Stitch & Glue boat longitudinal strength

Discussion in 'Boat Design' started by joe1947, Mar 27, 2013.

  1. joe1947
    Joined: Mar 2013
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    joe1947 Junior Member

    I am new here and I have a question:
    How I get longitudinal strength in a boat built from plywood in the “Stitch & Glue method as the boat has no longitudinal frame members?

    I had built small boats in the past from plywood on frame, but this will be the first project in this method as it’s written that is saving time and money.

    Asked the question from Devlin but get no answer.
    Asked the question on Bateau forum but my topic was locked with no real answer.

    Thanks
    Joe
     
  2. tom28571
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    tom28571 Senior Member

    Joe,

    The answer should be fairly simple but without knowing more about the specifics of the boat or boats you are asking about, I can't give a good answer. I have never known my understanding of "longitudinal" strength to be a problem in any boat but that is probably not what you are asking.
     
  3. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    The plywood skin provides the longitudinal atiffness and strength. The joints between the skin sections are an integral part of the system.
     
  4. messabout
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    messabout Senior Member

    Joe, the ply itself provides sufficient longitudinal stiffness if (and only if) you have adequate crossmembers, or frames, or bulkheads, to keep the ply from panting. When I say panting, I mean springiness in the sides of the boat. You can stiffen the sides in many ways, such as adding decks or inwales. The seats in a boat like a skiff or rowboat will also reinforce the sides and keep them from panting. The chine joint will be radiused on the inside and reinforced with glass. If done well, that constitutes a stringer of substantial stiffness.

    If you have plans from Devlin or other competant designers, you can build according to plan and have confidence that the boat will be sound enough.

    Describe the boat that you are building and you will get other comments that may be useful.....and welcome to the forum.
     
  5. joe1947
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    joe1947 Junior Member

    Hi Tom
    Thanks for the replay.
    My question is a general question.
    In plywood on frame boat the longitudinal strength is given by the stringers, keel and the plywood connected to them.
    As in the “Stitch & Glue” boat don’t have longitudinal frame members, I would like to know how such a boat gets the longitudinal strength.
    I know many people are building boats with this method, but I didn’t find a satisfactory answer to my question above.
    Sorry if my posts are somehow difficult to understand, I am not native English speaker.
    Joe
     
  6. joe1947
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    joe1947 Junior Member

    After writing (slowly) my answer to Tom I see the other answers, thanks.
    It is clearer now for me.
    I begin to build a small trimaran (3.6 meter length) and I want to use the “Stitch & Glue” method, at least for the Ama’s to save building time. I made the design myself as I didn’t find a design for a trimaran of this size.
    Joe
     
  7. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    Joe,
    As explained somewhat by others, you get longitudinal strength from the skin as well as the longitudinal frame members. In fact, MOST of the strength comes from the skin, and only a little bit comes from the frames. You still need some frames and bulkheads to hold the skins in their positions. Without internal stiffening, the skins will buckle.

    Eric
     
  8. joe1947
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    joe1947 Junior Member

    Again, thanks for the welcome and answers

    I am attaching some PDF files, any remarks are welcome

    Joe
     

    Attached Files:

  9. tom28571
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    tom28571 Senior Member

    Joe,

    It does depend on the type of boat as well as what the others have said. For instance, many boats like stitch and glue kayaks may have no internal parts at all although bulkheads are often used mainly for sealing floatation compartments. In that case the longitudinal rigidity (which is the kind of "strength" you are asking about) is supplied by the covering deck and the compound shape of the hull panels joined together in a unitized body shape, like an egg. In many boats, stringers only keep the panels from "panting" or flexing as was stated, much like spreaders in a mast or intermediate guy wires on a communication tower. Another analogy would be an automobile where the highly compounded shape of fenders might need no internal reinforcement to hold their shape but the top, hood and trunk lid will always have an internal structure to give the needed rigidity on such large and relatively flat surfaces.
     
  10. joe1947
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    joe1947 Junior Member

    Hi Tom
    The subject is clear now (after the answers I get on the forum) for me and yesterday I started to build the AMA’s.
    I understand now that because the AMA’s deck is completely sealed (with access hatch to empty water if) and have two sealed compartments it will be very strong.
    Same with the VAKA that will have front deck, back deck and other reinforcing members (seats).
    Thanks for all of you for the help.
    When I will progress a little more with the construction I will open a new topic at the relevant board and all remarks, advices will be very welcome.
    Joe
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    Hi Joe. I went through the same learning process a few years back.

