prop doing bad

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

  1. bit
    Joined: Feb 2008
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    Location: Trieste

    bit Student

    hi. A design ok.
     

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  2. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Nice illustration, thank you.
     
  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Nice schematic diagram.
    That about sums it up..nice one, Bit :)

    PS..i've just notice that image is from the paper "some current issues with cavitation and propeller rudder interaction"
    nice catch :)
     
    Last edited: Nov 11, 2009
  4. Guest625101138

    Guest625101138 Previous Member

    Bojan
    If you are still interested in gaining a better understanding of cavitation as it pertains to the situation you are looking at then this paper will be of interest:
    http://www.ichd2010.org.cn/ICHD-EC/ICHD/eighth/papers/ICHD2008_2B-02_50.pdf

    The prop modelling in section 4.3 and 4.4 will be of particular interest as it shows how cavitation zones are progressive and can shift around on the blades depending on their operating condition. In the case of an inclined blade the cavitation zone can be fully collapsing and reforming every revolution as shown in the example.

    Sheet cavitation is not something that happens suddenly causing a prop to instantly run unloaded as laymen think - that is aeration or ventilation. It is progressive and will occur on certain parts of the blades depending on loading condition, blade shape, blade depth and blade orientaion to flow.

    It is only reducing efficiency over what could otherwise be achieved with a better design/manufactured blade working under lighter loaded condition. As you want to get a higher speed for the same power you need to improve prop efficiency. One way is to reduce the amount of cavitation.

    A blade that has surface imperfections could have localised cavitation at those points. So examination of the blades and cleaning up if necessary could improve a little. If the leading edge has been blunted through impact this will have a detrimental affect.

    The paper demonstrates how Cp of the foil is tied into determining where cavitation is going to occur over the blade. This is what I mentioned in one of the earlier posts.

    The point of this is that understanding what is going on and being able to model it reasonably accurately from basic principles leads to much better knowledge than simply applying empirical corrections that are unlikely to match your application given the range of props and variation in application.


    Rick W
     
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Ahh..i see Rick is reduced to giving me negative hits again. (That's 100 neg hit points this week, i must be really hitting the "truth" nerves :):p)...it is the kettle calling the pot black "...dimwitted arguments.."...hahaha

    It is very very sad when someone -who claims to be an engineer- is so ignorant of facts and blinded by such belief in themselves that they do not wish to debate publicly and become silent when their flaws are exposed, or get their "big brother" to wade in...your only retort is to throw pathetic weak stones as neg hit points as a form of debate, shockingly shockingly pathetic and weak Rick. The most terribly immature and unprofessional behaviour as one can ever be....but there it is, you are not a professional engineer nor naval architect, so what can one expect, really??..honestly...just sad!

    It sums you up I'm afraid Rick. All mouth and no trousers....just weak and immature, very very sad way to attempt to take a higher ground, as if anyone really cares. It is not surprising you have no idea how to design real boats. Stay with the little models you do...

    Your typical MO is thus, make a bold statement, appear impressive and knowledgeable, thus (post#41):

    "..Cavitation makes a normal lifting type blade less effective than it would otherwise be. It only affects the lifting face (suction side)..."

    and, as always real professional engineers and naval architects point out your flaws. But, wait, what is your standard reply...., oh yeah, silence, and then you retort by saying (post #49):

    "...cavitation zones are progressive and can shift around on the blades depending on their operating condition..

    WOW, you go from one breath trying to appear impressive then, when countered, as always, you suddenly flip-flop and say the 100% opposite of your bold statement, before being questioned. Amazing!!!!

    He could have said "oh ok, thanks for letting me know, i didn't realise that", or something similar, as anyone wishing to learn or seek the truth on a subject would and even be grateful to those that point out the error. But naaah, just ignores the faux pas and and facts, and blindly carries on.

    You chop and change your position when questioned, ignore the flaws in your statements and then brazenly plod on expecting no one to notice your 100% about-face, clearly demonstrating your lack of comprehension and understanding of the subject at hand.

    What kind of pseudo-science-religion do you aspire to??

    I leave you with this:

    " Nothing is more dangerous than the certainly one is right.
    Nothing is potentially so destructive as the obsession with a truth once considers absolute"

    Francois Jacob.
     
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  6. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    This is the comment Rick Willoughby left me as motivation for negative reps he has just given me:


    "post #43 direct attacks rather than offering an alternative opinion. I do not like showing you up as a fool. Rick W"


    I invite everyone to read my post #43 and judge by themselves if my post is a personal attack on Rick Willoughby or is it rather a motivated technical explanation of cavitation, with pertinent bibliographical reference, and a note on usage of Wikipedia as a source of technical info.

