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Thread: Plywood stitching construction method

  1. #16
    Registered User cluttonfred's Avatar
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    Re: Plywood stitching construction method

    I was thinking more of monocoque-style construction and just use epoxy and cloth to hold the edges together. It would also be possible to use traditional longerons to tie the edges together.
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    Registered User BJC's Avatar
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    Re: Plywood stitching construction method

    Quote Originally Posted by cluttonfred View Post
    I was thinking more of monocoque-style construction and just use epoxy and cloth to hold the edges together. It would also be possible to use traditional longerons to tie the edges together.
    Or just stitch composite panels together, then laminate the seams.


    BJC

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    Registered User rtfm's Avatar
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    Re: Plywood stitching construction method

    Quote Originally Posted by FritzW View Post
    The elephant in the room is that the fuselage you drew in post #1 can't be made using the stitch and glue method. Your (very pretty) fuselage is the poster child for compound curves and you can't do compound curves with stitch and glue. It's all got to be flap wrapped.
    Hi,
    Here is a rough sketch of a part of one of the bulkheads (which will not be solid, of course, but will have the centre section removed, leaving a 2 inch perimiter). Notice, if the curve of the bulkhead is retained, we DO have compound curves for the plywood to bend around. However, simply by drawing straight lines between the longerons, we end up with flat wrapped plywood. The challenge is to accurately draw/cut the individual strips of plywood. One could assemble the bulkheads/stringers first, and then use this structure to draw each strip. Shouldn't be too difficult. Once the strips have been drawn, cut and verified, one would have accurate templates for any additional constructions.

    And no big epoxy fillets. Bond the plywood to the 15mm square stringers/longerons with T88 - very light, and very strong. I have seen videos of "how to build using the stitch and glue method" where they use epoxy bulked out with microballoons! CRAZY! A one-stop method for making weak joints. Plywood to longeron/bulkhead joins using only T88 is going to be way stronger (and lighter).

    Click image for larger version. 

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    DevFus (I'm not sure if you are familiar with the package) does a great job of drawing the bulkheads, as well as the building jig. Here's a screenshot...

    Click image for larger version. 

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  4. #19
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    Re: Plywood stitching construction method

    If you poke around the boatbuilding world, you will find free programs that figure out the plywood shapes and even show how to plot them on plywood sheets with minimal waste. Alternatively, you can find an aircraft with the right sort of fuselage shape and buy a paper model of it!


    I'm pretty sure the epoxy and glass used to join the sheets is lighter than using a stringer. Plus more of the fibers run the right way. (On a stringer, none of them run the right way.) The epoxy used for the fillets can have microballoons in it and probably doesn't need to be much heavier than the wood involved, especially if that wood is birch.

    Seems like the diagonal plywood strip method wouldn't have to be any heavier than plywood. And it, or something like it, has been used for aircraft many times. Weren't a bunch of Lockheed fuselages made that way?

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    Registered User Dan Thomas's Avatar
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    Re: Plywood stitching construction method

    Way back in the '70s I built two small boats with stitch-and-glue before I had heard about anyone else doing it. Those boats were heavy for their size, as I had to use thicker plywood to get the required stiffness. I'd have been better off (lighter) making a light frame and thinner ply. The resin and glass on the inside does add weight. I did the entire outside in light glass and resin, for waterproofing and weather resistance.

    I also designed and built a frameless 11' boat, and it was REALLY heavy due to the need for much heavier ply.

    Famous aircraft designer once said that lightness was not simple. It most often requires a complex structure. I've seen that in aircraft, having once started a Maranda with its 1/4" plywood wing and tail ribs. Very quick and simple to make, but much heavier than ribs made of 1/4" spruce capstrips and small 1/16" birch ply gussets, as my Jodel and taylor had. Lightness means placing the weight only where it will do the most good: along load paths.