    As you noted the ama gets strength by being a closed box. The vaka with its open top would distort easily unless measures are taken to prevent it. With fore- and aft decks it will be stiffer in those locations especially if they are closed boxes.

    These boxes are connected by the midships segment: an efficient way to stiffen it is to add narrow side-decks, gunwales are less effective but simpler. My sailboat has a foredeck but is too small for an aft deck, so I enclosed the volumes under the side decks to form longitudinal sealed floatation chambers which provide torsional stiffness back to the transom, and double as seats.

    As others have noted, for ply-on-frame construction the frame provides a support for a plywood skin during assembly. In glued assembly the skin is stronger and stiffer than the frame. The main function of the longitudinal frame members is then to provide adequate area for the glued joints between the ply planks.

    In stitch and glue this function is performed by glass tape and epoxy, which saves significant weight. The glass tape can be viewed as a long hinge, although epoxy fillets provide some stiffness, but the strength and stiffness mostly comes from two pieces of ply attached together at an angle, like the corners of a cardboard box. This is further improved in a boat by the multi-dimensional curves of the planks, thus preventing the “hinges” from opening or closing.

    I moved away from stitch and glue some time ago. For small boats where the thin planks are easy to break during assembly, I glue longitudinal members or chine logs to the planks while flat. The planks are easier and stronger to handle during assembly, and the chine logs eliminate the need to use glass. It adds a few percent weight but saves a great deal of time drilling, wiring, taping and filleting the plank-to-plank joints.

    A card model of a design is an excellent method for understanding how the various elements of the hull work together. Flex it this way and that and you will easily see how forces applied at one location result in flexing in another place, and this reveals where reinforcement such as decks and gunwales can most usefully be added.
     
  12. joe1947
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    joe1947 Junior Member

    Hi Terry
    Thanks for the advices. I just finished cutting the AMA’s skin plywood, not made the holes yet for S&G. Your method you have mentioned “I glue longitudinal members or chine logs to the planks while flat” sounds me very interesting as is the closed thing to my experience of building with plywood on frame, (starts from 1978 up to day; 3.3 meter boat demountable in two pieces covered with glass and polyester, 2.35 meter dinghy paint only, 3.1 meter round bottom boat covered with glass and epoxy, 6.8 meter small yacht covered with glass and polyester, 2.9 meter dinghy covered with glass & polyester and 3.5 meter trimaran, (never see the water because come out very heavy) with paint only.
    I have two questions:
    Can you give some more details regarding your method with some basic drawing or a picture?
    You are using screws also in the structure or taking them out when finishing?
    I always left them in as I am afraid of bonding failure.
    Joe
     
  13. tunnels

    tunnels Previous Member

    Shape of the hull gives strength longtitudinally !! a shallow vee will give less strength than a deeper vee !! you can make small hull shapes from thick paper or cardboard and lightly glue together to prove the point !

    Screws are only used to hold things together till the glue hardens after its all cured hard remove the screws and fill the holes ! the glue holds a lot more than what a screw does !!
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    There’s a blog of a my canoe build at http://theancientkayaker.weebly.com/index.html - the build part starts about halfway down the page.

    The advantage of the method is, the chine logs can be thinner than the longitudinal members of a frame, which have to be stiff enough to maintain their shape while the planks are being attached. I use chine logs 2.5 to 3x the ply plank thickness, which provides an adequate gluing surface using epoxy. The total thickness of ply plus chine log should be less than 1/150th of the bend radius IMO, especially if the wood has grain runoff or small knots. Don’t even think about using wood with knots near the edges: I used questionable wood for the small sailboat also on the blog and it gave me trouble when the first log cracked: I had to make lots of cuts to prevent more damage.

    These remarks apply to flat bottoms. I don’t bother with screws for canoes which have narrow bottoms and seats, so the load is supported adequately. Wide bottoms can flex and have room to walk around on. That can concentrate the whole load on a small length of joint, so I used screws as well as epoxy to attach the bottom. The screws also helped clamp the joints which minimizes bonding failure. Some builders use fairly heavy screws with a sealant in the joint, but that calls for thicker ply on the bottom. You can use steel screws if you back them out before the epoxy has fully set and either insert bronze screws or just fill the holes, or stainless steel screws for freshwater only use.

    Having said that, I agree with Tunnels about the added strength of a vee bottom.
     

  15. joe1947
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    joe1947 Junior Member

    Tunnels
    Thanks for the answers.
    Terry
    Thanks for the answers and your blog is amazing!!!
    Joe
     
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