prop doing bad

Discussion in 'Props' started by bblagonic, Oct 24, 2009.

  1. bblagonic
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    bblagonic Junior Member

    Hi guys,
    I`m calculating the right prop for a boat of my friend.
    The boat`s performance is quite bad in comparision to a boatbuilder chart and the trials data I saw in one article (but with different engines).

    There are few things that I`m quite concern about:

    1. How could I predict the wake and thrust deduction factor for a planing power craft with a single prop?
    I´ve found on this forum a diagram but didn`t understand what Wt and Wq means? I need only W as a factor. Those diagrams are form Hadler 1971. paper and are used for twin prop configuration.
    See the diagram in att.

    2. When reading the Kt and Kq factors from KtKq diagram of Wageningen B-series, should I enter into the diagram with J as advance coeff. or with apparent advance coeff. Ja (Ja=J(1-w)?:confused:

    and the third

    3. What would be the best suitable calculation for checking if my prop will cavitate or not?


    Sorry for so much questions but I`m spending weeks trying to get inside the world of propellers.

    Thank you in advance.:)

    Bojan
     

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  2. gonzo
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    gonzo Senior Member

    Cavitation will be produced by too high a propeller speed or too small a blade area
     
  3. bblagonic
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    bblagonic Junior Member

    tnx Gonzo,
    the thing is that I would like to listen from you your experience about the way of accurate prediction of cavitation.
    Is this from the Burrill charts or using the Gerr`s equation which include the MWR (mean width ratio), or some else empirical formula?:cool:

    regarding the thrust deduction factor and wake factor...
    If somenone has the pdf of Hadler and Hubble paper: Prediction of Power performance of the series 62 planning hull forms" 1971,
    I`ll be very grateful sending me this.:eek:

    thanks guys
    Bojan
     
  4. gonzo
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    gonzo Senior Member

    Gerr's book is simple enough. Just plug in the numbers and it gives you propeller dimensions.
     
  5. baeckmo
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    baeckmo Hydrodynamics

    Bojan, here my personal view:

    1/ For the planing region, monohull, ~constant deadrise, use (1-w)=0.97.
    2/ Understand Ja as the working environment for the prop; Va=Vship*(1-w), ie.; Ja=Va/(n*Dp).
    3/ As I am unfamiliar with Gerr's algorithms, I suggest you use Burrill's limits. If it is a leisureboat you may accept a 10 % cavitation level according to B's calculation method.
    4/ The Hadler/Hubble paper is a recalculation of the original Clement/Blount test results; if you can't locate it, try to find the original paper:
    "Resistance Tests of a Systematic Series of Planing Hull Forms" SNAME 1963.
     
  6. Guest625101138

    Guest625101138 Previous Member

    Bojan
    If you really want to understand what is going on you should make use of a good analytical tool like JavaProp. It will take a few hours to appreciate how it works but it will give give you much greater insight into what is happening than looking over empirical charts. You can run the Applet from this page:
    http://www.mh-aerotools.de/airfoils/javaprop.htm

    You will not be able to get the exact foil of the prop being considered but there will be inbuilt ones that are close to the one you are looking at.

    The thing with JavaProp is that it can give you Cp for the prop. From this you can determine if the blades are cavitating by considering the flow velocity over the blades at various radial positions at your speed of interest.

    If it is a typical inclined shaft installation then the shaft inclination will have much greater influence on cavitation than any wake factor on the planing hull.

    Another thing that often gets scant consideration is appendage drag. Are protrusions minimised and streamlined. Also how does the actual weight compare with the boats you are comparing it with.

    Rick W
     
  7. daiquiri
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    daiquiri Engineering and Design

    Hello, may I ask you please to elaborate a bit more on the following (not very clear) passage: from Cp obtained from JavaProp to the cavitation estimate?
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Well Rick, are you going to reply to daiquiri's simple question??..or doesn't JavaProp know how to answer simple questions?
     
  9. baeckmo
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    baeckmo Hydrodynamics

    Bojan,

    To gain the required "engineering feel" for the variables in propeller selection, I strongly recommend that you first continue along the path you are following now, before you start using computer programmes! The graphical overview you have when working along the manual way will give an understanding of the optimizing process, that is not a natural result from just flipping numbers in a software.

