How can I determine which prop to use?

Discussion in 'DIY Marinizing' started by Ed-H, Mar 16, 2010.

  1. Ed-H
    Joined: Mar 2010
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    Location: Montana

    Ed-H Junior Member

    Does anyone out there have some means of calculating which prop to use if certain parameters were known about the motor used and the boat used, or, is it just a trial and error endeavor?
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  2. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    well you can usually guess what is going to be about right ...90 hp 13x19p etc but is it a big boat small engine or big boat big engine ?? ....The engine maker has a suggested prop or look at in the end you have to put one on and take it for a run at full throttle and trimmed up and measure the rpm ..then check the rpm range for your motor ..somewhere in the middle is about right and the rpm will change approx 400 rpm for every 2 inch change in the prop pitch
  3. Ed-H
    Joined: Mar 2010
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    Location: Montana

    Ed-H Junior Member

    Thanks PnB, I should have been more specific in my request. I am building a longtail like you did, but with the free 4hp BS (perhaps you remember me from the PMs I sent you). Given this info. how did you go about selecting the one you used on your 6.5hp? I know you used a 2.1 reduction, if memory serves me correctly, which would provide you with greater torque, and the ability to use a larger prop. What size rig are you using yours on? I will be using my on a 14ft. sneak/layout boat.
  4. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Generally, you cannot select propeller without knowledge about hull performance, but a useful rule of thumb for the first guess is:

    Diameter=630*power^0.2/rpm^0.6; where power in HP, rpm is prop rotation in rev's per minute and the resulting diameter in inches.

    A two-blade longtail screw is partly ventilating, meaning that you may get closer with an increase in the constant to, say 640.

    To find the value for pitch, you have to have a basic idea on attainable speed. Without that, start with a first guess of a Pitch/Diameter ratio of ~0.8.

  5. jonr
    Joined: Sep 2008
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    Location: Great Lakes

    jonr Senior Member

    Also depends on what you are optimizing for. Most people go for maximum speed, but acceleration or MPG may make more sense for you.

    Acceleration - less pitch (reach higher rpm and output earlier)
    MPG - more pitch (keep load high, rpm and total output low)
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