Help- High pitch propeller in displacement boat

Discussion in 'Props' started by yodani, Aug 20, 2016.

  1. W9GFO
    Joined: Dec 2014
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    Location: Olalla, WA

    W9GFO Senior Member

    If there was a real benefit to be had you would want the rudder to have an asymmetrical foil shape with the top half being opposite the bottom half to take advantage of the direction of prop wash. A little bit of twist would help too, I think. Surely this has been tried before though...
     
  2. MikeJohns
    Joined: Aug 2004
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    Location: Australia

    MikeJohns Senior Member

    Are you saying you don’t believe that the rudder position inline behind the prop increases thrust, or don’t you like the simplified explanation ?

    Prsuming it’s the latter; it’s hard to provide a popularised explanation of something as complex as the prop rudder interaction without quickly getting into complex hydrodynamics. But if you look to the far(ther) field and ignore the immediate prop rudder interaction you find a reduction in swirl and an increase in thrust from nothing more than an inline rudder behind the prop. Even additional horizontal vanes on the rudder can further increase efficiency.

    Maybe read Marine Propellers and propulsionm and Mollands Marine Rudders and Control surfaces you’ll also be able to find the papers cited on this by Molland.
     
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Obviously it is not a case of a foil behind the prop always being beneficial, otherwise they would be standard on outboards/outdrives as well. It is a little difficult to get the head around it, but I am not going to argue against test results.
     
  4. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    "Obviously it is not a case of a foil behind the prop always being beneficial, otherwise they would be standard on outboards/outdrives as well. "


    The big money folks oil tankers and box cargo ships already fit devices on the hull and rudder to reduce the fuel burn at cruise speed..

    Of course they are all custom builds , not an OB stuck on a boat.
     
  5. johneck
    Joined: Nov 2011
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    Location: New England

    johneck Senior Member

    Might be too late, but I would suggest that you look at a different gear. The ratio is too deep if you are limited to a 19" diameter. The problem with going to high P/D ratios is cavitation and low end torque. If had a racing hydrofoil or a high speed naval frigate a 1.5 pitch ratio might be OK, but for a low power, low speed vessel you are very likely to run into problems.
    I would suggest a gear 1.5 - 2.0 for this case.
     

  6. yodani
    Joined: Nov 2010
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    Location: Danube Delta

    yodani Senior Member

    Hi johneck,

    Just today I have received my propeller from the "trimming" as I have found a Vetus 21x25" for 200 EUR and have brought it down to 18". I have now a Kanzaki 2:1 gearbox and I think this will go well. The boat is going to travel at slow pace so I don't think there will be problems. I will keep you posted.
     

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