Metal casting and propellers (the boat kind),

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Comments

  • Cavitation is not an issue with my slow turning, low thrust propellers. They will only need to absorb between 100 and 150 Watts of power on average.

    As far as surface finish, the fairest and smoothest surface possible should give the best results. It is something I would like to test.

    My approach will be mostly "guess and check", starting with known good designs and foil sections.
  • idbruceidbruce Posts: 6,134
    edited 2020-09-03 - 07:14:04
    My approach will be mostly "guess and check", starting with known good designs and foil sections.

    Perhaps you could approach this problem on a much smaller scaller, with miniature model and a bath tub :) For example, revolutions versus distance traveled or distance traveled versus energy exerted :) Just 3D print a scale model of your floatation device, then mount a motor with a battery and encoder, and make it chase the rubber ducky around the tub :)
  • idbruce,

    Good thought. But to perform a test in miniature like you suggest, you'd also have to "miniaturize" the viscosity of water. I've learned the hard way that scaling down a sailboat, say, to make an RC model, does not yield performance characteristics anywhere near in proportion to the scaling coefficient. In fact, just to make it perform at all requires a radical departure from the boat's original parameters (e.g. deeper keel, different sail configuration). I suspect the same is true of a propeller design.

    -Phil
  • ceptimusceptimus Posts: 116
    edited 2020-09-03 - 11:41:46
    In some model aircraft competition classes, the competitors choose to use single-blade propellers, with a weight concealed in the spinner for balance. The idea is that the tip of any 'wing' is the least efficient part (because of the tip vortex), so by getting rid of one blade and tip, and making the remaining blade a little larger so that it can provide all the thrust by itself, you improve overall efficiency. Anyway, it must work in those classes, because that's what the winners use, and everyone else then copies them!

    I don't know if this has been tried for boat propellers - and whether it would work or not, because obviously water behaves differently to air. Food for thought though.
  • Phil

    Okay.... Point taken :)
    Good thought. But to perform a test in miniature like you suggest, you'd also have to "miniaturize" the viscosity of water.

    Instead of miniaturizing the viscosity of the water :) How about testing in a liquid with a thinner viscosity, such as rubbing alcohol?
  • W9GFOW9GFO Posts: 4,002
    edited 2020-09-03 - 18:01:55
    I think it will be far less work and produce far more accurate results to test in the open water. Testing a scaled down version would certainly work, that is how the big boys do it, but to get good results the test tank would need to be able to present the propeller under test with undisturbed, turbulence free water. Anything else is just playing. Not to mention I would need to learn a lot more in oder to make proper sense of the results. I've read quite a lot about scale model testing and I what I do know is that I do not have the level of math background needed to be able to do it in a meaningful way.

    A singled bladed propeller is theoretically the most efficient. If the counterbalance could be contained within the spinner it might be worth a try, otherwise the balance weight would be out in the stream creating drag while producing zero thrust. There is also the issue of vibration since the thrust force is unbalanced but that may not be an issue on my slow turning prop. Actually water behaves remarkably similar to air (they are both fluids), one of the reasons why model airplane propellers are a popular choice for human powered pedal craft.
  • I wonder whether a superhyrophobic coating such as NeverWet would improve efficiency by eliminating boundary-layer adhesion.

    -Phil
  • I wonder whether a superhyrophobic coating such as NeverWet would improve efficiency by eliminating boundary-layer adhesion.

    I don't know, my guess is that the effect would be insignificant and/or short lived. I know I have read about using hydrophobic coatings before but I cannot recall what the details were, I think that is because the results were not very encouraging. I wouldn't mind testing it though.
  • Look at (FAI) F1B propellers, and it would be a useful feature
    if they folded too.

    Lots of examples of single blade propellers used in other rubber
    powered events.


    Bill M.
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