prop doing bad

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

  1. Guest625101138

    Guest625101138 Previous Member

    Bojan
    JavaProp determines what is occurring from the basic physics. It is primarily an analytical approach. The prop curves you are using are based on extensive test data. They are empirical. A good analytical model should have close agreement with good test data for given conditions. As you rightly point out if there were no cavitation you would have the actual thrust being achieved by either method.

    My model is similar in construct to JavaFoil only I have more detail that can automatically calculate for different Re# and cavitation. So my model takes more real world factors into account.

    The basis for my calculation for cavitation limits the lifting face pressure as actually occurs in the real situation. JavaProp does not have this capability.

    I determine the point of cavitation using the Cp for the approximate blade shape and the velocity over the blade at the various radial positions. I provided you with the Cp curve for a 5% foil as is the AU series. This curve can be produced using JavaFoil. It shows the Cp on the lifting face is around 0.3.

    As far as the hull goes it has to be operating essentially in the planing regime at 20kts given the length, beam and weight. Although the curves I attached to the last post were not intended to represent the hull you are looking at, they do indicate what you can expect as the hull goes from displacement mode to full planing. The lift to drag ratio is increasingly related to the angle of trim as you get into planing mode. The trim is easy to measure at various speeds. From this you may be able to determine where the boat is on the drag hump. If it is under 5 degrees then it likely over the hump. If it is above 6 degrees it is likely still coming off the hump.

    I have my doubts that the motor has sufficient power to get the boat much faster with a different prop having the same diameter. The risk is that the motor becomes torque limited and you actually get lower top speed.

    You are better off doing more testing so you get more data. This will build a more comprehensive picture. Trim, speed and engine revs are all easy to measure and help build a picture of the drag curve. At slower speeds you will not have prop cavitation so this removes one of the difficult variables.

    Getting rid of weight and moving any moveable weight forward usually helps getting fully on the plane. So have a look at what can be done in this regard.

    The aim of further testing is to get confidence that the engine will have the power to get a higher speed with a different prop.

    The reason I like analytical methods is that they lead to understanding the basic physics rather than blind faith in test data that might not relate well to your case. In the case of the curves you are using they are related to a particular blade section and plan shape and at a Re# only applicable to the scale and speed used for testing. They are bound to have errors or need correction for shape and Re# even without allowing for any hull interference and cavitation.

    I have seen some useful analytical tools for determining drag on planing hulls but I have not found one I can afford. Hence Savitsky has to suffice although it is not ideal and it will help if you understand how the dynamic lift interplays with wave drag and viscous drag.

    Rick W
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Bojan

    If you wish to fully understand your problem, you ahve to be able to "think" like a naval architect/engineer too. For example:

    "..JavaProp determines what is occurring from the basic physics. It is primarily an analytical approach...The prop curves you are using are based on extensive test data.... They are empirical".

    So looking at this statement. Starts off by saying the approach is analytical. Oh wait..then it is taken from testing (ie not analytical)...then..oh wait, more....it is empirical.

    This is a classic statement made by those that are not professional engineers, nor posses the mental analytical tools to dissect data to establish what is what objectively, as one is trained to do. Since a procedure cannot be purely analytical and empirical at the same time, it is one or the other.

    Since
    "...I determine the point of cavitation using the Cp for the approximate blade shape and the velocity over the blade at the various radial positions.."
    How is this analytical..it aint...it is pure guess work.

    There are currently no 100% correct analytical tools that can give exactly prop designs, such as any RANS programs. There are some which are getting closer, but there is far far too much going on, in the region, in respect to fluid dynamics, to say analytically what is correct.

    You may want to pursue some aspects further by reading:

    "Design of Propeller Geometry Using Streamline-Adapted Blade Sections", by Kim, Kim, Pyo, Suh, 2009
    "simulation of unsteady flow free-surface flow around a ship hull using a fully coupled multi phase flow method", by Zwart, Godin, Penrose, Rhee, 2008,
    "Investigation on the Vortex structure of propeller wake influenced by loading on the blade", by Paik, KIm, KIm, 2007
    or even
    "Basic design of a series propeller with vibration consideration by generic algorithm" by Chen, Shih, 2007.

