Over propped? 18x4 3 blade vs 18 x 13 4 blade

Discussion in 'Props' started by W9GFO, Aug 20, 2016.

  1. W9GFO
    Joined: Dec 2014
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    W9GFO Senior Member

    Just picked up a 26ft motor whaleboat. It has the original Westerbeke 4-107 and Paragon transmission but it has an 18 x 13 four bladed prop instead of the standard 18 x 14 three blade. I won't be putting it in the water for several months. What pros or cons can I expect from this prop?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Won't be overpropped. You are going down in pitch. Probably won't make much difference.
     
  3. JSL
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    JSL Senior Member

    If sized right the 4 blade may be more efficient if this is a heavy boat
     
  4. W9GFO
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    W9GFO Senior Member

    7% reduction in pitch, 33% increase in blade area.

    For any given rpm I'm guessing that there will be less slip, but more power absorbed by the prop. My feeling is that the speed/rpm relationship will be about the same, but require more horsepower to swing the extra blade. Resulting in a slightly higher fuel burn.

    So the question is, does the reduced slip make up for the extra power required to turn it? If it does, why wouldn't the four blade prop be the standard?

    My understanding of prop efficiency is that you use the largest diameter you can, then if you have more power available you add blades. Adding blades without the extra power to drive them not being a good idea.

    I'd like to be wrong though. It would be nice if this four bladed prop was a good thing rather than a detriment.
     
  5. FAST FRED
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    FAST FRED Senior Member

    IF you Chane back and you will never notice the difference.

    With the existing prop run full throttle for a min or two..

    IF there is black smoke note the RPM and pull back 10% or 300RPM.

    Enjoy
     
  6. W9GFO
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    W9GFO Senior Member

    The previous owner commented that he could not reach full rpm. The thing is I'm not sure if he meant 3,000 rpm or 2,500 rpm. If he meant 2,500 then that points to being over propped, if 3,000 rpm then I think that's okay. Also, since the tach has been replaced - it runs off the alternator, I am not sure that it is accurate.
     
  7. FAST FRED
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    FAST FRED Senior Member

    "it runs off the alternator, I am not sure that it is accurate."

    NO tach that runs off an alt is accurate or will give repeatable results.
     
  8. W9GFO
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    W9GFO Senior Member

    After some research I am able to mostly answer my own question.

    The main benefit of a four bladed prop (vs 3 blade) is less vibration. For any given rpm there will be more pulses per second as the blades pass by struts/ rudder etc., and potentially greater tip clearance - when the four bladed replaces a larger diameter three blade prop. In my case the four blade is the same diameter as the three blade, so no benefit there.

    The downside is less efficiency. There is more wetted area, more turbulence and greater chance of ventilating.

    I can come up with no scenario where a four blade is more efficient than a properly sized three blade.

    The only case where a four blade would beat a three blade (excepting vibration issues) is when the three blade cannot make use of the available shaft horsepower - and a larger diameter propeller cannot be fitted. In that case you must add blades.

    Something else interesting, when I input the data into this prop calculator http://vicprop.com/displacement_size.php it recommends a two blade 17.6 x 11.4. Which is close to what I already have - if I chop two blades off, but a long ways off the original 18 x 14 three blade. :confused:
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't think you need to worry about ventilation. It would likely be less prone to cavitate. I'm no prop guru, but my inderstanding is an odd number of blades is better for less vibration ?
     
  10. W9GFO
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    W9GFO Senior Member

    The increase chance of ventilating is due to the increased amount of turbulence due to the extra blade. Each blade is biting into "dirtier" water than if there were fewer blades. Ventilating is an issue when reversing if not heavily loaded. I don't think cavatating is much of an issue at the relatively low rpm that my prop turns at.

    I can see an odd number of blades being and advantage if there is a vertical structure near the prop both above and below the centerline. In my case there is nothing on the underside so there is not the situation where both the top blade and bottom are creating pressure pulses at the same time.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I know 5-bladed props appear to work better than 3 in aerated water, so ventilation does not come into it that I can see.
     
  12. W9GFO
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    W9GFO Senior Member

    Well, I don't know anything about that, but I can guess that if the water is already aerated, the more blades the better. Also, that's probably high speed props, not displacement speed.

    In theory, a single bladed prop would be most efficient, it is always biting into clean, solid water. As you add blades, the less clean the water is that the blades are operating in.
     
  13. FAST FRED
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    FAST FRED Senior Member

    A single blade prop is more efficient as it has less induced drag.

    4 blades is 4x the drag.

    The hassle is getting enough blade AREA to absorb the shaft power.
     

  14. Easy Rider
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    Easy Rider Senior Member

    Hi FF,
    You wrote;
    "A single blade prop is more efficient as it has less induced drag."

    Only in theory because a ballance weight will need to be opposite the single blade. And the drag of the ballance weight will more than destroy the efficiency advantage of the single blade. So the single blade is off the table. That leaves the two blade as the most efficient.
     
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