    I have submitted this to the attention of the moderator, and have invited him to take a decision on this arrogant, patronizing and uncivil kind of treating persons who are trying to give help at the forum.
    I am appaled by this behaviour. You should be ashamed of yourself Rick Willoughby. I have never given you a single negative point, though I did have occasions to do it for the technically inaccurate and inconsistent stuff you have written on several threads.
     
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  7. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    daiquiri

    There is none, non at all in your post #43. It is a statement of fact, unlike Rick's #41.

    Rick just does not want to be questioned, he does not want to be shown as a "dabbler" or "amateur" to those he is trying to preach too. He is not open to reasoned argument nor when shown the error of his ways he ignores and childishly hits with endless neg point ...it is very irrational behaviour and suggest a deep rooted mental issue with "authority figures" and "control"

    Whatever it is...it ain't engineering and it ain't professional.

    How someone like Rick can be allowed to be called a "guru" is laughable. He wouldn't last 5mins in the real world of design/engineering. But this "cyber world" of his is probably all he has and covets attention and the need to be seen as a "pro"....just pathetic immature behaviour. Totally agree daiquiri, as always :)
     
  8. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Bblagonic,

    The problem you face when adressing a real world problem on a forum like this, is that you have to discriminate the input from those who are misunderstanding and misinterpreting plus the ethernal wannabee scharlatans, from those who have factual knowledge. There is only one way that will tell; final results and experience.

    I am an enthusiastic advocate of the principle of free speak, but must admit that sometimes it becomes frustrating to see skilled persons like daiquiri and others having to spend so much effort in negotiating pure incompetent nonsense that pops up from wannabees in each and every thread. I wish that Apex1's WROM were a reality.........!

    Now to your problem, adding to your dilemma as to wether I am thrustworthy or not....:

    Checking the data of the setup you have described against Wageningen B peformance, using a wake factor of 15%, we get the following results for noncavitating operation:

    Ja=0.69, SHP=257 hp, Thrust=11034 N. With a depth to shaft cl of 0.7 m, the cavitation coefficient ("cavitation number", Sigma07) at 0.7 radius is 0.17 and in order to limit cavitation to 10 % of the blade area, you need a BAR of 1.92! Obviously, this does not work in reality. To understand what actually goes on, please see the engine power diagram attached, where I have added some data.

    Your engine is very close to this variant. You have the line of nominal power, below that a shaft power line. If you look at the point 257 hp/3500 rpm (the noncavitating result from prop calculation), you will see that it is far above what this engine can produce. The maximum noncavitating rpm possible would be 3100 rpm.

    With an engineering guess of hull drag, we can check cavitating performance with reduced rpm and speed of advance. We do not enter the 10 % limit until about 2400-2500 rpm, so from there and upwards the cavitation intensity is increasing (thrust and efficiency going down). This is illustrated by the broken line, starting at 2400 rpm/96 hp and crossing the available shaft power at 3500 rpm/198 hp. This is the explanation why your engine is working at 3500 rpm and still cavitating heavily, as Daiquiri noted.

    The Sigma07 value corresponds to the lowest acceptable pressure coefficient (-Cp) anywhere locally on the blade profile at 0.7 radius. If foil analysis (test or numerical) indicate anything lower than -0.17 on the suction side, there will be cavitation from there on. A value of -0.3, as mentioned somewhere, is telling us either that the profile studied is not suitable for this application, or that the method of analysis is incorrect; the selection is yours!

    If I may give a piece of advice: listen to Daiquiri here, he knows what he is talking about (....as long as he is not contradicting me of course...).
     

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  9. Guest625101138

    Guest625101138 Previous Member

    Slavi
    I delayed my response to you on this as I felt it was simply a cynical question with the intention of you making good on your threat highlighted in the attached. You and I know you readily accept misinformation from others without offering your view. We also know you have a propensity to rush in without giving things much thought - as you do occasionally like this particular question.

    To make this clear for you, the Cp can be determined from JavaFoil for the particular prop section being considered. The method used to determine the area of cavitation using Cp is explained in post #53 above or in the paper linked to post #49.

    And I suppose you do disagree with the information given in post #53 as it discusses cavitation as being progressive. There is nothing here suggesting the motor runs away at the point of cavitation! So lets see you take a stance against this as you have made it clear you believe the engine will suddenly speed up when the prop goes into cavitation.

    My response to your silly comments are shown in red-

    Rick W
     

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    Last edited: Nov 16, 2009
  10. Frosty

    Frosty Previous Member

    Ad hoc you are mistaken when you suggest that someone can repeatedly hit you with neg points. The forums computer will not allow neg or pro rep points twice in a set period of time - 3 months.

