Correct prop size and speed

Discussion in 'Props' started by rfleet1066, May 7, 2021.

  1. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    As a general rule on heavy barge like vessels (similar to a lot of our fishing boats that tank down) we like to slow the prop down and keep the pitch in a more normal range. I.e. I'd rather a 3:1 gear with a 24x22 than a 2:1 with a 24x10. Shallow draft boats are a tough one especially when heavy. One of my boat more than doubles in weigh whem tanked down with fish, it's also limited in draft. It originally had a shallow gear ratio with a small pitch prop. We bumped the ratio up to get a little more modest pitch and it does much better against the tide and wind than previous.

    Reality is on a heavy slow boat prop slip is just a part of life, everytime that prop goes around is compounded inefficiency up and to a point. In our wetfish boats that are very heavy and slow it's been best to slow up the prop a little and give it a bit more pitch.


    As to proper engine install I've never seen a new engine company sign off on a setup tha doesn't allow the engine to spin up to max rpm (usually 25 to 50 rpm over nameplate). In your case it's tough as your talking an inch to a half inch of pitch having hundreds of rpm difference. That's a tough one to tune.
     
  2. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    The propeller loading in terms of the classical Bp value is extremely high, indicating that you are running the prop at too high rpms, the efficiency is very bad with the resulting pitch/diameter ratio. Two question come to mind here: first is this a barge with a longtail shaft arrangement, if yes, any pictures of the prop installation?

    Second: do you have a power curve for this engine? Asking because the John Deere ("PowerTech") engines are tractor engines and at least one of the 75 hp variants has a no-load max rpm at 2400; tractor engines often have max power setting 200 to 400 rpm below max speed, and a very flat power curve at top. When checking your propeller data versus rpm and advance speed, there is something out of range. Also, is your tacho calibrated?
     
  3. Adler
    Joined: Jan 2010
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    Location: PIRAEUS - GREECE

    Adler Senior Member

    Good Day Ryland,

    Take a note for a Propeller type: GawnAEW 4 blades (pls see to the next table).
    Issue due and in respect to Propeller's Immersion (Im) and Blade Area Ratio (BAR).

    Ryland_Calc.jpg

    In conclusion ,
    It's clear that the crucial factors in question subject are consisted of a reduction to the Pitch/Diameter ratio (P/D) and a rise to the Blade Area Ratio (BAR).

    Ryland_Boat.JPG
     
  4. rfleet1066
    Joined: Feb 2012
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    Location: New Kent, VA USA

    rfleet1066 rfleet1066

    Thanks for that. I was running a 12 pitch 24" for some time with satisfactory results. I mistakenly surmised that I needed to run it faster to make peak RPM. Clearly that was an error as you point out above. I will repitch it back to 12" and be happy with that. That gave me 6 MPH @2.8 GPH 85000 gross weight. That's full deck, full fuel, full freshwater. I also have a 5 blade that went to the shop today to be pitched at 11". Perhaps more blade area will provide an answer that does not include changing my gear reduction. Oh, these are new props from Michigan Wheel. Want to know how bad it can get? I originally made an ordered for two props, one for a spare. I waited 8 weeks and gleefully installed and tested them. Wrong rotation. Mistakenly ordered left hand.
    Tach is calibrated. Disclaimer........when I started building this boat I didn't know the first thing about boats. It's all been a learning experience from stem to stern. This boat was born and raised on this forum. You guys are so patient with my ignorance.

    Ryland
    IMG_3061[1].JPG
     
  5. rfleet1066
    Joined: Feb 2012
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    Location: New Kent, VA USA

    rfleet1066 rfleet1066

    Thanks for your interest. You make an interesting point about gear ratio. Inevitably I may change the ratio, as you suggest. I will try first to optimize the existing ratio by dialing in the prop performance.

    Ryland Fleet
    Expert Since 10 AM
     

  6. rfleet1066
    Joined: Feb 2012
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    Location: New Kent, VA USA

    rfleet1066 rfleet1066

    Thanks for that. I'm thinking that we had the sweet spot and didn't know it. 85000 lbs @ 6 mph @ 2.9 gph. Sometimes less depending on water temp. The data you provide is noted and logged. Any success will be blamed on you and people like you.
     
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