Determining the proper gear ratio:

Discussion in 'Boatbuilding' started by paulpoz, Jun 23, 2014.

  1. paulpoz
    Joined: Mar 2011
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    Location: Middle River, MD

    paulpoz Junior Member

    Greetings Everyone. Does anyone have a link or resource for trying to setup the proper gear ratio for an inboard cruiser?

    I have a 7000 pound boat with twin 350s and 1 to 1 ratio velvet drives. I'm calculating a 36% slip rate. I'm hoping to go to 1.5 to 1 and swing larger props and cut down on that slip rate.

    Thanks.
     
  2. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    go to a prop mfr and give them all the data- they can run several scenarios for you
     
  3. paulpoz
    Joined: Mar 2011
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    Location: Middle River, MD

    paulpoz Junior Member

    I have went to several. They all give me the props I'll need based on all my information, however, no one will come right out and say, "A larger, slower turning wheel should reduce your slip".
     
  4. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    You should probably hire someone who is qualified to do the calcs for you. Generally, bigger is better* when it comes to a prop so the blade loading is ideal. But, too big (prop and reduction) on a fast(er) boat can lead to cavitation/erosion

    *( one critical factor is blade area so you keep the loading within limits. An on-line pgm I have seen does not allow for this aspect and says so in their 'comments'. Such programs are probably handy for a check but not sufficient to do a proper calculation.
    Other options such as increasing the number of blades (ie: from 3 to 5) or going to a higher DAR or BAR to get more area may not work. And, increasing pitch to make up for insufficient blade area or diameter is not an option.
     
    Last edited: Jun 25, 2014

  5. paulpoz
    Joined: Mar 2011
    Posts: 5
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    Location: Middle River, MD

    paulpoz Junior Member

    Thanks JSL. The boat is too light to go to anything more from a 1 to 1 to 1.5 to 1. Thanks to online prop calculators, with the 1 to 1 I'm swinging 13"x12" wheels, they are saying about a 15" x 17" is what I'd need. I'm hoping that is going to work.
     
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