior Member

    Okay. I’m not getting any specific information as to a recommended layup schedule so, from the ground up, here’s what I’m planning:

    1. Grind out the cracks, fill with thickened epoxy and reinforce with cloth tape.

    2. Cover the complete floor with a layer of mat.

    3. Lay in a sheet of 3/8 ply, saturated, and set in thickened epoxy, from chine to chine and beveled along the leading edge.

    4. Add new chine logs of 1x2 lumber to tie plywood into the boat sides.

    5. Lay in a second, longer sheet of 3/8 ply, with a beveled leading edge, between the new chine logs

    6. Fill and tape all raw edges.

    7. Add a layer of roving and finish with a layer of mat and paint with polyurethane if I can find it locally.
    If anyone can see a glaring error in this, please bring it to my attention.

    Much thanks, everyone…

    Bry
     
  13. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Your plan is over kill.

    Remove loose pieces with your fingers no grinding.

    FG with mat and roving [mmrmr]

    OR

    Plywood set in bogging

    Paint

    No need for both fg and ply.
     
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  14. fallguy
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    fallguy Senior Member

    x2, 100%
     

  15. bcripps
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    bcripps Junior Member

    Okay, and thank you. Overkill is good, right? But on the other hand, I don’t want to do any more work on this project than I have to.

    So I’m led to believe that two layers of 3/8 ply should be sufficient for stiffening, not guaranteed I understand, but a place to start. I want everyone to also understand that this is a cruising dinghy, relied on to supply passage ashore daily. It is in the water all day, everyday. Often with an inch or two of water sloshing about in the bottom. The ply has to be sealed permanently against water ingress. So the solid laminate of mat, mat, roving, mat, roving suggested is perhaps preferable, if not more expensive. I guess that’s a decision I’ll have to wrestle with during the wee hours.

    Again, thanks guys. I’m a lot closer to getting this thing done.

    Bry
     
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