Human Powered Airboat

Discussion in 'Boat Design' started by alan craig, Dec 4, 2014.

  1. alan craig
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    alan craig Senior Member

  2. daiquiri
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    daiquiri Engineering and Design

    Technically, it doesn't make sense. An air prop in those conditions is a very loaded prop, hence inefficient. If they can get 50% efficiency from that one, they can claim to have made a bingo.
    On the other side, a correctly optimized water prop can arrive to 75% under the same power, hence giving a higher boat speed. Or can consume less power for the same speed, hence allowing more endurance.
    If the goal of the project was simply to make something different, then ok - they have made it...
     
  3. alan craig
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    alan craig Senior Member

    I couldn't say how efficient it is but the prop looks big, with quite large blade area and is only transmitting a few hundred watts at most. The ability to explore very shallow or weedy water could be useful though.

    I would have to admit from my own experience that an air prop at low speeds is very vulnerable to both wind strength and direction; you can hear the blades stalling as the apparent wind changes direction.
     
  4. daiquiri
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    daiquiri Engineering and Design

    Stalling blades put even more load on the prop, because it implies high blade angle of attack, hence high drag, hence low efficiency.
    Ok for the argument of shallow waters, but as about weed - with an average 150 W (and short bursts of perhaps 300-400 W) coming from the "human motor" it won't get very far sliding over the weed... ;)
    Cheers
     
  5. dinoa
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    dinoa Senior Member

  6. daiquiri
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    daiquiri Engineering and Design

    One thing you have to understand is that propellers' efficiency increases as the disc load decreases. That fact is valid regardless of whether the prop works in the water or in the air. Now, the disc loading decreases as the advance ratio J=V/nD increases. However, for a given fixed input power, there is a relationship between n (revs per second) and D (diameter) which at the end give only two ways of increasing the J (and decreasing the disc loading) - either by increasing the diameter D or by increasing the axial speed V.

    A second thing to bear in mind is that Decavitator was designed to be a speed-record breaking machine, and Gossamer Condor was designed to fly on human power for a very short time. See this excerpt from the Condor Paper: "The 2-1/2 minute flight a month later was powered by a strong, championship-level cyclist."

    So the efficiency figures they give for their air props are relative to that particular design condition, which means about 800-1000 W burst of power for a very short period of time. In this case, an air prop of diam. = 3 m can be designed which gives 75-78% efficiency. It is then perfectly comparable to what can be squeezed from a water prop of diam. = 0.4 m, for example.

    But for a prolonged period of cycling, say an hour or so, one has to do his calcs with no more than 400 W of human-motor power, which in turn give a much smaller boat speed. And the boat speed is the axial flow speed of the prop. Now recall that smaller axial speed means bigger disc loading - hence, lower efficiency. In other words, at such low power levels and consequent forward speeds, the efficiency of an air prop will drop significantly, down to 50-55% - where it cannot compete with a well-optimized water prop, the diameter of which is much smaller and scales down better to accommodate the low power input.

    Cheers
     
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  7. dinoa
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    dinoa Senior Member

    Well said
     
  8. messabout
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    messabout Senior Member

    Excellent Daiquiri.
     
  9. philSweet
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    philSweet Senior Member

    Yes Daiquiri, but I'm not sure I agree with this part.
    To operate a hydro prop at 3 knots and 75 percent efficiency and on a small scale is just as hopeless as the air prop.

    Normally, it is the hydro prop that suffers the most from perturbations when compared on a Reynolds number basis with an air prop. This becomes an increasingly larger problem as speeds decrease.

    Another consideration is that pedal driven hydro props suffer greatly from a lack of momentum to carry the crank over top dead center. The hydro prop basically tracks the torque of the pedals, and this is very wobbly unless you have a great deal of training. An air prop can be set up to carry quite a bit more momentum and this will moderate thrust fluctuations and thus suffer a much smaller loss of efficiency.

    The flip side is the environmental factor. Wind perturbations, in a absolute sense, are likely to be much greater than the water perturbations. And you would need gears to deal with head wind and tail wind conditions since we invariably measure performance with respect to the water; and that bias is probably the real reason for preferring the hydro prop.
     
