The Controllable Pitch Propeller, a summary.

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

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  1. RHough
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    RHough Retro Dude

    At the risk of getting slapped again... :)

    I think the hybrid guys could learn from you.

    This comment from that presentation says it all:
    Lower shaft speeds and different torque characteristics are where CPP's shine is is not?

    It seems to me that much like a diesel, these systems are most efficient when loaded properly. A CPP would allow the torque requirement at a given speed to match the optimum system loading if I understand this thread correctly.

    In fact for a system that seeks to recover some energy from the drive system whilst sailing a CPP would do that better than the fixed prop options that were tested also.

    They are looking at a 55kw system diesel/prop combination. A CPP designed for that power might make their system more viable.

    Sorry if you feel this was off topic, I see a parallel here.

    Randy
     
  2. apex1

    apex1 Guest

    While we wait for Marco to catch up.............:cool:

    a short "Howto"

    For the novice a CPP system seems a bit irritating, it offers far too many choices. You have too many possible combinations of pitch/rpm.

    During the first hours or even days of operation a lot of guesswork is indeed what you do.
    After becoming a little more familiar with the CPP installation, the pitch/rpm that´s best in a given condition, vessel loading, or speed will become virtually automatic.

    Having a pyrometer to check the EGT (Exhaust Gas Temperature) is helpful, as mentioned above.

    Opposite to the recommendation above, the correct way is to choose the pitch and adjust the throttle!

    First select the pitch that you guess appropriate for the requirements of the situation. Then increase the throttle to raise the engine rpm until there is no further gain in rpm.
    Now allow the boat to come up to speed with the power being applied until desired speed is reached. Now vary the throttle setting slightly making sure rpm is at its maximum for that pitch setting. You can back the throttle until you see the engine loses rpm, then set it just under maximum achievable rpm for that pitch. (of course nothing is wrong when you adjust / finetune pitch as well)

    Watch the pyro to be sure you stay well below the manufacturer´s recommendation for EGT. If it´s too high, just back off the throttle a bit.
    At this setting, the engine is powering the prop at its maximum ability, and runs well loaded at max. efficiency.

    Do´nt set the pitch too shallow, the engine will not be loaded by the prop and will run straight up to its maximum rpm.
    Do´nt set the pitch too steep either, for the given rpm, that will overload the engine. Dark smoke and a increasing EGT are a signal for overload.

    When trolling or net pulling, set low rpm, high pitch, to load the engine sufficiently at low power range (and low fuel consumption).
    When going uphill in adverse weather set high rpm, shallow pitch, to allow the engine to come up to higher power output.

    With a single prop vessel go for higher rpm (about 60 to 80%) when docking! You´ll need the thrust when you do the appropriate HARD manoeuvres. Do´nt fear to push or pull the prop in either direction at high revs it does not harm the system! (of course do´nt let the engine “overturn” at zero pitch during manoeuvres)

    I hope that was clear enough to understand (no SI units).

    Baeckmo
    any complaints?



    Randy
    I do´nt feel any need to slap you!
    And I do´nt think your post is off topic, though some wrong points are found.

    A description of a sample installation and the merits of it will follow!


    Regards
    Richard

    PS and thanks Moderator for cleaning!
     
  3. apex1

    apex1 Guest

    Yes bntii, ignorance is bliss.
    Though for the ignorant only, all others around feel much less of the joy.
    oops, why did you remove your post?



    Ok lets talk some basics:


    Let me try to give a example when and why a CPP installation is the first choice.

    To make it easy for me I have choosen my long range motoryacht as a example. This vessel will be built for unlimited cruising worldwide from the tropics to the high latitudes. She will have icebraking capability and is fully classed as a icebreaker. (makes hitting reefs and containers less a issue too)
    Range is well above 5000nm at cruising speed (SL / 1.1), endurance 3 month at sea.
    The boat is about 31 to 32 meter loa. The final design is not done yet for some personal reasons.

    The appearance will be roughly as in this sketch:

    see attachment!


    Though the hull below waterline will be redesigned for single prop and ice bow.
    more here:http://www.boatdesign.net/forums/me...o-anywhere-trawler-build-side-side-28678.html

    The engine choosen is a Grenaa Diesel of 95 liter displacement and 750hp at 500rpm. This one:
    [​IMG]

    Due to the low revving engine there will be no reduction gear necessary.

    Prop diameter is 1,7 meter (or 67 inch), shaft diameter is 18cm (or 7 inch), remember, the bigger the wheel you turn, the more economical you can operate.

    The whole shebang, Engine, shaft, prop is about 14 tonnes weight.
    Overkill I hear some say. Not really, when you bear in mind that this is a vessel operated all year round and worldwide. And for a given service life of the vessel of around 30 years, the cost of 550.000$ is a fair one. Much less than the average replacement of a smaller engine.
    And there WILL come a situation you would give your leg for as much power as you could have, be sure. Have it installed, it does´nt eat bread while not on duty.

