The Controllable Pitch Propeller, a summary.

Discussion in 'Props' started by apex1, Dec 26, 2009.

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  1. TeddyDiver
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    TeddyDiver Gollywobbler

    Fanie.. the list of suppliers is in the end of the first post..
    RHought.. It just don't work like that. The idea is with lower speed to lower the engine rpm even more to maintain the most efficient engine loading. Let's have an imaginary example. With a fixed prop at quarter throttle you get half the speed and the rpm of the full throttle but the engine load might be round 30% which means unefficient.. With CPP you increase the pitch and be able to use much lower rpm and have higher engine loading (~70%) and efffiency so the bottom line is saving ~50% of the fuel and longer engine life..
    2 people like this.
  2. Marco1
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    Marco1 Senior Member

  3. apex1

    apex1 Guest

    Thanks Teddy.

    I will go deeper into it, once the nay sayers and ignorant have settled down.
  4. jonr
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    jonr Senior Member

    Agreed, it is the same on airplanes. The smaller, less expensive ones have fixed pitch, the larger more expensive ones have variable pitch.

    Given the wider range of conditions, I'd say variable is even more important on a boat.
  5. baeckmo
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    baeckmo Hydrodynamics

    Naa, burrp, but maybe tha sscchpellingk...
  6. jonr
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    jonr Senior Member

    Is is correct that engines are most efficient at full load. For example, if you wanted maximum MPG efficiency in a gas engine, you would over-prop it such that at full throttle it would be "bogged down" to around (this varies) 2/3 of max rpm.
  7. RHough
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    RHough Retro Dude

    Okay. Say a 10P prop at 300 RPM gives 6 knots and you want to run at 3 knots.

    Do you change the pitch to 20 and that increases the load on the engine? Thus reducing the shaft RPM?

    Is the idea to maintain the % of load on the engine and slow the boat down by increasing pitch? Basically keeping the engine at peak torque more of the time?

    At higher load the fuel used each cycle is higher, but since the RPM is lower, less fuel is burned?

    If you want higher speed, you reduce the pitch and the RPM gain at the same engine load increases speed?

    I'm trying to learn, my experience is with high performance gasoline engines, where HP from RPM is more important than peak per cycle efficiency. Peak HP occurs at some RPM above the maximum torque RPM where BMEP is lower than at peak torque, but the rate of torque production is higher due to the higher RPM.

    I associate load with throttle position and RPM. The idea of limiting RPM by increasing load and keeping throttle constant sounds like "lugging" or overloading the engine. I understand that it is not, as long as you don't over do it, it is just a new concept for me so I am struggling with it.

    Thank you for helping me understand. + to you!
  8. jonr
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    jonr Senior Member

    You can call it "overloading", but it doesn't hurt the engine. On a gas engine, having the throttle plate at anything other than wide open is just causing the engine to work harder and not get a full charge into the cylinder.
  9. brian eiland
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    brian eiland Senior Member

    Electric Motor Drive

    The internal combustion engine produces its best torque characteristics at the higher RPM.s

    While our needs generally like to have the torque come on at lower RPM's.

    So we mate up these reductions gears, variable speed transmissions, and even CPP's to get a better match.

    The electric motor on the other hand produces its best torque at the lower RPM's. I believe the future in propulsion is electric drive. Sure there are some hurtles to be worked out in the D/E drive technology, but I'm counting on the automobile sector to foster a lot of this improvement in electric drive, as it suits their needs very well also.

    This was posted not to change the subject from CPP, but rather as an observation that is often overlooked by many people.
  10. TollyWally
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    TollyWally Senior Member

    Could you expand a bit on your views regarding pitch, effiency, etc. I'm not trying to overthink this stuff but reading your posts I sense just the tip of the iceburg. :)
  11. jonr
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    jonr Senior Member

    Back to CPPs - what is the smallest one can get? I only see them in 1000+HP boats.
  12. dskira

    dskira Previous Member

    I am not sure but if you read the all thread you will have the answer.
    I think it is in the 50hp range, but don't take my word for.
    The best is asking Richard, he knows better than everybody else about CPP.
  13. apex1

    apex1 Guest

    Thanks for adding that Brian (though completely off topic), we know already, there are other systems available!

    And please! This is NOT another elctric propulsion thread!!!

    It would be nice to remove the post.

    I will elaborate on the "Howto" issue later. There is no overloading in the example given.

    Excuse me, I doubt you have seen many? If any:cool:

    Daniel you´re right. About 50hp is the lower range.

  14. thudpucker
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    thudpucker Senior Member

    Constant Speed prop would automatically keep the pitch/Rpm/load at the most efficient working speed.

    Controllable pitch is used best when the Rpm is kept low so the Pitch can be changed rapidly to permit maneuverability almost instantly without Wrecking the Drive train.
    As if you had twins, and were moving the stern around to help catch a Net, or keep the fish in the Purse, or Docking, or a small boat used to Herd logs around in a Log jamb.

    Controllable pitch can be used well in Cruising at low speeds. Just set the power/Rpm to what you want to cruise at, and run the Pitch up till it loads the engine down. Your getting the best speed per Best fuel mileage engine performance that way.

  15. jonr
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    jonr Senior Member

    Are you aware that the WWW has pictures? While 50HP was mentioned earlier, why aren't CPPs common in small boats?
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