Controllable pitch prop, effective range

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

  1. Magnus W
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    Magnus W Senior Member

    One of the arguments for CPP is that a FP is only at its best at a certain rpm/load/etc. But since a CPP "only" can compensate for differences by adjusting the pitch it leaves so many other variables untouched (diameter, blade area, gearing etc).
    I'm just starting to wrap my head around this CPP thing but I assume that a CPP propulsion system must be designed for a certain set of conditions just as a FP (I mean we can't just slam a generic CPP on any boat and adjust for differences with pitch).
    So if a FP has a very narrow window when it's really efficient, how much wider is a CPP? (CPP efficiency should go down the drain with to much towards either fine or coarse pitch.)
     
  2. jehardiman
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    jehardiman Senior Member

    A fixed pitch propeller is designed to achieve maximum efficiency at a selected Advance Ratio (J). J= Va/nD with the understanding that pitch distribution is a fixed function of D, i.e. the pitch varies from the root to the tip to work with the expanded blade area to provide the desired torque (Q) and thrust (T) (i.e efficiency= [T*Va]/[Q*n]). Similarly, a CPP prop also has only one speed-rpm-root pitch combination where it achieves its maximum efficiency. Because the pitch distribution (root to tip) is built into the blade, any other combination will not reach maximum efficiency. While you can change the root pitch, this will change the tip pitch too much or too little, leading to a loss of thrust or an increase in torque, both of which decrease efficiency. Whether this is better or worse than a FPP at some off J is open to debate depending on the situation.
    Generally there are only three situations where you should select the added cost, complexity, and maintenance problems of a CPP: first is where the drag of the vessel varies dramatically for a given speed such as a towboat, second is where the prime mover changes dramatically such as a CODAG patrol craft, finally where there is the need to immediately start, stop, or reverse thrust without changing engine speed or rotation direction.

    Edited on a second read through to make clearer.
     
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  3. Magnus W
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    Magnus W Senior Member

    I understand the basic arguments for using CPP in these three situations. But all three call for large changes in pitch in order to deal with huge differences in conditions or demands. The question I'm trying to ask is more narrow and I'll try to explain what I mean.

    Consider a boat that's being used in a large speed range and with large differences in mass (much like the opposite of a patrol boat). Most of the time the boat is planing and it's primarily in this speed range we would like to increase the efficiency. We don't want a large reduction in speed with increased mass but at the same time we don't want to pay the efficiency penalty of running the engine at low(er) load most of the time.

    In other threads arguments have been made that a CPP installation is neither more expensive nor more complex that the corresponding FP. Assuming this is mostly true, what are the differences?
    I can see that a CPP powertrain should be heavier (in order for you to able to actually use it at high load/low rpm so increasing weight with increasing max load span) and as pointed out earlier also with more drag and a less efficient prop (or should I say without the sweet spot of a FP) even though a CPP designed for a narrow range should be better (in this range) than a broader counterpart. But data from engine manufacturers suggests that efficiency really goes down the toilet when load goes down, I looked at the Scania DI16 and fuel consumption increase in the neighbourhood of 20 percent for a given output at 50 vs 100 percent load.

    Could a CPP be used as a way of optimising the engine load for a certain speed and variations in mass (certain speed in this case would be the planing cruise speed) with a net gain in total cost of ownership?
    The question is asked with a workboat that runs a lot, say at leat 1000 hrs per year, in mind. And difference in drag for the "normal" mass change is 20 percent up and down. (Drag calculations taken from a specific boat but I'd like to keep the thread generic if possible.)
     
  4. jehardiman
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    jehardiman Senior Member

    Are you using an internal combustion engine (ICE)? This is important to your question because how a propeller absorbs power and how the prime mover produces it. A vessel will always go as fast as the power available lets it go. And a propeller will always find its most efficient point if RPM is free vary based on torque needed. The reverse is true for an ICE, RPM will vary to deliver the needed torque. So because the FPP power absorbsion curve and an ICE power delivery curve roughly track each other, the propeller is mostly always operating at the most efficient J. There are very small gains to be made with a CPP, but that is in an ideal situation. "GENERALLY" this ideal increase is not sufficient to warrant a CPP.
    Secondly is the matter of specific fuel consumption (SFC). SFC is power delivered/mass of fuel in. Yes, an ICE increases SFC when operating off optimum, normally to improve cylinder and valve cooling. But, as you question, is a 20% increase in SFC when operating at 1/2 power going to cost you more than a 5% loss of efficiency at top speed? For that decision you will need to dig down into the very specifics of the economics of the operation. I have seen a situation where the selection of equipment was based on the cost of the required lube oil, not the hardware.
     
  5. Magnus W
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    Magnus W Senior Member

    Thank you.

    Yes, the original question is for an ICE (diesel). And my operation is problematic since I operate in both deplacement speeds and planing speeds and in a large weight span. We have used two weights for the initial calculations, 9500 kg (lightship) and 12500 being on the high side but still normal.
    Over 14000 will not be common and 15000 is kind of a limit. All weights are based on a single jet installation, power train weight is 2700 kg and it would seem that a twin installation with props in tunnels will be less heavy (although not by a game changing amount).
     
