Crouch vs Bp-delta

Discussion in 'Props' started by Walt Elliott, Oct 21, 2012.

  1. Walt Elliott
    Joined: Oct 2012
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    Location: Kingston WA

    Walt Elliott New Member

    I'm re powering a sail auxiliary hence re-propping with a given SHP and prop diameter. Crouch calculations give a pitch of 6.7 in while Bp-delta is 9.7 in. Calculations from Campbell propeller agree with Crouch while MWC agree with the Bp-delta. Campbell has an unusual blade design with some cupping while the MWC is the ogival with some airfoil at the root. Both are narrow three blade. Any ideas on how to resolve?
     
  2. johneck
    Joined: Nov 2011
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    Location: New England

    johneck Senior Member

    The answer probably depends on how much you know vs how much you have guessed. The crouch method is very empirical versus the Bp-D charts are based on test data. So if you know, or have a very good guess about vessel speed and wake fraction, the Bp-D charts are more accurate. If you don't, then using the empirical result may be better.
    Adding cup to the propeller can significantly change (reduce) the pitch requirement. The Bp-D charts are for a specific propeller series that have constant pitch and semi-airfoil blade sections, no cupping. So if you are selecting a certain propeller from a manufacturer, you probably ought to follow their guidelines, especially if they have experience with a similar vessel.
     
  3. Walt Elliott
    Joined: Oct 2012
    Posts: 2
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    Location: Kingston WA

    Walt Elliott New Member

    Thanks for the very helpful advice.
     
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