    I understand that you are using diagrammes in the (kt, kq, eta) versus Ja format. A better way to get preliminary results, and to see the possible alternatives, is to use the Taylor diagramme, ie P/D over load factor Bp. This format is available for a range of configurations for the Wageningen B-series propellers. This series covers most applications with a reasonable precision; special props are a bit further down the road for you right now, I guess.

    As for the cavitation limits, the Burril method can easily be transferred to an Excel sheet, becuse it just involves routine arithmetics based on loading figures from the propeller selection. Of course, it doesn't hurt to have a clue on what is meant by the basic terminology (cavitation, NPSH, cavitation number a.s.o).
     
  10. apex1

    apex1 Guest

  11. daiquiri
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    daiquiri Engineering and Design

    I think it would be really about the time to start educating people about the concept of VALIDATION of the software before using it for the evaluation of new designs, perhaps in a new thread dedicated only to that issue.
    And that thread should (imho) pop-up each time a person connects to the forum.

    Now, there is a page with validation data at JavaProp site. It can be found here:
    http://www.mh-aerotools.de/airfoils/jp_validation.htm
    coloured lines are test data from NACA Technical Report n.594, thin black lines are results from JavaProp.
    How to read diagrams:
    - Fix the advance ratio J on the horizontal axis, draw the vertical line through it, up to the curve of interest.
    - Read the corresponding values of Ct, Cp and Eta (efficiency).
    Ok, so let's read what the Author says about the results of the validation:

    1) Efficiency vs. Advance Ratio:
    He says:
    "It can be seen that the overall characteristics are met quite well; typical deviations are in the order of 5%"?​
    Well sorry but, based on the graphs I can see, I have to disagree. All the lines start to diverge severely above the point of max. efficiency, quickly becoming completely useless. Below that point, it seems that anything can happen.
    The orange line follows pretty closely the test results up to J=0.5 and becomes very imprecise above that value.
    The magenta line is reliable only in the range between J=0.8 and J=1.2. Below it there is a 10+% deviation and above it the error goes skyrocketing.
    The blue line is out of target by nearly 10% along it's enire length, diminishing the error to some 6-7% only in the region of max efficiency.
    So not only the error is not constant, it also follows a very irregular pattern with respect to J - depending on blade angle of attack. So, with such a herratic behaviour of the error function, how can you use this software for a reliable prediction of design chracteristics of a new prop?

    2) Thrust Coefficient vs. Advance Ratio (but the same is valid for Power Coefficient vs. Advance Ratio curves):
    The Author says:
    "Here we can see a good representation of the region where the curves are almost straight"
    Where is that? The only thing well-rappresented is the slope of the curves, in the linear region. But all the calculated curves are shifted to the right, and that shift induces errors of the order of 30%.

    The errors observed above are due to the fact that JavaProp uses a strip-theory (which becomes "blade-element theory" in case of props) method for the analysis, combined with empirical airfoil data. It means that the flow is esentially considered to be bi-dimensional at every station, thus neglecting all the very important effects of radial flow components along the blades (particularly near the blade root or hub). It also relies on the the correction factor for tip losses (Prandtl's, Goldstein's or else), which generally becomes less accurate with increased blade loading or Advance Ratio. That is the main reason for the big errors observed in the region of low Advance Ratios and for the shift of the curves at high J's. Back in my university days I had developped a very similar algorithm (written in Matlab, for a good old i386 :) ) for direct and inverse aircraft propeller analysis, the same strip-theory and (consequently) exactly the same kind of errors encountered. I'm looking here at the graphs at the JavaProp's validation page and it seems to me too look at those old printouts of Matlab graphs... We are talking about 1997-1998, so its nice to know that fundamental physics have not changed since then, and the algorithm errors have not become smaller just because graphs are calculated and plotted much faster and in colors now.

    Now, the authors of JavaProp (dr. Martin Hepperle et al.) have put a big effort and a considerable ammount of their time into that site. Big respect to them for the job done. But they have been a little bit optimistic when claiming the precision of JavaProp's output results - at least from what can be seen in the page above. And I'm pretty sure that JavaProp was never really intended to be a primary tool for prop design, if not only at a preliminary stage.

