Propeller for hydro-bike

Discussion in 'Props' started by malith, Nov 12, 2014.

  1. malith
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    malith Junior Member

    I'm designing a hydro-bike. Currently i'm having troubles with selecting a propeller. My requirements are follows

    Hull type- planing catamaran
    maximum speed - 10knts
    power- 400W

    What type of propeller i have to use and any suppliers of propellers??

    Thank You!!
     
  2. gonzo
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    gonzo Senior Member

    400W will not make your boat plane. How are you calculating required power?
     
  3. malith
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    malith Junior Member

    i used both savitsky spread sheet and maxsurf resistance.
    both gave me the same results.
    it's a single person craft. so i think results are accurate.

    Thank you
     
  4. daiquiri
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    daiquiri Engineering and Design

    As Gonzo said, the numbers don't sound correct. 400 W is a very low figure for a planing boat, even if light one. Unless it is an RC model.
    And then, is it 400 W of shaft power or effective power?
    May I suggest you to post the data of your hulls, so that a double-check of your figures can be done?
    Cheers
     
  5. gonzo
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    gonzo Senior Member

    You may be one order of magnitude off. 4000 W seems more reasonable. I am not sure about 10 knots though.
     
  6. malith
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    malith Junior Member

    i have attached spread sheet and maxsurf data(in zip file) here. it gives 250W per hull. so i think total power requirement will be around 500W. i'm planing to use a electric motor as power source. i want to know where i can find a propeller that match to my requirements.

    Thank You
     

    Attached Files:

  7. daiquiri
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    daiquiri Engineering and Design

    I don't get it. I am reading 15 lbf at 10 kts, which means 66.7 N at 5.14 m/s in SI units. And that gives 343 W of effective power. Where does the figure 250 W come from? :confused:
    I think that you will have to make your own prop. Your requirements are too far off from what is commercially available. If you need a help in designing it, you can PM me.
     
  8. malith
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    malith Junior Member

    Maxsurf gives the required power 250W. I thought it is more accurate than spread sheet since it is a commercial software.

    I have written to some of propeller manufactures. according to their specifications the smallest propeller they have is about 3hp. how anout using that kind of propeller in low rpm

    Thank You
     
  9. gonzo
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    gonzo Senior Member

  10. baeckmo
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    baeckmo Hydrodynamics

    Looking at your spreadsheet numbers, I find that you use a weight of 44 kg per hull. And, as Daiquiri has shown that will give a required thrust of 133.4 N in total, demanding an effective shaft power of 686 W, provided you use a single prop (which is better for efficiency than twins).

    Now, if you really go for a high propeller efficiency, let's use "grandpa's way" and use the Wageningen B type diagram, and look what is needed to get, say 80 % propeller efficiency (and that is high, really high). If you want to understand what you are doing, this is better than just stuffing numbers into a propeller calc program.

    Since two blades are more efficient than three, we use the 2.38 screw diagram. In order to come to 80 %, the Taylor coefficient Bp must not exceed the value about 4.8, but the efficiency curve is nearly vertical in this region, so you can aim at a value for delta ~100.

    Since delta=(n*D)/Va, and Bp = (n*P^0.5)/Va^2.5; where n is shaft rpm, P is shaft power in hp (ie effective power/efficiency), and Va is speed of advance in knots, you have all the input necessary to calculate the diameter and shaft speed for your application! (Note that D is given i feet!).

    Using the figures you have come up with, you will need a two bladed propeller with area ratio of 0.38, running at 1406 rpm and with a diameter of 8.5" and pitch 10.5". The necessary shaft power is 1.17 hp just for the propeller; add the margins for bearing friction, shaft friction in the water et c., and you will have to produce some 1.35 hp at least to reach 10 knots.

    Now that you know the method, you can check the options available for reducing the shaft power by reducing the propeller loading.

    Hardly a comfortable sunday picnic performance in my view, but good luck!
     
  11. baeckmo
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    baeckmo Hydrodynamics

    ......and what makes you believe that a certain program is better because it is Commercial????

    There are application limits in all calculation methods, since they are all based on a combination of experimental data and analytical methods in the form of algorithms. The thing is to know the limitations, and Dingo's spreadshet is better than most.
     
  12. daiquiri
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    daiquiri Engineering and Design

    Malith, what is the max. prop diameter you can fit on your boat?
     
  13. malith
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    malith Junior Member

    both systems use stavitsy method for planining resistance calculations. spread sheet does not sat anything about limitations. But my values are in the limits of maxsurf. so i thought results are more acceptable

    thank you
     
  14. malith
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    malith Junior Member

    distance between my demi hulls is 800mm. so i can fix anything smaller than that.
     

  15. baeckmo
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    baeckmo Hydrodynamics

    In Connection to my post #10 above, I would add that the lift/drag ratio of your hull setup seems to be a bit off optimum. The angle of attack is on the high side for a high L/B hull, so test a few other combinations of slenderness and center of mass to see if you can find an improvement.
     
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