Aluminum Kayak?

Discussion in 'Materials' started by Tedd McHenry, Dec 14, 2020.

  1. DCockey
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    DCockey Senior Member

    How did you you select the panel thickness in your preliminary CAD design?
     
  2. upchurchmr
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    upchurchmr Senior Member

    Cad analysis only tells you what you already put in as data and choices.
    Useless for real world issues.
    My analysis is based on previous examples.

    Including a piro a friend built. I helped him build a similar plywood boat and it was 40% lighter.

    Please publish your assumptions - stiffiness and strength of the aluminum you choose (what aluminum), allowable stress, factor of safety, assembly method, tooling required to shape the hull and deck.
     
  3. Tedd McHenry
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    Tedd McHenry Junior Member

    I started with a panel thickness that gave the same hull mass as 4 mm plywood, which seems to be the most common thickness for stitch and glue kayaks. Then I looked at loading per unit area on aircraft wings with similar unsupported area of panel and the skin thicknesses used there. That suggested that a panel thickness on the kayak thinner than the one that gives the same mass as 4 mm plywood would be suitable.
     
  4. Tedd McHenry
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    Tedd McHenry Junior Member

    C'mon, @upchurchmr , get a grip. This is just a simple question about a very preliminary analysis. I get it, you don't like the idea. But, please, none of us are impressed that you could think of names for some details I haven't investigated yet. Your "analysis" based on previous examples is hardly an analysis in your own terms. Heal thyself, first.
     
  5. Barry
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    Barry Senior Member

    https://www.ellero.com/wooden-kayaks/ultra-sport-wooden-kayak/

    The biggest issue as I see it is making up the joints. Looking at the kayaks on the link above, you would have to make every joint meet with an overlap, obviously but to do so near the edge, say 1/2 inches in, you will have
    to form this lip and the angle of this lip and the adjacent lip from the next piece will be changing their angles from front to back. Now when you are say attaching the deck to the side, the lip is going to be close
    to 75 to 80 degrees which will impart a stiffness to the edge/ panel and then it will be difficult to bend/wrap to the shape that is needed without buckling.
    Without an extremely tight fit, the panels will leak.

    Many years ago some of the riveted aluminum boats used a mastic tape between the lap joints. The rivets spaced on close centers then would only have to hold the mastic tight to form a watertight seal.
    This was on slightly thicker aluminum so there was some rigidity in the panel to hold the mastic in compression. With a thin panel, this would not be possible so the spacing of the rivets would have to be extremely close.
    Maybe a few inches.

    Ellero sells just a set of plans, I did not get into what comes with the plans, but perhaps full size paper patterns or a table of offsets, to make up the panels

    It will be interesting to see a finished project. Whatever sealant that you use, it should have some elasticity to move with the aluminum, from bending due to loading and also temperature induced length changes.
    Good luck
     
  6. gonzo
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    gonzo Senior Member

    The load assumption should take into account puncture resistance and point loads from rocks, etc. It is different from an airplane wing.
     
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  7. upchurchmr
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    upchurchmr Senior Member

    Tedd,

    The problem with a forum like this is that the OP rarely provides enough to info to understand what has been done with an "analysis".
    So I ask question (just a few) that might show what has been done and not done, or how it was applied.

    The basic question is what is your background / profession?

    Lets start with your aircraft analogy. Aircraft don't usually use the aluminum alloys used in boats. They use higher strength alloys.
    What did you assume?

    Second is that aircraft generally use a margin of safety much lower than boats. A higher margin will drive up the weight quickly.
    What did you use?

    Third would be what loading did you use? Assumed weight load times a factor to account for how the boat is used. For aircraft typically from 1.5x to 9x (the G force expected).
    What did you use. Actually I don't know what you should use, but I'm interested, and if you didn't use anything that would be very un-conservative.

    I don't need to "get over my self" this is an open forum. Like you, I can say anything I like - but it is better to be dealing with facts as much as possible.
    So I'm going to ask you for some facts. I notice you ducked most of the questions.
    Just to be up front, I was an aircraft structural dynamics engineer for military aircraft. Not qualified to design boats, but the basic engineering is at least similar.
    I never was a analyst, but I dealt with them and had to be able to understand them for 35 years.

    CAD analysis has been misused lots of times. The analysts I worked with with never use that alone, since it doesn't tell you anything about if the design will fail due to too high of a stress loading or buckling, etc.

    You really should be happy that I asked questions about "terms" you don't understand. It will help you work to get a better understanding about why your "design" will work or not.
    Then you can adjust the design without having to spend a lots of money on repetitive build tries.
    That's the only real value of using engineering.

    In spite of your attitude, I do wish you the best if you are serious.
    There are some very good questions above about the build process, and important issues about damage. Ever looked at an old aluminum canoe?

    Tedd, never mind, I finally saw your response in #18. Your "analysis" ignores everything I asked about.
    Sorry I was giving you more credit than I should.
     
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  8. Kayakmarathon
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    Kayakmarathon Junior Member

    For a first attempt with a material you are not familiar, I would recommend borrowing as many concepts from well-established designs. I would also build a model, which will be displayed on the fireplace mantle. When I think of an aluminum canoe, I remember the one I used to learned to paddle. It had many rivets, ribs every 12 inches, 2 bulkheads that contained flotation, a center thwart, t-beam gunwales, and two seats that also served as thwarts to hold the gunwales together.
     

  9. Rumars
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    Rumars Senior Member

    To get the same weight as 4mm okoume ply you need 0.8mm Al. This thickness has been used before for canoes, but those were shaped to gain stiffness. A flat panel design would need to use lots of ribs to achieve the same. It's doable, but I doubt you can gain any weight advantage.
    Simplest way to join the plates at the chine is to use an extrusion (or similar preformed from plate) and rivet the plates to it with some elastomeric sealant/glue in between.
     
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