    So I would like to advise averyone to be careful when using the numerical analysis software for design purpose, be it JavaProp, JavaFoil or whatever. You better always check out the validation results first and try to understand the limits of validity of mathematical models used by the software. Unless you intend to use it just for playing around with some numbers in your spare time.

    P.S.
    Bojan, if I were you I would stick to the much more reliable propeller charts and to Baeckmo's sound advices - Taylor's chart for prop sizing and Burril's chart for cavitation estimate.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    The method described by baeckmo is exactly the method i use when designing a prop. I ahve a "fist full" of charts and as he rightly points out
    ...The graphical overview you have when working along the manual way will give an understanding of the optimizing process, that is not a natural result from just flipping numbers in a software.....nuff said!

    As for validation.
    Whole heartily agree with Daiquiri. Without validation from an independent source, once is ok, twice is much better, the output is as much use as a chocolate tea pot!...nice pretty colour plots though :p
     
  13. baeckmo
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    baeckmo Hydrodynamics

    Very good observation daiquiri; and a very important note on validation!!! While you were writing I was in fact checking some of the values in the JavaProp validation section. If we just look at the 30 degree setting, which equals a P/D of about 1.26 (depending on definition), and take values for Ct at J= 0.75, 1.0, and 1.25, we get for JProp Ct~ 0.13, 0.1, and 0.042. The corresponding test data are: ~0.155, 0.112, and 0.065.

    The absolute difference is then 0.025, 0.012 and 0.023; the error compared to the test is then 16%, 11% and 35%. The error is systematic and far to great to be acceptable even in preliminary calculations. And dreaming of the use of any kind of pressure coefficient to predict cavitation is just nonsense!

    Although I like the working principles of the JavaProp and admire the effort put into it, it still has to see some further development before it is of any use. The observed deviations from test data suggest to me that the lattice/solidity effect of the vane configuration is not fully taken in. Both the fact that the JProp data show a stall at about 75% of the J values and the too low circulation values after passing the vanes, point in that direction.

    When you test a rotor (pump, propeller or turbine) with fewer vanes (less solidity) than optimum, you will see deviations in performance, very similar to the deviations between calc and test in this validation example.

    If this is correct, it should not be too difficult to introduce a better compensation for the lattice effect into the program, making it a very useful tool for us, instead of a toy.
     
  14. pistnbroke
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    pistnbroke I try

    maybe I am stupid .but if this boat exists can we know the length and the engines its got on it them maybe someone can tell you what prop to put on it ...as a ball park figure
     

  15. bblagonic
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    bblagonic Junior Member

    well I can`t say I`m surprised about the discussion arrised from my questions as I had clear view about the knowledge of members of this forum...tnx

    now...back on subject

    baeckmo...I will follow your suggestion to first introduce myself in theory and empirical equations dealing with propulsion
    the fact is that I have the basic knowledge in mechanics and hydrodyinamcs as I graduate as Marine Engineer but till know I was dealing mainly with mechanic system of boat and ships:cool:

    pistnbroke...your question is far from stupid but my intention is to fully understand the problematic and to be capable resolving the future problems that will arrise in my career

    as your advice is to go intro the Bp delta diagram for better estimation could you also advice me where can I found (buy, download) all the these diagrams. I have few of them but those ones are far from all the chart numbers that Taylor made?


    the boat is a round chine, round stern, single screw (5° angle shaft) and dimensions are as follows:

    Lwl = 6,5m
    Bwl = 2,45m
    BHP = 220 hp at 3600rpm
    screw DIA. 17"
    screw PITCH 23"

    the screw is light scewed, AU series, three bladed with B.A.R. of 0,73

    DISPLACEMENT (supposed) - 4t = 4.000 kg

    the goal is to reach the max speed of 23-24 knots
    Now the boat is performing only 20 kts at 3500rpm what is the max. achievable

    producer trials data says that with 220hp and displacement of 3,4 tons (3400kg) the boat max speed is 26knots

    these days I will try to measure the exact displacement :idea:

    As the eninge is old, I`m not quite sure if it still develops the design power. Do you have some advice how to check this? I`m also ready to buy a device (dynamometer) in order to be capable of checking the real power the engine is generating. (Some advice about the model or producer of a small power meter?)

    once more time my acknowledgments to all of you
    I`m sure we will collaborate well in future
     
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