Controllable pitch prop, effective range

Discussion in 'Props' started by Magnus W, Aug 30, 2019.

  1. DogCavalry
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    DogCavalry I aim to misbehave.

    Magnus, did you come to a clear answer?
     
  2. DogCavalry
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    DogCavalry I aim to misbehave.

    For what it's worth, another field determined empirically that there was no legitimate argument to be made in favor of FPPs, other than simplicity and cost, and the performance sacrifice was so great that only the cheapest installations could justify one.
     
  3. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    I like the idea of an adjustable pitch but not on-the-fly, it's too expensive as DogC says.
    But adjusting dock-side is attractive.
     
    Last edited: Nov 27, 2020
  4. DogCavalry
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    DogCavalry I aim to misbehave.

    To address one of Jehardimans's points the change in pitch relative to the plane of the disk is a function of disk advance rate, and distance from axis of rotation. So CPP has only 1 perfect rpm/advance rate ratio, like a FPP. But unlike a FPP, the ratio is still acceptable over an enormously broader ratio range. The FPP is just *** outside of it's narrow band.
    The application I'm broadly hinting at uses a centrifical governor to keep the engine at it's optimum rpm, so that feature won't really work in boats, since the usable range of advance rate in aircraft is so small in comparison.
     
  5. jehardiman
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    jehardiman Senior Member

    I believe that vast majority of vessels are better served by a FPP than a CPP, just because the weight, cost, maintenance, and failure modes added by the CPP are not worth the less than 5% gain when operating just off optimum when a 10% gain can be made with a FPP at full throttle. <shrug> To each his own, but my requirement is to recommend the best solution based on the operational scenario.
     
  6. DogCavalry
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    DogCavalry I aim to misbehave.

    Can't really argue with that Jehardiman. Most boats spend most of their time at 1 throttle position and 1 load state.
     
  7. jehardiman
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    jehardiman Senior Member

    Actually, most Naval Architects do. Rather than just repeatedly parrot your own opinion, you should read for understanding and not try cherry pick numbers without understanding their meaning. In my very first post I said
    As can be seen the last three vessel types fall into the first or third case I listed. Considering that the worldwide merchant fleet over 100 tons is 55,000-56,000 vessels with 2000 -20004's new construction order book of ~100,ooo,ooo DWT of tankers, bulkers, and containerships per year (say 500-800 large vessels), less than 10% of those had a CPP. Dramatic Decline in 2020 Shipbuilding Orderbook, Increased Demolitions https://www.maritime-executive.com/article/dramatic-decline-in-2020-shipbuilding-orderbook-increased-demolitions For an example from the recent news the EVER GIVEN, one of a class of 13, is 199,629 DWT, laid down in 2015, 79,500 shp, and a FPP. That leaves the "General Cargo" class. Realistically, I bet that very few of those installed CPPs were on ships above Small Handysize (~25,000 DWT) and were mostly fitted on 3000-8000 DWT coastwise LOLO feeder ships for unimproved ports, and are expected to spend half of their time with empty containers but need more thrust loaded due to significant change in draft when laden with full containers, which means a CPP is an acceptable alternative. Container Ships For Sale | NautiSNP https://www.nautisnp.com/container-ships
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    But why would you wish to fit a CPP to a vessel which has as its raison d'etre to run at its service speed for some 90% of its time at sea?
    This I think you're being some what obtuse with those examples....since there is never a reason to justify a CPP for these type of vessels.
    They use tugs etc for those "times" when they need that addition benefit of thrust at low speed without overloading the engine, whether for manoeuvring etc.

    The choice of a propulsor does not always make logical sense. If the client wishes to use X, contrary to ones recommendation, then so beit.

    I now have many clients that actively want a CPP merely for the operational point of view, rather than a FPP.
    In the long term, it saves them money on fuel, maintenance etc...thus any other points, for them, are mute.
     
    Last edited: Mar 30, 2021
    DogCavalry and BlueBell like this.
  9. BertKu
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    BertKu Senior Member

    Thanks for an very interesting thread. In my view the only way a cheap variable pitch sort of propeller system could be made by a thin flexible screw which can be pressed in and out hydraulically around a shaft. Just an idea. Thanks for our explanations Ad Hoc
     

    Attached Files:

  10. popobowa
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    popobowa Junior Member

    Good day, does anyone know what method is mostly used to keep the motor running on a constant load, which input governs the pitchcontrol.
    1.if a diesel is used
    2. electric drive

    I can imagine a diesel might be controlled by EGT, and maybe a torquesensor?

    An emotor is probably governed by an electronic controller ..but is that controling rpm, torque...pitch....lots of possibilities...?
     
  11. BertKu
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    BertKu Senior Member

    I cannot comment on a Diesel engine, but for Emotors, it differs whether you use an Brushless motor or AC motor at High AC voltage or plain brushed motor. Here for an Brushless motor the controller normally only controls temperature and speed i.e. rpm. But you should realize that the inductance from the coils are extremely low at very low revolutions and extreme high pulsed currents. The result is that the Torque is slightly higher at low revolutions than at full speed. But the Torque is for easiness regarded as constant by most electronic people for a brushless motor.
     
  12. Barry
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    Barry Senior Member

    An interesting article and one that provides quite a few suppliers of CPP's
     

  13. popobowa
    Joined: Apr 2012
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    popobowa Junior Member

    who are actually producers of smaller (max 1m) cp props?
    I only found "West Mekan"...maybe "Hundestedt"..all Danish.
    Oceanvolt has one they sell in conjunction with their servoprop 15kW electric drive.

    If anyone knows further producers, pls post...
     
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