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    Re: Plywood stitching construction method

    The "big boys" in stitch and glue, talk about reasonable weight. Not light weight. Now.. if you do stitch and glue, with frames, you end up with a very strong structure...... :-) But then, if you're doing frames.... might as well do a strongback and build off of frames right?

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    Re: Plywood stitching construction method

    I could carry my 14 foot stitch and glue rowboat over my head. I think it weighed about 75 lbs. It had enough displacement for 5 adults, but that was bad for the handling. The 8 foot, tubby stitch and glue dinghy I built was even lighter. Both of these were either 1/4 or 6 mm ply, with no bulkheads except the transom and, in the case of the dinghy, the bow. These were fairly sturdy boats.

  8. #23
    Registered User Dan Thomas's Avatar
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    Re: Plywood stitching construction method

    Of the little boats I built, the first weighed 36 pounds with its oars, which clamped across the gunwales lengthwise and had padded saddles on them for carrying on your shoulders. It was made of 1/4" ply for the bottom and transom and 1/8" for the sides, and had 1/4" x 1-1/2" spruce gunwale stiffeners. Did the interior corners with strips of glass cloth and poyester resin, and the entire outside with the glass and resin. The seat was a 1/4" piece of ply sitting on 2" styrofoam transverse ribs and attached to the sides.

    The second boat was of identical shape and size. I made it of 1/8" throughout and used cardboard formers as stiffeners along the gunwales and across the transom, and covered it with old fiberglass curtains and resin, and the curtains and cardboard soaked up a lot of resin. It weighed 39 pounds. It was really rough-looking compared to the first. I have a picture of that one only, with my Dad in it. It was just over five feet long:

    Click image for larger version. 

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    The first boat was the same shape but much nicer. I built a third boat, six feet long, out of urethane foam sliced into boards an inch think and glued up into a boat shape and glassed inside and out. It weight 26 pounds with its oars. We used all of them to get into a trout-infested lake that had no roads into it. We had formerly been trying cheap inflatable dinghys, but you either inflated them at the lake, which took forever, or inflated them first and packed them in, and crashing though the forest would puncture them. And on the lake there were numerous deadheads with sharp, broken snags sticking out that would deflate your dinghy real quick, and the short little oars and cold boat bottom (on which you were sitting) made life miserable. These little wooden boats bashed though the bush and took the snags without any trouble.

    Youi didn't dare stand up in one. Doing so shifted the CG so that the transom would go under and you'd ship water quck. I watched Dad try that, to reach a snagged lure in a tree reaching over the water, and he instantly understood what I'd warned him about. He was used to his 11' aluminum rowboat, and one time he caught a fish in the tiny boat, and after landing it he reached behind himself to drop the net in the boat like he always did in the aluminum boat. With the next fish he reached for the net but it wasn't there. There was no boat there! Short, they were. Very maneuverable.

    If I had built frames and planked them I think I could have built them to come in under 30 pounds. One could eliminate the wire and interior cloth and resin, and eliminate the double or triple layer over the exterior corners. All ply would be 1/8".

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    Registered User Victor Bravo's Avatar
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    Re: Plywood stitching construction method

    Sorry to drift the thread off into a different direction, but if a stitch and glue plywood boat needs to be fiberglassed and "buttered" with cab-o-sil then why not just carve the hull out of foam instead of stitched wood and use the same time and effort to fiberglass it with heavier cloth? That way you would have a curved hull cross section in whatever was the best shape for hydrodynamic efficiency, instead of a polygon shaped boat hull?

    I cannot imagine the effort required to lay out a multi-planar stitch and glue boat hull made out of a dozen flat strips, done by manual measurement and mathematics. A CNC design program would almost be a requirement!
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    Registered User Dan Thomas's Avatar
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    Re: Plywood stitching construction method

    Quote Originally Posted by Victor Bravo View Post
    Sorry to drift the thread off into a different direction, but if a stitch and glue plywood boat needs to be fiberglassed and "buttered" with cab-o-sil then why not just carve the hull out of foam instead of stitched wood and use the same time and effort to fiberglass it with heavier cloth? That way you would have a curved hull cross section in whatever was the best shape for hydrodynamic efficiency, instead of a polygon shaped boat hull?