    If you have 100 neg points in this one week as you say you have then they must have all be from separate individuals.
     
  11. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    i didn't say i have been hit by the same person in a short period of time. Just by the same person again, but within the time allowed on this system.....still weak and pathetic regardless.!
    All saying the same thing....sad really. Just like the poster above...seeking redemption.....!
     
  12. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Regarding thrust breakdown due to cavitation, it is a well known fact that the steepness, or gradient is varying primarily with rotor specific speed. A high specific speed unit, like a low solidity propeller will have a gradual performance reduction with decreasing cavitation coefficient, while a radial pump (=low specific speed) has a sharp threshold.

    So, in bblagonics case, we must expect thrust and efficiency gradually decreasing with propeller load, as shown in my graph.
     
  13. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Rick, it looks like you have decided to take aim on me and you don't want to let go. Well, no problem...
    The words you have highlighted were not a threat. I was simply telling you that I would not stay silent (I know you would like everyone to bow down to your feet but... sorry, I'm not that kind of person) if I saw that you were writing untrue or unproven information. If you see me saying some crap in some thread, and you can prove it, it is your right (or better, an obligation - if we want this forum to inform and educate) to do the same.

    This funny patronizing style of writing had made me laugh. Thank you. :D :D :D

    Now let's go technical.
    As far as my knowledge goes, the ONLY thing you can estimate with JavaFoil is the probability that the cavitation will start in a given point, via Cp value. Once you have decided that Cp at a given point of the foil is low enough to allow you to assume that the cavitation will begin there, you have no means of estimating neither the extent of the cavitation nor the type of cavitation, nor the quantitative loss of foil's performance.
    That is, unless you have an access to the source code of JavaFoil and you know how to modify it.
    I'm citing from the paper you have linked in the post #49:
    "...the sheet cavitation model presented here uses transpiration velocities to deviate the flow as to represent the cavitation sheet. These transpiration velocities v* are equivalent to additional sources strengths which are simply added to the part of the surface carrying the cavitation sheet. As a consequence, the implementation in the code is relatively simple since it is only a matter of modifying the slip condition on the body surface where the cavitation sheet is located. The problem is how to determine the strengths distribution of the additional sources needed to correctly simulate the cavitation sheet. The core of the model is the function used to determine this distribution and it is the topic of the next section."​

    Translation: once the cavitation starts (based on some Cp criterion), the model of the flow field around the foil will have to be modified by adding a number of discrete sources of unknown strengths. For the purpose of that research they have been modelled by a function the Authors have called "transpiration velocity v*". That function is a part of their computational algorhitm and, in case of JavaFoil, would need to be implemented at the level of source Java code. Are you telling us that you are able to do it? Sorry if I have my doubts that you have access to the source code of JavaFoil...

    There is nothing in Baeckmo's explanation that goes against my knowledge on cavitation, why should I disagree? The cavitation is a progressive phenomenon, in a sense that it grows with blade loading. The graphs given by Baeckmo should, imho, be inserted in the "Hall of Fame" section of the forum, if there is anything similar to that. He has demonstrated, in various threads, to have rock-solid balls when it comes to hydrodynamics and propulsion.

    I have used the expression "sudden drop in Kt and Kq coefficients" and I admit that the word "sudden" should not have been used in that context. It is my fault.

    But I should also have known that you would end-up playing a lawyer, desperately trying to hang to every word said, just to get yourself out of this unpleasant situation which has been created.

    What a pitty that every discussion where someone disputes your claims has to take the same ugly direction.
     
    Last edited: Nov 11, 2009
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  14. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Daiquiri

    Just thought i would add the "typical" graph of performance one obtains showing the effects of cavitation on Kt, Kq and efficiency, as you have rightly noted.

    This is pic attached, primarily for those that are unfamiliar and wish to understand a bit more. Daiquiri and baeckmo have posted very detailed stuff above already, and the 'words' are shown in this pic of prop performance.

    Rick, as always shall ignore, then chop and change his position....not worth responding to the "RickW website I am always right don't question me" forum D, waste of time, his computers cannot be questioned.
     

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  15. apex1

    apex1 Guest

    To Bblagonic,

    unfortunately I did not manage the WROM* going into production by now. You otherwise should transfer Ricks statements and Play Station numbers directly towards it.
    Ad Hoc, Baeckmo, daiquiri give you proper and sound advice, as always. Rick makes bubbles, as always!

    Regards
    Richard
    * WROM = WRite Only Memory
     
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