Finding delivered power

Discussion in 'Software' started by souravsandesh, Oct 8, 2014.

  1. souravsandesh
    Joined: Aug 2014
    Posts: 8
    Likes: 1, Points: 0, Legacy Rep: 33
    Location: INDIA

    souravsandesh Junior Member

    Hello,

    I would like to know how the delivered power of a propeller is calculated...

    My vessel is a catamaran with speed of 8 knots

    I calculated wake fraction and thrust deduction fraction using holtrop's emprical formula viz Wake fraction is 0.42 and thrust ded.fraction. is 0.15

    my resistance of a demi hull is 1400N...my ship advanced speed Va is 2.358 m/s.
    My thrust calculated is 1662.70 N.
    I chose Bseries wagninegen series propeller with 3 blades and my Ae/Ao ratio is 0.3 and my Thrust co efficeint Kt, advanced coeff J and Torque Coeff Q are 0.52,0.38 and 0.02. P/D ratio is 0.7 and propeller diameter is 0.5m.

    I used open water diagrams of B series wangninegen propeller for the respective propeller and AE/AO ratio.
    I got only 53% propeller efficiency..is it ok??is not normal to have propeller efficiency that low??..no matter how many parameters I change i couldn't maximum of that...please help me

    P.S am attaching necessary documents
     
    Last edited: Oct 11, 2014
  2. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,003
    Likes: 207, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    I don't see anything wrong in your work, and your results are as expected. Typically, we expect a propeller to be only 50-55% efficient, and you're right in the middle of that range.

    So now you have learned one of the basics of naval architecture--propellers are only about 50% efficient, and yet even though we have been using propellers for about 180 years, no one has been able to come up with a normal powerboat propeller that is more efficient than that. And the reason has to do with the physics of propeller blades and they way they generate lift, and the fact that they do this in a fluid (water) which has an interface with another fluid (air) close by. There is also air trapped in the water. No matter how fast you spin the propeller, or how much blade area and pitch you have, you can only get so much energy out of the device. Airplane propellers face similar, although less complicated problems because they operate only in a single fluid. They suffer from sound barrier and flow separation limitations.

    Eric
     
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