Correct prop size and speed

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

  1. rfleet1066
    Joined: Feb 2012
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    rfleet1066 rfleet1066

    They told me that it takes three tries to get the prop right when building a boat. I used the online calculator and ordered mine from Michigan Wheel. It's 24", four blade with a 12" pitch. I have a 2:1 reduction. My engine's max speed is 2400 rpm . So, right out of the box, the engine would only go to 1900 rpm. I had the prop repitched to 10" . Now I can turn 2300 rpm engine speed. Is this the elusive sweet spot? I am only armed with ignorance. OK, heavily armed.

    Ryland
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Sounds like great news for propeller vendors that you need to get three of them ! :)
     
  3. gonzo
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    gonzo Senior Member

    Does the boat float on her lines?
     
  4. rfleet1066
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    rfleet1066 rfleet1066

    Gonzo, I'm not sure what that means. I'm thinking that loading the prop at or about the peak RPM is proper because that's where she develops max horsepower. Am I doing this right?


    Ryland
     
  5. Adler
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    Adler Senior Member

    Why you turn lower the pitch so much. Should never be changed more than 7% ; specially in your case where you have to deal in large variance between in the Velocity of the boat
    and the Va Velocity of the Sea Water Entrance to Propeller's Disk.
     
  6. gonzo
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    gonzo Senior Member

    It means if the boat floats on the designed waterline. If not, the weight or the center of gravity are not as calculated. That would make a huge difference in the drag of the boat at different speeds.
     
  7. rfleet1066
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    rfleet1066 rfleet1066

    Yep, In that case she does lie on her lines well, and this does not change much at all during operation at different power settings. Due to her slow speed, about 6MPH, she leaves almost zero wake also. The anti-ventilation plate made a huge difference in speed increase and vibration reduction. I feel like we are dialing it in, but I'm not sure when to be satisfied.

    Ryland
     
  8. rfleet1066
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    rfleet1066 rfleet1066

    I am not sure if seawater velocity is a huge factor because we only make around 6 to 6.5 MPH This speed is satisfactory especially with only 2.8 gallons per hour fuel consumption.

    Ryland
     
  9. rfleet1066
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    rfleet1066 rfleet1066

    I find asylum here where ignorance is welcome. I thank those of you that coached me through the creation of this vessel. Perhaps you recall , The Sebastian Marie was supposed to be a sternwheeler when conceived. Considering efficiencies and low draft capability I chose to build a long tail drive instead, which has traveled over 4500 river miles so far with no problems. The center pontoon was an afterthought, too. Anyway, she evolved into a fine ship, 97' stem to stern. Now back to this propeller thing...........how does one know when it's as good as it gets?
     
  10. rfleet1066
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    rfleet1066 rfleet1066

    You said center of gravity. I noticed when we recently refitted her, removing a Casita camper and replacing that with an exquisite, adult sized Airstream, that shifting the CG aft 1"/65' lowered the prop a bit and seemed to push more. We carry 800 gallons of fuel and 250 gallons of fresh water, so the load can vary. I intended to have a slightly aft CG. We have the capability of carrying our freshwater in a starboard or port locations.
     
  11. Adler
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    Adler Senior Member

    How much heavy is she ?
     
  12. rfleet1066
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    rfleet1066 rfleet1066

    The vessel weighs 85000 lbs.
     
  13. comfisherman
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    comfisherman Senior Member

    Guessing really low power relative to weight if your swinging a 24 inch wheel with a 10 pitch.

    2 inches of pitch fit you 400 rpm..... that's a lot.


    It would seem that a 24 inch 10 pitch prop would indicate the need for a deeper ratio with a bit more pitch. Slow that thing down a little. When moving weight slowly does better as a rule with a deeper ratio and some more pitch.
     
  14. Adler
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    Adler Senior Member

    Is your engine power between in 55 to 60 hp ?
     

  15. rfleet1066
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    rfleet1066 rfleet1066

    Good Morning Adler,
    Thank you for your interest in this matter. This vessel is powered by a John Deere 75 HP turbo diesel. Max RPM is 2400. I have a 2:1 gear reduction. Originally, I used a 24" four blade prop with a 12" pitch. The engine would not turn faster than 2000 RPM and the vessel would travel 6 MPH. Perhaps mistakenly, I surmised that the engine would produce max HP at max RPM, 2400. So, I reduced the pitch to 10" and she does turn 2400 now, but propulsion is reduced.
    I'm not sure what the next step is, but I'm thinking of re-pitching the prop to 11" in hopes of finding the sweet spot.

    I also have a 24" 5 blade prop 12 " pitch. When I tried this, the engine would only turn 1800 RPM. I will experiment with fine tuning this one, too. When I find the best performance between the two props, one will be the primary, and the other a spare.

    Ryland
     
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