Boat overpropped - need help please.

Discussion in 'Props' started by yodani, May 20, 2016.

  1. yodani
    Joined: Nov 2010
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    Location: Danube Delta

    yodani Senior Member

    I have built a new boat and the prop I put in it is abit big. The calculated prop needs to be 21x15".

    I plan to reduce its diameter. Do you have any experience with that? Prop calc reccomends a prop 21X15" and I have a 22x17" now. The engine achieved a max of 2000RPM when it should go up to 2700 RPM. Do you have any suggestions on how much dia should be taken out?

    By my calculation I should reduce the diameter with 2" so that will give me a 20x17" prop. Is that correct? Or should I reduce 1" and try again to see how it works? in that filed?

    I have read somewhere that one inch of dia less will give you about 400 RPM more. That sounds about right. In theory the engine needs to reach 2750 RPM. Is that correct?

    Here are the characteristics of the boat:
    Length - 10.3m
    Width at water line - 3m
    Draft without keel - 70cm
    Weight - about 6 metric ton
    Engine - 100hp@2700rpm (Diesel)
    1:2 reduction ratio gearbox
    Two bearings on schaft

    I can definitely modify the prop dia but the shop engineer is not sure about pitch as we are not sure about the material used in it.


    Cheers,

    Daniel
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Ideally, you could trade in your propeller and swap it for the correct one. Otherwise, they can be repitched. You could try going to 21 or 20 1/2 before cutting the blades.
     
  3. yodani
    Joined: Nov 2010
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    Location: Danube Delta

    yodani Senior Member

    Hi Gonzo,
    The prop I bought from Ebay and it was 100$ so no trade in for me :). I will modify it in a specialized shop here but need some feedback from someone who did this before. I could repitch it but I'm afraid they will brake it :).

    Is the 1 inch 400RPM theory true?
     
  4. gonzo
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    gonzo Senior Member

    Repitching is a common adjustment. Any reputable shop will have no problem. Cutting the diameter increases the blade loading. Have you calculated it? Too much will cause cavitation.
     
  5. yodani
    Joined: Nov 2010
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    yodani Senior Member

    Sorry... No blade load calculated... I'm not so knowledgeable:)

    How do I do that?

    I found this on Vicprop:

    Every two-inch increase in pitch will decrease engine speed by 450 RPM, and vice versa. This is a good rough guide for moderate- to high-speed pleasure craft, passenger vessels and crew boats. Like all rules of thumb, though, it is no more than a rough guide.

    One inch in diameter absorbs the torque of two to three inches of pitch. This is a good rough guide. Both pitch and diameter absorb the torque generated by the engine. Diameter is, by far, the most important factor. Thus, the ratio of 2 to 3 inches of pitch equals1 inch in diameter is a fair guide. It is no more than that, however. You could not select a suitable propeller based only on this rule.
     
  6. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Befoe you do anything; what speed did you get with the existing propeller?
     
  7. yodani
    Joined: Nov 2010
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    yodani Senior Member

    Hi Beckamo,

    I got about 19 km/h with the existing prop at 2000RPM. It was hard to calculate as I have tested on the Danube with current but that's roughly what I got.
     
  8. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Ok, I guess you have a 3-blade prop of "normal" proportions, ie something like 60% blade are ratio. If so, you should get reasonable efficiency with 20" x 17", meaning that you can reduce the diameter. Note that the blade shape in the tip region should be adjusted as well.

    But with that said, I must add that something is not adding up with the figures you have given. A 22" x 17", 3-blade screw at 1000 shaft rpms and 19 km/h (~10 knots) in a hull with a 10% wake is only using ~50 hp, while your engine should have at least 70 to 80 hp at 2000 engine rpms.

    So, what kind of hull are we talking about? Have you run the speed trials both ways? How deep is the trials course? Is the engine ok (type?), and rev counter checked?
     
  9. yodani
    Joined: Nov 2010
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    yodani Senior Member

    Hi Beckamo,

    You can find my build log here - http://www.boatdesign.net/forums/boat-design/build-log-10m-tour-boat-54574.html

    There are details about the boat and engine etc.

    The trials I have done are not ideal and the RPM pickup is from the W terminal on alternator. It seems to be a fairly accurate rpm meter.

    I have modified the hull and added a gozo boat like plate that helps with the speed of the boat. I have to say this is a great addition.

    Attached you can see the propeller.
     

    Attached Files:

  10. yodani
    Joined: Nov 2010
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    Location: Danube Delta

    yodani Senior Member

  11. whitepointer23

    whitepointer23 Previous Member

    how do you know the rpm reading is accurate, have you tried a master tachometer. you need to calibrate your tachometer when running it off an alternator.
     
  12. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Ok Daniel, I see from the building log that the deadwood in front of the propeller is very, did I say VERY!? thick! The propeller is operating in a huge wake, which has a dramatic effect on its performance. I'll do some number twisting and come back later today.

    And, as Brendan says, check the tacho calibration!
     
  13. yodani
    Joined: Nov 2010
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    yodani Senior Member

    I have calibrated the tachometer with a strob tachometer and I have set it in the InteliDrive from ComAp and I think it has an error of 5% max. When accelerated with no load the tach shows about 3000 rpm. When in gear it will not exceed the 2000rpm mark. And you can feel the engine is struggling.
     

    Attached Files:

  14. yodani
    Joined: Nov 2010
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    Location: Danube Delta

    yodani Senior Member

    The deadwood is indeed thick but much thinner than it was as I have modified it. There is no vibration what so ever in the boat at any speed so no complaints there.

    I'm not looking for speed but for efficiency and reliability.
     

  15. whitepointer23

    whitepointer23 Previous Member

    thats good, it will aid beackmo and others knowing the rev readings are accurate. fwiw your boat looks pretty fast in your video.
     
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