Nesting Proa Strength

Discussion in 'Boat Design' started by Cedric Oberman, Mar 15, 2021.

  1. Cedric Oberman
    Joined: Nov 2020
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    Location: uk

    Cedric Oberman Junior Member

    Hello All,

    I'm building a simple 3 plank hull for a 13' proa as a lockdown project. The hull is in two halves for convenience (I don't have access to my garden apart from through the house) I've looked at a number of nesting boats and dismantlable boats which bolt together and they don't seem to have a great deal of reinforcement around the joint. I have reinforced the bulkheads which will bolt together abouve the waterline, and I'm thinking of maybe having a a 2/3ft section of the protective wooden skid that I plan to run along the bottom of the hull which is removable and fits across the joint through bolted through the bottom of the boat onto nuts epoxied into the spine inside. Tightly lashing the crossmembers together both sides of the hull where they pass through the hull just below the deck, ought to add strength to the top of the joint I think.

    I'd be grateful for your thoughts. Am I over building this? I intend sailing on light flatish days in the sea, but I have to land on the stony beach, so I want a robust craft and I imagine this is the weak point.

    Keep safe everyone
     

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    Dejay likes this.
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Hello Cedric,
    Re your previous posts, have you finished building the smaller outrigger for the proa?
    Which design did you choose in the end?
    Having a bolted on shoe on the bottom sounds like a good way of transferring loads into the hulls, rather than having everything just on the bulkhead joint.
    And the gussets / brackets shown on your bulkhead in the photos above should help re strength.

    You mention "Tightly lashing the crossmembers together both sides of the hull where they pass through the hull just below the deck, ought to add strength to the top of the joint I think."
    Could you elaborate a bit on this please, perhaps with a photo or a sketch?
     
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  3. Zilver
    Joined: Nov 2007
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    Location: Amterdam the Netherlands

    Zilver Junior Member

    Hello Cedric,
    It looks to me like you are overbuilding some parts, and underbuilding others. The epoxy filet between the bottom ply and the sides looks small, and there's no glass tape on top of it ? In the Gary Dierking canoes the chines are from fir wood, and they essentially form a box together with the frame from the bulkhead. If you make something like that (the boat section like a fir batten box, covered with plywood) it should be strong enough and you wouldn't need the knees (although they won't hurt of course), and the bottom skids could be on either side if the hull connection, only for protection and not for strength.
    If you haven't got it already it might be helpful to buy the "building outrigger canoes" book, it's full of good ideas for these kid of boats.
    Good luck, Hans
     
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  4. Cedric Oberman
    Joined: Nov 2020
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    Location: uk

    Cedric Oberman Junior Member


    Hi Bajansailor,
    None of this is based on a set of plans, I couldn't find one that really answered my rather restrictive requirements on the maximum dimensions of each component, (6'6") so I've just approximated the hull form design of the waapa with more freeboard, a deck on the top and a cat rig. Here's a picture of the mostly completed outrigger (only the taping of the seams and an access hatch to do).

    re, lashing the cross-members; I really can't draw, so I'll have a go at describing what I mean a bit more clearly. The crossmembers pass right through the hull via a reinforced box bonded to the bulkhead, (see pic 5785) so, a strong bit, I figure that joining them during assembly using a tightly lashed rope parallel to the sides of the hull would compress the joint between the two hulls a little and hence resist any left and right forces that might stress the bolted bulkhead joint between the two hull halves. Possibly not necessary?
     

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  5. Cedric Oberman
    Joined: Nov 2020
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    Location: uk

    Cedric Oberman Junior Member

    More strength.
     

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  6. peterAustralia
    Joined: Mar 2006
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    Location: Melbourne Australia

    peterAustralia Senior Member

    Am sure your boat will be OK. One typical way of building is called stringer frame. A stringer placed at a corner is called a chine. A stringer placed at the top of the deck is called a wale, you can have inwales or outwales or both. You build the frame first, add plywood later. Epoxy resin is used. For really cheap boats you can dispense with the epoxy and use gorilla glue and paint, downside is lower boat life ,,, probably wont last 15 years, The wood (say 2 x1 for a boat of this size), forms the inside of the joint. You then epoxy the plywood on, use thickened epoxy to ensure good adhesion. It can be useful to add some stainless steel screws by drilling from the outside of the plywood and into the stringers and chines.

    The screws are optional and probably not needed if using fiberglass on the outside. Once you have secured the ply, you can then give it a radius by filing and sanding and then add fiberglass tape on the joint (seam) using 4 or 6 inch fiberglass tape.

    In the photo above, to secure the bulkhead to the bottom and side panels, I would have used 2 inch x 1 inch timber on all three interior joints, and then used small stainless steel screws to both secure the timber to the bulkead, and also to secure the timber to the hull panels. If you choose not to use fiberglass, you can just glue your ply to the stringers. Then use stainless steel screws from the outside to secure the ply to the timber. SS screws are pretty cheap nowadays. Galvanised screws would start to rust after say 18 months, based on my experience

    Building outrigger sailing canoes book has been pirated and can be downloaded for free off the internet. Another book that I really recommend, is by Gavin Atkin, Ultrasimple boat building. I have both books, Both are about 15 dollars each, I think these books are best for simple low tech boats that you are building

    Your attachments for the crossbeams to the mainhull are probably too complicated. It is done now, It would have been way easier just to secure the crossbeams to the top of the hull on each side. It does not need to be super complicated

    Below is a similar sized boat
    tacking-outrigger.com e15 tacking outrigger http://www.tackingoutrigger.com/e15.html
     
  7. Cedric Oberman
    Joined: Nov 2020
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    Location: uk

    Cedric Oberman Junior Member





    Hi Peter,

    Thanks for taking the time to reply to my post - I appreciate it. The 15ft boat you sent the link to looks good and seems very much like mine - a lot more authentic looking actually and slimmer. I'm new to building, so I've probably overbuilt mine in some respects but the joint between the two sections seems an obvious failure point and I thought I'd solicit the wisdom of experienced builders. I've beefed up the fillet between the bulkheads and the sides and added a layer of glass tape. Do you think I should also tape the outside seams of the hull? I have the materials, but I'd rather leave the hull with its attractive wood finish rather than paint it white, and I think the tape will spoil the looks rather. I'll do it if I need the strength of course, but the boat's made in 6mm marine ply with a hefty fillet joining the panels, so I'd say it's fairly robust. Maybe I could get away with just taping the bottom?

    I'm also adding stainless steel screws on your suggestion.

    All the best from the uk.
     

  8. peterAustralia
    Joined: Mar 2006
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    Location: Melbourne Australia

    peterAustralia Senior Member

    I think it is strong enough as it is. I actually broke the joint on my previous boat at this exact same place. I was puzzled at the time where all the water was coming from. At that time I knew little about boatbuilding and had no fiberglass tape, just the fillet, combine that with using the motor going straight into a steep short chop.. and... . You probably don't need screws, plus you can only use them if you have timber to screw into, versus only ply at moment. . Summarizing, as is it is strong enough... Don't forget to down load the outrigger book off the internet for free in PDF format
     
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