Microcatarmaran

Discussion in 'Fiberglass and Composite Boat Building' started by sebaseba, Apr 14, 2020.

  1. Dejay
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    Dejay Senior Newbie

    If you use a 56 to 10 sprocket you'd get 5.6 ratio. Those are the most extreme I've found. With a more normal and cheaper 48:11 you'd get 4.3 ratio. I'd rather use a bigger prop though.

    A bit off topic but using a chain could also be a great way to build a DIY saildrive or outboarder with an electric motor. Chains and sprockets are both efficient and cheap as replacement parts for motorcycles. Unfortunately you can't use the sprockets as part of the gearing since most electric motors most likely rotate too fast for efficient props.
     
  2. sebaseba
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    sebaseba Junior Member

    This is a good site for all the mechanical movements: 507 Mechanical Movements http://507movements.com/

    I think with a flexible shaft you can make the whole thing much lighter and simpler. Also easy to make the propeller turn in the direction you want (vector thrust?).

    Chains and belts seem OK, not sure if I should stick to gears or those. Nylon gears are supposed to be good, quiet and rust proof, also self-lubricating. Although it's recommended to add lubrication. I think I'll have to study a bit more, but I just wanted to start somewhere to not be only in theory. :)

    I bought also some epoxy, some rowing fiberglass, styrodur (XPS). I'm waiting for the styrofoam cutter tool to arrive. I've seen that balsa is harder to find, I guess due to COVID-19? Also PVC foams like Divinycell aren't that hard to find, they just have different brand names. I've seen that they are used for making signs.

    The rowing which I've found is folded. From what I've read that's not good as it's better that it's rolled, otherwise some strands could be broken. So I have to find also that. ^.^
     
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  3. sebaseba
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    sebaseba Junior Member

    I've checked this hydraulic jack IRL yesterday.
    [​IMG]

    It costs 12,34 € and underneath you have two holes with threads. One is covered with "DO NOT ADJUST", it has a screw. Probably to set maximum pressure. The other one is a screw, that you can turn withot tools, to release pressure, but you can remove it. I've tried in the store and hydraulic fluid comes out.

    I presume you could remove it and add a pressure gauge/sensor + a valve to release pressure.

    The idea is to maybe make an universal test machine, to test composite materials' theory to practice, to see when they fail, to find the strength of yield.

    I was thinking for some time to make an universal test machine, to finally settle this XPS/PVC/PS/PU foam debate, but how to do it was always a tad expensive. Hydraulic jacks mostly needed to be drilled to add the gauge (according to youtube and the other models I've found), but this option is cheap as hell and already has a hole together with threads. :)

    They had also models that look the same, but are bigger and for more tons, but I want to make the device small, so to not take that much space. Maybe I'll get the 4T model, I don't know.
     
  4. Dejay
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    Dejay Senior Newbie

    There also was some discussions and ideas in the thread "Epoxy over XPS method". Basically it comes down to low shear strength which you can somewhat compensate for by making the hull much thicker (see boston whaler in that thread). There were also ideas to add corrugations or struts into the XPS foam which should in theory make XPS or PU much more usable.

    I think to measure force you can also use various types of scales.
     
  5. sebaseba
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    sebaseba Junior Member

    Yes, that's what I've found when I did the calculations, that the limiting factor is shear strength of XPS, but still it would be kinda cool to be able to test things. :) :) I'll see if I'll do it or not, maybe I should just start building the boat ^.^
     
  6. upchurchmr
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    upchurchmr Senior Member

    Don't forget to measure deflection when you measure force.
    Most designs require or assume a certain amount of stiffness.
     
  7. sebaseba
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    sebaseba Junior Member

    If you make the sandwich panel thicker, it automatically becomes also stiffer. But yes, I'll do calculate that as well. :)
     
  8. upchurchmr
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    upchurchmr Senior Member

    Just make everything bigger. Even more stiff.
    And heavier.

    Why do you want to do sandwich at all?
     
  9. sebaseba
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    sebaseba Junior Member

    Sounded like a good idea at start. Now the more I learn/calculate the more I see that core shear is a big issue if the foam density is low.

    https://plesko.si/beam/s.html here. But I'm not sure yet if it's fully correct.
     
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  10. upchurchmr
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    upchurchmr Senior Member

    I haven't calculated anything, but from experience, that pedal boat probably needs no more than 1 ply of 4 oz cloth on each side of 1/8" of PVC foam.

    You might also try to determine the bond strength of the foam to a ply of glass.
    Of course, guys have made boats from eps with no glass, so it might be really easy.

    Check the weight, it is really really easy to make a heavier boat than just solid laminate.
    You are doing it right by trying to calculate the proper design.

    I'll leave you alone now. Have fun.
     
  11. sebaseba
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    sebaseba Junior Member

    That sounds correct.

    Bond strength is an another thing I haven't addressed yet. PS has lower surface energy than PVC, so it would bond worse to XPS than to PVC foam, but I think that wouldn't be an issue.

    One of the ideas is also to have it unbonded. The equations are slightly different I think, you mostly keep the rigidity, but aren't (I think?) that limited by core shear. But I haven't researched this that much.

    Of course.

    I think I'm doing a huge overkill right now, but it's more that I want to learn how to calculate stuff to get a feel and to be able maybe in the future make something bigger. :)
    Insofar I mostly built stuff (non-boat) "by feel" and qualitative "equations", which mostly led to overengineering, but slowly over the years less and less.

    I think when I'll get my foam cutter I'm going to make a test sandwich beam and just load it with weights in between, to see if the calculations are correct, but I'm confident now that XPS is not really suitable for the shell except in maybe some low load boat areas or if you design in a special way: very thick sandwich or honeycomb structure with XPS.

    Also balsa seems to be an awesome material, especially for its price.
     
  12. Dejay
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    Dejay Senior Newbie

    Yes, you're slowly coming over to the dark side, let the hate for XPS flow through you ;)

    You could also use the XPS just a former to lay up solid fiberglass. But with developable surfaces, thin plywood with light fiberglass is probably the easiest and cheapest way to build a small hull.

    PS: Nice calculator, I'll have to look into that and learn how to do that myself.
     
  13. upchurchmr
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    upchurchmr Senior Member

    Sebasba,

    I like your idea of sandwich beam testing. Simple and good correlation.
    If you make a XPS beam, draw a vertical line on the side of the foam.
    Underload, you will see it makes an S-shaped deformation instead of a straight line deformation.
    XPS does not really react to load the same way typical sandwich construction calculation assumes.
    What you will see is that the beam is mostly supporting the load weight by bending the skins, instead of acting as a unit and putting the skins in tension and compression.
    Actually, I did this but supported the beam from one end (cantilever). So the deformation might look different if you support from both ends.
    Either will work.

    Balsa is great, but heavier, and prone to rot. Seal it well with epoxy, and immediatly fix any skin damage.
    Cheaper but heavier would be to use spruce or cedar as a core.
    I really like your approach.

    Marc
     
  14. sebaseba
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    sebaseba Junior Member


  15. sebaseba
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    sebaseba Junior Member

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