Propeller size

Discussion in 'Props' started by Duco84, May 26, 2015.

  1. Duco84
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    Duco84 Junior Member

    I plan to increase pitch on propeller, please advice propeller pitch. I have propeller D=25.6'', P=22.8 ", Z=4. Lwl-12m, D-12t, Engine 110HP.

    Max rpm of engine loaded and unloaded is same 2200 RPM. Gear ratio 1:2.9. Current speed max 8.5 kn
     

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  2. gonzo
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    gonzo Senior Member

    The maximum rpm is what the governor is set up for. It doesn't mean that the propeller is loading the engine properly. Try towing something to increase the load and see what happens.
     
  3. daiquiri
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    daiquiri Engineering and Design

    Lots of HP for a 8.5-kts displacement boat and too few for semi-displacement or planing speeds.

    Looks like you have added a Menorquin-style transom extension. A 12-m, 11-t Menorquin C120 has 2x300 HP for 17 kts max speed and 14 kts cruising speed. With your engine power, you can hope at best to reach approx. 9.5-10 kts.

    So the first thing to ask is: what is your goal?
     
  4. Duco84
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    Duco84 Junior Member

    Correct, I have added a Menorquin-style transom extension. My plan is to reach 9.5, 10kn. But after installation extension, I fix problem with big trim, now is zero, but the speed is same, 8.5kn.
     
  5. daiquiri
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    daiquiri Engineering and Design

    Do you have any photos of the transom extension during the construction, or with the boat out of water? Or do you have a drawing with new hull lines?
     
  6. Joakim
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    Joakim Senior Member

    In the other thread I calculated that the propeller is using in the order of 70 hp now. I also asked what is the recommended WOT rpm. If the governor is correctly set, then this is probably about 2000 rpm.

    If you are optimistic and think you can reach about 10 knots, then you could go for 28-29"pitch. If not, you need a few inches less.
     
  7. Duco84
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    Duco84 Junior Member

    Please see below some photos
     

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  8. daiquiri
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    daiquiri Engineering and Design

  9. johneck
    Joined: Nov 2011
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    johneck Senior Member

    If there is room, there would be a benefit in increasing diameter a bit. I think that you could get 6-7% more thrust with a diameter of 28". Something like 28X25 should get you at least 1 more knot than the current prop.

    You WILL use more power and thus burn more fuel.
     
  10. Duco84
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    Duco84 Junior Member

    There is no space for bigger diameter. I can only increase pitch. I will do that on same propeller, but I'm not sure about max pitch for my engine and gear. Now I have same rpm in idle and load conditions.
     
  11. daiquiri
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    daiquiri Engineering and Design

    Looks like your prop operates in a pretty disturbed flow. Since you want numbers - I would increase the pitch to 25" and use 5-bladed props. Same diameter as what you have now.
    Good luck
     
  12. Duco84
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    Duco84 Junior Member

    Thanks a lot, I will try, but I will bend my propeller.
     
  13. johneck
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    johneck Senior Member

    Then Daiquri is on the right track
     
  14. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Propeller tuning is not a simple task because there are many parameters in the equation. One of the biggest unknowns is the actual resistance of your new hull.
    Try with the new pitch only and see what happens. But chances are that the gear ratio will also have to be increased to approximately 3.1-3.5 for the optimum operation. You will discover it through sea trials of the new prop. In that case a ZF 45-1 (1:3.03 or even 1:3.74) could do for you.
     

  15. Duco84
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    Duco84 Junior Member

    I excepted that the resistance will be smaller with spoiler, because now I don't have trim and such big wave resistance, as before. Now, I don't have waves on stern, but I have bigger wave on forward part of boat. I'm not sure about friction resistance and wetted area, probable is little bigger because the stern is not so deep in water due the big trim but I add 4m2 of spoiler.

    Having on mind results of first sea trial I can conclude that resistance is almost same, because with same engine, gear and propeller I achieve the same speed.

    I will try with pitch 25" on same propeller, engine and gear. With current propeller I have slip 0.35. Not so bed for this type of vessel. But having on mind that engine achieve same rpm, idle and load, I guess that I can increase pitch.
     
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