  10. markdrela
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    markdrela Senior Member

    I'm not sure what you're trying to argue here. Whether a given boat driven by either an air prop or a water prop, the prop will be equally well matched to the boat for any speed. More specifically, the various quantities of interest have the following variation with speed V. All these assume that there is no wind, and that the boat water drag is dominated by friction drag -- reasonable for a slender HP hull:

    drag ~ V^2
    prop torque ~ V^2
    prop rpm ~ V

    drag power = drag*V ~ V^3
    prop power = torque*rpm ~ V^3
    prop blade Re ~ V

    thrust coefficient = Tc = T / 0.5 rho V^2 A ~ constant
    power coefficient = Pc = P / 0.5 rho V^3 A ~ constant
    prop blade lift coefficients = cl ~ constant
    prop blade drag coefficients = f(cl,Re) ~ constant (if Re effect is weak)
    prop induced efficiency = eta_i = 2 / [ 1 + sqrt(1+Tc)] ~ constant
    prop profile efficiency = eta_v = f(cd/cl) ~ constant
    prop efficiency = eta_i * eta_v ~ constant

    Note that in terms of the dimensionless coefficients, the prop doesn't see the effect of speed V, aside from a Re effect on the blade airfoils.
     
  11. markdrela
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    markdrela Senior Member

    More on the problem of matching the prop and boat over varying boat speeds:

    The main "problem" here is that the pilot needs to increase pedaling rpm linearly with V. However, this is less of a problem than it appears, since as power is reduced, the optimum pedaling rpm tends to drop also. Let's say the prop is geared such that we need the following pedal rpm versus boat speed (assuming rpm ~ V and Power ~ V^3):


    V(kt) rpm P(W)
    10 100 300
    _8 _80 192
    _6 _60 108
    _4 _40 _48

    For a cyclist, those are very comfortable rpm/power combinations. The 40 rpm is quite slow, but the power at that point is so low that it doesn't matter much.

    Back to the water prop / air prop issue. A water prop can theoretically be made more efficient than an air prop because its dimensionless disk loadings Tc and Pc can be made much smaller for a practical prop diameter.

    On the other hand, an air prop will have a gear ratio close to 1:1 and hence much smaller gearing losses, and it will have significantly smaller drag because there is no underwater mounting strut and prop shaft. This is why we went for an air prop on the Decavitator.
     
  12. myark
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    myark Senior Member

    It would go extra well with an electric bicycle system attached.
     
  13. Clarkey
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    Clarkey Senior Member

    Seems somewhat similar to the crosstrek:

    [​IMG]

    How much might something like this benefit from variable pitch?
     
  14. daiquiri
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    daiquiri Engineering and Design

    Very simple. I am saying that in this particular case (the boat in the OP, not the Decavitator), with a maximum practical diameter of the air prop fixed at 3 meters, the maximum continuous power input fixed at 150 W (leisure cruising), and the maximum boat speed with that power equal to 5 kts (approximately - to be verified), the air propeller cannot reach the efficiency of a water prop of a, say, 0.4 m diameter.

    The relationships you have given are valid if the diameter can be varied at will. But the problem here is the max. diameter limit of the air prop. If that parameter can be increased to around 4.5-5.0 meters, then the efficiency of an air prop can match the efficiency of a water prop (based on the above approximate numbers). Not a case here, where the diameter of the air prop is 3 meters - which is IMO a practical limit for this kind of boat.

    Increasing the power input closes the gap between the air prop and the water prop because it increases the boat speed and allows the diameter-limited air prop to work closer too the optimum point. Above a certain critical maximum speed, the air prop becomes more efficient and preferable, because it eliminates the cavitation issues - and the example of your Decavitator shows it very well.

    Cheers
     

  15. daiquiri
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    daiquiri Engineering and Design

    Actually, if you try to design two optimum props for the conditions given in my previous post, you will discover that they both operate at very close Re-number points. Of course, that situation will change by changing the ratio of their diameters, but for practical prop sizes, it won't change too much. :)

    That is probably true. Check the video in the OP, where the guy is pedalinng at a slow pace. See how the boat struggles to move in a straight line. I don't know if that is a steering-input issue, a vortex-street issue, or a wind (or waves) perturbation issue. Or all of them together. But it does visibly weave through the water.

    Cheers
     
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