    I cannot imagine the effort required to lay out a multi-planar stitch and glue boat hull made out of a dozen flat strips, done by manual measurement and mathematics. A CNC design program would almost be a requirement!

    The tiny boats needed hard chines for stability. Hydrodynamic efficiency didn't enter into it, or they'd have been pointed at both ends. Canoes and kayaks, of course, are better with rounded bottoms.

    Not many boats are made in sandwich fashion anymore, I don't think. The one I built was light, but I only used a single layer of cloth and resin over the whole thing, while the outside would have needed more layers to improve puncture resistance. Airplanes don't deal with snags and branches, but boats do. making it strong enough to really bash around would end up with a boat as heavy as a fiberglass or aluminum affair.

    CNC. Yup, would be nice, but there are plenty of us that like to work things out for ourselves. It's the same sort of motivation that causes guys to spend ten years building an airplane to their tastes rather than just buying a used airplane for the same money and be flying it the same day instead of ten years on. There's a personal, or human, touch to it that the machine can't offer. We could build a computer to read and play classical music and sound like an orchestra, but it wouldn't have that same human feel to it.

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    Re: Plywood stitching construction method

    When I was younger I used to do a lot of whitewater canoe and Kayaking. Borrowed molds and we built 3 glass Kayaks, use an narrow aluminum canoe with a raft tied down in the middle, one friend built a 17" two hole fiberglass kayak that weighed 35 lbs. I put class 5 as my limit in the canoe.
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    If its not there, it cost nothing, weighs nothing, and is 100% reliable.

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    Re: Plywood stitching construction method

    Unless you mold custom ply like an Eames chair, you are going to have a mess with curves going in two directions. The boats are only single direction curves. The boat thing with stitches is kind of a traditional take, but kind of strange for an airplane. Why are we not just stapling the ply to the stringers when gluing? Way less work.

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    Re: Plywood stitching construction method

    You're right that if there ARE stringers, it would be easier to staple the ply to the stringers, though I think it would still make sense to use tape on the outside. However, that assumes that there ARE stringers and that they coincide with joints between pieces of ply. I think glassed joints between ply pieces would act as stringers anyway.

    Victor Bravo:
    The Mirror dinghy dates back to 1962. I would bet that they didn't use a computer for the design. OTOH, they didn't have that many strips, either. In any case, a dozen strips is probably too many. Probably less work than a bunch of other construction methods, though.

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    Re: Plywood stitching construction method

    I have built two ships in stitch and glue. A racing sailboat and a micro-runabout with an E-Bike drive as the propulsion.

    There is one SW available that is being used by almost everyone developing stitch and glue:
    https://sourceforge.net/projects/fre...Version%202.6/

    It is easy to use and when clicking on develop plates, you get ready to cut DXF files or patterns you can print out. It was later evolved into the SW Delftship, but I think developing plates is not free anymore in Delftship.
    Freeship/Delftship also has a lot of nice features, including showing you the weight when you enter thickness and relative weight.

    In addition I would not encourage to use stringers, as the strength comes from the bends. You should rather have a thicker sandwich (then use foam instead of plywood), but then we slowly drift away from stitch and glue to positive mould composites ;-)

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    Registered User rtfm's Avatar
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    Re: Plywood stitching construction method

    Quote Originally Posted by Scheny View Post
    I would not encourage to use stringers, as the strength comes from the bends. You should rather have a thicker sandwich (then use foam instead of plywood), but then we slowly drift away from stitch and glue to positive mould composites ;-)
    Hi, and thanks. The only reason I am considering stringers is to assist in lining up the plywood strips. Just something to staple and then bond the plywood edges onto. So, not actually stitching at all. I was thinking in terms of 10mm square...

    I did take a look at Delftship - but I'll take a look at Freeship also. Thanks for the tip.

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