    With a conventional FPP you would need to run your engine at about 80% power setting to keep her happy and within a sensible range of load.
    (we remember, a underloaded Diesel has glassing, coking and other problems and does not live long)
    At 80% we have a power output of nearly 580hp! Much too much for cruising speed. Our demand there is about 150 to 200 horses.

    Now, with a CPP, we can run the engine at a tad above idle under sufficient load for years, without any problem. And we have all the power needed, when the going gets tough and we have to escape a front, or steam uphill.

    Firing rpm for this engine is 120. That is idle 180 rpm (50% above firing revs.)
    At 200rpm the engine has about 180 hp cont. output. The perfect setting for a economical cruise.

    580 x 160g = 92,8 kg of fuel per hour, or 111 Liter
    180 x 160g = 28,8 kg of fuel per hour, or 35 Liter

    No doubt about the advantage.

    Of course one can install much smaller engines, save weight and tankage space and just operate at half throttle or less. But that gives a service life of a few thousand (very few) hours and never these small engines can turn a wheel of this size.

    Service life of such a displacement monster is about 50.000 hrs between basic overhaul, and probably 4 times that before a remanufacturing is needed.
    The average annual running hours are designed / calculated being 8.000 !!! Thats a long trip to go for even a enthusiastic boater.

    There was a comment on starting these systems. Yes there will be no clutch. (higher revving engines have a reduction gear which commonly comes with a clutch, though) And there is NO movement of the vessel when the prop is at zero pitch.
    The entire commercial fleet worldwide has NO CLUTCH. And worse, they have to stop the engine and start in reverse for backing.
    They DO move when the engine is started, but are usually tied to the docks while warming up.
    All of them are started by air, and a 20 or 30 bar air starter turns your engine, be sure.

    Remember, what is not there does not wear!

    Another point in favour of the CPP:

    Manoeuvring with a large prop (and accordingly high masses) can be a real task in tight quarters. The dynamic forces of the rotating mass do´nt allow to shift the gear as fast as you wish. A shaft brake is the common solution, but the pads do´nt live long, they heat up the engine room quite much, and still the shifting takes time.
    A CPP system needs one or two seconds to shift from full ahead to full astern!
    Try that with your gearbox, you´ll find the pieces all over the marina (when there is´nt a clever system to prevent such idioty).

    Even a tiny little putt, putt installation, so common on most sailboats is pretty slow, compared with a CPP.


    Have I forgotten something?

    Enquiries of the industry about my bank account, per private message please.
    Any amount is welcome!

    Regards
    Richard
     

    Attached Files:

  4. jonr
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    jonr Senior Member

    Guessing at a pitch is not going to result in optimal (however you want to define it) performance. And diesels and gas engines are optimized differently (the throttle does different things).

    A BSFC map is nice to have as is a GPM/MPG meter.
     
  5. TollyWally
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    TollyWally Senior Member

    Whoa Apex,
    I fail to see the need for the tone you take in your response to my post. You're a wee bit abrasive.

    Mentioning the possibility of broken parts in passing is no reason to get snippy. It's not out of the realm of possibility that more moving parts and rotating blades might have some differences in reliability and long term maintenence compared to a simpler system of machined and cast stationary parts.

    I can see no reason to castigate a person for being more familiar with horsepower than kilowatts. While it may be the norm in many places it is not here. Every ICE built on the planet that is sold in this country is rated in horsepower.

    I gave as many specs as I could for my set up in the metrics with which I am familiar. Specs that would assist a knowledgable man to make a reasonable rough estimate of the strength requirements needed. We use horsepower and footpounds and measure our shafts in inches. There are 300 million of us and that is how we roll.

    Lastly I don't expect anyone to pester a manufactor for an exact price quote on the strength of my whimsical request. I was hoping for an intelligent rough estimate off the top of an experienced man's head.

    Something like; 300 hp motor that's now using a 1.25" shaft, hmm well a set up of of this size ought to reasonably handle that load, lets see I think those go for 6 or 7 thousand. Perhaps I will be fortunate enough to be able to get some idea by extrapolating from Mark's numbers if it isn't a secret.

    As you have mentioned there has been a bit of midwinter unpleasantness here recently and we have perhaps lost a kind and wise contributer to the forum. Reflect upon your own postings and see if you have been as charitable and pleasant as you could have been. Collectively we all contribute to the tone and tenor of the correspondence here.
     
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  6. apex1

    apex1 Guest

    Well if you would read what is written, you could have skimped on that!

    Guessing is the only way to get familiar with such a propulsion! MPG meter? May I ask if you have any experience with boats?

    And we do not really talk petrol (gas) engines here. The marine world is Diesel.


