New propeller for a water bike

Discussion in 'Props' started by MauritsDWT, Jun 4, 2015.

  1. MauritsDWT
    Joined: Jun 2015
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    Location: Delft, The Netherlands

    MauritsDWT New Member

    Hi all,

    I am a member of a student team that is building competition water bikes (to get an idea: http://www.delftwaterbike.nl/). We're currently trying to upgrade our previous boats, and are looking at using a different propeller.

    The propeller we are using right now is a two-bladed one (no idea what series, but looks quite custom).
    We do not have CAD/3D model drawings of the boat, therefore we cannot just use that to calculate wake and thrust deduction factors. We assumed them close to 0 for propeller calculations.

    The first problem we run into is that the Wageningen B-series is not suited for the low load we impose on the blades since it is completely human powered. What other propeller series could we use for these low loads?

    Regards,

    Maurits
     
  2. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    I am not sure you will find a standard series of propellers for such a craft. Human powered propellers turn relatively slowly, so in order to get the most power imparted to the water for the fastest speed, you need a large diameter. Such propellers also have two blades because 3 blades or 4 blades does not add that much more power for their increase in drag that they produce. If you do basic propeller calculations, you'll find out this is true. The blades do not have to be too sophisticated in blade shape, but they should have twist relative to the radius in order to generate uniform power. That is, in the inner half of the blade the sections will have a lot of angle of attack, and as you move out the radius, the sections will have progressively less angle of attack.

    You see this in wind turbine blades--you can model your propeller similarly so such blades, as a matter of fact, because wind turbines also have to generate lots of power for little wind speed. So you can look to the wind turbine industry for suitable propeller blade types, and use those for modelling a two-blade propeller for your craft. Generally, the inner half of the radius of wind turbine blades have sections with lots of twist distribution, but in the outer half of the radius the sections have practically little or no twist distribution.

    I hope that helps.

    Eric
     
  3. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    As Eric said.
    One option is to use one of many models available for RC aircraft. But there is an inherent problem with them - they are designed to operate in air. When working in water they are subject to a root bending moment which is nearly 4 times higher than what they were designed for. So you will have to verify that the model you eventually choose has a necessary structural resistance to not break under the high thrust during the competition.
    If you need help with choosing one, just blow the whistle. :)
    Cheers
     
  4. keysdisease
    Joined: Mar 2006
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    Location: South Florida USA

    keysdisease Senior Member

    If you could get a close look at the prop they used for the human powered plane that crossed the English channel you could see what Eric and Daiquiri are agreeing to. Of course the diameter would have to be reduced appropriately but the basic concept of a slowly turning prop that produces max thrust is what you need, here's a peek:
     

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  5. MauritsDWT
    Joined: Jun 2015
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    Location: Delft, The Netherlands

    MauritsDWT New Member

    Thanks for all your inputs guys!

    We are indeed looking at two-bladed propellers, since like you said the efficiency would only drop when choosing more-bladed propellers.
    The propellers we have now are bent aluminium plates and worked by hand (some years ago, without leaving some kind of blueprint or anything, so we don't know how they did it back then).
    The thing keeping us from trying to bend one ourselves again, is that it will take considerable time until we master doing that as well as the ones before us did. We are second year Maritime Engineering students, currently facing finals... Ideally we could turn to Wartsila / MARIN if we had some kind of legit (3D)design to have it manufactured...

    The suggestion about aviation propellers is solid indeed, we are looking into that! RC planes are indeed a place to start, however I was under the impression that the propellers that are used there are made of plastic, which will just bend underwater?

    A lot of the other teams (mainly from Germany) are using (3-bladed) outboard propellers. Although they seem to be faster than the ones we are currently using, because of the small diameter and the very high RPM they require I don't think they are the most efficient option.

    EDIT: We asked a guy working at our University about it, and he said we could try to figure out the efficiency of our current propeller by determining if the thrust and flow velocity vector are parallel. However, doing this by hand seams extremely hard (Blade element method?) and we don't have any experience with CFD, so I guess that is a dead end...

    Thanks again for all your wisdom!
     
  6. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Send me a PM if you need help with that.
     
  7. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    I would use the bath tub method.

    Buy a cheap digital bathroom scale, power the prop with a thin belt drive and a DC motor. Use a beam across the bath tub with a pivoting construction so you can read the static thrust on the scale. Measure the current consumption of the motor at several voltages and draw a graph with one axis current, the other thrust.

    With this setup you can compare the efficiency of various props without computer simulations that still leave room for doubt.
     
  8. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Sorry CDK, I respectfully disagree.
    If they need any meaningful numbers from the measurements, then a bath tub is too small for this size of props because nearby walls would alter the flow too much. A recirculation would soon (after a couple of seconds) transform the static thrust into some unknown and totally unrealistic kind of operation.
    If they had an access to a sufficiently big swimming pool, that would be much better. In that case a similar kind of measurements could be arranged.
    And I would still make computer simulations and comparisons with prop charts in order to assess the difference between measured and calculated values, and to try to understand why are those differences present and which method gives the most reliable numbers.
    Cheers
     

  9. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    OK, so it has to be a big bath tub then.
    These guys have a really big one nearby in a laboratory where water flow was studied until 2008 using scale models. The institute is now part of "Deltares" with several test sites in the Netherlands.
    The basin in Delft already has a moveable construction where the scale models were attached to.
     
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