  6. johneck
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    johneck Senior Member

    It seems to me that the first question is can a FPP even meet all the design points that will be required? If you have to set the design point for the max speed/max load condition, you will not be able to use the installed power in a lighter condition even if you would like to. As Mr Hardiman stated, the FPP will follow a curve that follows a resistance curve (in general), but with greatly different displacements you will need to look at several resistance curves and thus several RPM vs power absorbed curves. That is where the CPP excels, the ability to shift the power vs RPM curve left or right without very much loss of efficiency. A well designed CP (and there are lots of them) can probably operate from 80% to 120% design pitch with very little loss in efficiency.
     
  7. jehardiman
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    jehardiman Senior Member

    But remember what efficiency we are talking about... the FPP's or the CPP's. Generally CPP blade shape and pitch distribution is developed to give that ability to vary design pitch. In doing that they have a lower peak efficiency and an overall flatter efficiency curve for all achievable root pitches. So while a CPP may be able to vary pitch without much change in their maximum efficiency, that maximum will be appreciably lower that what can be achieved by a FPP.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    So this begs the question - what is your objective?

    Since you're debating CPP v FPP, but in the holistic nature of the operation, what is the objective at each design condition?
     
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  9. baeckmo
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    baeckmo Hydrodynamics

    Yes, but the CPP will have a higher efficiency at off-design operation due to its wider (=less "peaky") efficiency curve. On top of that, at all off-design loads, the CPP can be operated at the minimum fuel consumption (BSFC) point for the engine. This is more important than a percent propeller efficiency up or down. Practically all commercial fishing vessels in Northern Europe are equipped with CPP and there is a lot of info on how to optimise the pitch setting at various loads. First step is to have engine fuel map available, second step to estimate an operational load histogram to see where "the money goes".
     
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  10. Magnus W
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    Magnus W Senior Member

    Thank you. Interesting "hard" numbers, although I'll treat them as a ball park figures.

    I'm contemplating whether CPP can be the best solution for mer or not. (But at the same time trying to keep the question as generic as possible so that others may gain from this thread.) So that's my objective for this thread.

    I don't understand (language issues) what you mean by "objective at each design condition". Can you explain so I can reply (I would very much like your input)?

    This is what I mean. An indication that the lack of peak might (and perhaps even probably) be well compensated for by a more efficient use of the engine.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Let's say your target speed and displacement in
    condition 1 is 10 tonnes and 5 knots
    and
    condition 2 is 15 tonnes and 20 knots

    So, the design driver to satisfy, in terms of power and fuel consumption etc for no.1 will be very different from no.2 - this is obvious. BUT, and the question at hand, if the objective is to minimise fuel consumption in each condition, by fuel consumption i mean by actual amount in litres of fuel burnt, nothing to do with SFC of the engine, this will dictate the solution.

    Sine to minimise this one attribute means all others shall be compromised. But so what?..if the objective is to minimise fuel consumption for both conditions, a compromise will be required and thus you must ignore these competing requirements, as they will exist no matter what and you will end up chasing your own tail and never arrive at a satisfactory solution. In simple terms this is the SOR - the statement of requirements. If you have no objective, what are you aiming at to satisfy?...too many competing elements is the answer and never the 'twain shall meet.

    Thus define the objective and satisfy that objective alone.

    Does this make sense now?
     
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  12. Magnus W
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    Magnus W Senior Member

    Yes, it makes perfect sense. Thank you.

    The spread between my two condition extremes is roughly what you mentioned in your explanation above. But the width of my objective is more narrow which is why I named the topic "... effective range".

    In short, my objective is to minimise fuel consumption in the area defined by 10000 – 12500 kg and 22 – 28 knots since this is where I'll mostly be.

    With the risk of being blown out of the sky:
    I estimate that more than 90 percent of the fuel burned will be in this area which in my calculations add up to 1.620.000 liter of fuel based on 20.000 h and 90 l/h. Yes, it's a very crude estimate but in my uneducated opinion reasonable. A 1 percent efficiency increase in this area is (in todays fuel price) worth about 57.000 SEK or circa 6.000 USD. Based on my objective and questionable math, is it possible to make an educated guess about which propulsion is most suitable (jet, FPP or CPP)?
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Ok.
    So you then need to look at the cost and weight implications of using a CPP v FPP as well then.
    Increase weight = lower speed or more drag, which ever way you wish to look at it. Also more cost, so higher initial capital costs.
    But it does suggest that you have an objective.

    So, the exercise will be select/design a simple FPP for your design condition. Work out the f/c at the target speed/displacement.
    Then do the same, with the same set up but at the different displacement and speed you cited above.

    That is your condition no.1

    Then repeat the above but using an FPP, taking into account the aforementioned increases.

    Until you run the numbers, you wont know which works best for you.

    But, in either case, you are seeking a best fit compromise, not an absolute.
    Absolutes do not exist in naval architecture/design.

    A successful design is greater than the sum of its individual parts...
     
  14. Magnus W
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    Magnus W Senior Member

    Again, thanks.

    About the "simple FPP for your design condition". I assume you mean that I should use/optimise one FPP for my entire envelope and see how far from optimum it performs in the other corners. But which corner to use as starting point, heavy/fast?

    You then say "repeat the above but using a FPP". Do you mean CPP? If you mean CPP, should the starting point (for lack of a better word) be in the centre of the envelope?
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Correct.

    What does the SOR say, which is the most dominant feature to be satisfied? And that'll be it!

    Yes, sorry typo - i type too fast and often don't double check or my speller checker takes over and corrects, incorrectly and I don't spot it.

    That's again for you to decide. Based upon what is the dominant feature to be satisfied. Your No.1 priority.
     
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