    Tolly Wally,

    it was´nt my intention to offend you.

    When something brakes it brakes, so what? When there are less parts to brake, as is the case with a CPP, well, better so. I did´nt see the sense of the question.

    The marine world is a international one, and internationally it is agreed since ages that the SI unit is the only one to use.
    Nobody cares that the US of A are not willing to adopt that. Be happy with your system and do the extra math when converting the units. You cannot expect others are willing to do that for you. That is just arrogant! My comment was a bit snipe according to this (worldwide hated) sort of arrogance.

    It was not meant to insult you personally.

    Yes I will post the quotation for Marks installation, why should it be a secret. And NO I will not do any calculations or estimations for any other setup, sorry.

    Regards
    Richard
     
  7. TollyWally
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    TollyWally Senior Member

    Richard,
    I accept your apologies. I'll be able to extrapolate what I need from Mark's answer well enough. The assembly of bids and quotes is an inherent part of my livelihood. Painting with broad brushstrokes I can get to within 20% very quickly. Only when the need for detail is important and the seriousness between parties mutual do I spend the time on the minutia to narrow the numbers down to 5% or so. Not everyone is comfortable with that approach. Arrogance can be a two way street, it certainly is a double edged sword.
     
  8. jonr
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    jonr Senior Member

    Evidently apex1 is always talking about just the small subset of the marine world that he is familiar with.

    There are a lot of boat owners that would have a really good laugh about the idea of using a diesel (or a CPP).
     
  9. Submarine Tom

    Submarine Tom Previous Member

    Glad you two have sorted that out.

    Speaking of arrogance, how many Fox Islands do you think there are in the world Tolly?

    I am to assume from the text you are somewhere in the US of A.

    Richard, great posts on your project. When do you arrive in south western Canada?

    An additional attribute of circumventing a clutch is you'll never have a clutch fire!

    -Tom
     
  10. Submarine Tom

    Submarine Tom Previous Member

    jonr,

    They sure would have a good laugh and ignorance is bliss!

    -Tom
     
  11. apex1

    apex1 Guest

    Thank god I must not communicate on this level when I do´nt like!
    And building boats and Yachts from 6 to 46 meter, I am completely biased.

    Your expertise is too obvious.

    And learn how to avoid double postings! (after learning about the real marine world) ahh no before! We do´nt have the 20 years to wait.

    Tom

    you´re right on the clutch problem! Though the average boat has a clutch of course. As I stated very clear, my boat was just a example of ONE possible setup.

    And the Vancouver area is on my schedule!

    Regards
    Richard
     
  12. TollyWally
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    TollyWally Senior Member

    LOL Tom,
    I've made no secret of my location is past postings even down to the lat lons. In my younger days when sailing appealed to me more it was about a day and a half running continuos @ 5 or 6 knots with the tide and against, riding the back eddies when neccessary, from my house to Vancouver BC. We didn't bother with customs in those days and they didn't bother with us.
     
  13. baeckmo
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    baeckmo Hydrodynamics

    Quote Richard: ....Baeckmo, any complaints?

    What's this, am I enlisted among the grumpy ol' complainers now Richard.....grrrr! Well maybe not completely wrong today for various reasons!

    About finding the pitch/rpm balance: best efficiency in a diesel is often found slightly above max torque rpm's. This fact is used to reduce the amount of guesswork needed to find the optimum setting. So, with low to medium pitch setting, give her throttle to slightly above max torque rpm, then adjust pitch until you have the speed you want. The centrifugal regulator will adjust fuel flow up to maximum possible at this rpm setting.

    If you don't reach the speed wanted with max pitch at the selected rpm, that tells you she needs more power (kW, hp) than available at this rpm. That means increasing rpm's; reduce pitch and increase throttle position, say 10 % of the rpm range. Of course, having a complete fuel map at hand, or a fuel flow meter, makes it perfect, but the max torque method is often a good starting point.
     
  14. powerabout
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    powerabout Senior Member

    Apex 1
    I like your boat and the concept
    but when you start up a 750hp 500rpm engine...everybody on board is going to know its running especially if its conventially mounted, i.e crankcase to bedplate ( hull)
     

  15. RHough
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    RHough Retro Dude

    Maximum Torque occurs when BMEP is highest.
    Maximum Power occurs when RPM (rate of torque production) can no longer compensate for the lower BMEP at reduced volumetric efficiency.

    As I understand it, a CPP allows you keep BMEP at or close to the peak torque value at much lower RPM. This loads the engine at it's torque peak values at low RPM so power output can match vessel requirements at a wide range of speeds.

    What the range of RPM that allows full torque per cycle to be developed is off topic for this thread, but it reinforces the idea that a CPP and engine must be well matched to derive the full benefit of the system.

    I'm struggling, but I think I have this bit right ... :(
     
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