Prop size

Discussion in 'Sterndrives' started by wireguy, Jul 9, 2007.

  1. wireguy
    Joined: Jul 2007
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    Location: Ontario

    wireguy New Member

    How do I find out the correct prop diameter and pitch?
    I just bought a 1987 Sylvan 17.5' Bowrider (Fibreglass) with a 2.3L 4cyl. Ford engine and OMC Cobra outdrive. I think all the previous owners have strayed too far from what should be on the boat but am not sure how to go back?
  2. timgoz
    Joined: Jul 2006
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    Location: SW PA USA

    timgoz Senior Member

    Dave Gerr wrote "The Propeller Handbook". Though I have not read it, his other book "The Nature of Boats" was quite good, and I've heard the prop book is very informative & well written.

    Hope this helps.

    Welcome to the forum Wireguy.

    Take care.

  3. switlikbob
    Joined: Apr 2007
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    switlikbob Junior Member

    Maybe you can tell us your speed, correlating RPM and what the max rated RPM is as well as the current prop you are using...
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Find out what the proper operating WOT (Wide Open Throttle) RPM is for your engine and check this against the actual RPM's you do pull at WOT. If you're off by more then a couple of hundred RPM, you could benefit from a new prop. The idea is to size the prop so your engine can pull full RPM at WOT.
  5. DaveClark
    Joined: Jul 2018
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    Location: Canada

    DaveClark New Member

    bought a replacment leg for an 87 omc 2.3L but the "bullet" is smaller diameter, I need a new prop. anyways will the upper and lower gear ratios be compatable?

  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Assuming they mate up mechanically OK, you should be able to find a prop with the right pitch for the new combination. You will need to know the overall reduction of the old set-up, versus the new, you might be able to search the web to find that out, or even by taking the plugs out and turning it by hand, counting engine and propeller shaft revolutions, obviously while in gear. You need this info to work out the right prop size. Obviously even if the reduction turns out to be the same, you won't be using the same pitch as with the bigger diameter old prop, you would go up in pitch, or find the boat speed down, at you old rpm setting.
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