    These if you wish to peruse an analytical method, will provide more background.

    But first, look at the basics. What is the real true displacement of your hull (website has just 'sales' data, ie not reliable)...the real true power delivered, and the type/shape of boat. Without this basic data,....you will go no where fast..unless, you wish to pursue just an analytical understanding of prop's.
     
  3. baeckmo
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    baeckmo Hydrodynamics

    Sorry Bojan, but basic info on hull shape is missing. Speculating on drag etc (not to mention some unvalidated "numerical analysis"), is a waste of time until we know more of her shape and have correct figures on displacement and projected frontal area for air drag.

    And regarding the amount of cavitation, I feel inclined to doubt the test data you refer to as well, unless the AU prop has quite extraordinary cavitating qualities.

    Cavitation performance is very individual for the various propeller series. As per today, there is no realistic numerical method available for engineering prediction. In order to get an idea of the reduction as f of cavitation number, you must have test results, either in the usual "kt/Ja" format with sigma as parameter, or in the "tau/sigma07" format as seen in Burrill's cavitation diagram. Unfortunately, the usual annotation of sigma as f of advance velocity is hiding the real physical effects of the governing velocity, which is the local relative velocity. With this in mind, the Burrill format should be preferred.

    Attached you will find a compilation of data for some propellers that have "crossed my trail" (real world trial data). The Gawn-Burrill series behaves quite similarly to the Wageningen props, while the Newton-Rader is a typical high speed transcavitating propeller with the cambered pressure face, necessary for good cavitating performance. It is just to demonstrate the variations and general trend (veeery empirical...), so use with caution!
     

    Attached Files:

    Last edited: Nov 4, 2009
  4. bblagonic
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    bblagonic Junior Member

    Producer of the boat in subject told me that if cavitation was my problem than the engine would go to overspeed.

    But my engine is not reaching even max rpm ?!?!:confused:

    What do you think about that? Theoretically looking it seems to have right and I think that I read in some article that the cavitation would result with overspeeding of the engine.
    But as I told, the actual engine is not reaching even max rpm insted my calculations give results that the propeller is in high (more than 50%) back cavitation area.
     
  5. daiquiri
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    daiquiri Engineering and Design

    If you cannot reach the max. rpm, it means either that the engine is under-performant or that the prop is over-pitched.
     
  6. Submarine Tom

    Submarine Tom Previous Member

    Change the prop and see what happens...
     
  7. Guest625101138

    Guest625101138 Previous Member

    Many people use the term cavitation when they are referring to aeration. I expect this is what the boat producer thinks you are talking about.

    Props can have localised areas of cavitation. I determine that the prop you are looking at is in partial cavitation. This is a localised process over a portion of the blades that lowers efficiency. Some props are designed to work in cavitating condition.

    This might give you some better understanding:
    http://en.wikipedia.org/wiki/Cavitation

    I roughly agree with your calculation. Cavitation does not kill the prop performance it causes an increase in slip and consequential reduction in efficiency.

    Rick W
     
  8. Submarine Tom

    Submarine Tom Previous Member

    Some people use the word aeration when they mean ventilation.

    Tom
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    quite right Tom.

    A prop with a hull 'above' it...this is cavitation.
    A prop with no hull above..it is ventilation, the air is literally sucked down from the surface, which cannot occur when the prop is under a hull (possibly cavitation too, but that depends on the load factor on the props, ie the principal causes of cavitation).
     
  10. daiquiri
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    daiquiri Engineering and Design

    Rick, I am sure that mr. Vicenzo Catarsi, after many glorious decades of boat design and building, knows well the difference between cavitation and ventilation.
    And, more important, knows very well the performance of his best-selling boat and what steps should be taken to fix this problem.
     
  11. Guest625101138

    Guest625101138 Previous Member

    Bojan
    The attached photo compares a CFD model prediction of cavitation and an actual photo showing the partial cavitation of the blade. As you can see it is only affecting the outer portion of the blade.

    Cavitation makes a normal lifting type blade less effective than it would otherwise be. It only affects the lifting face (suction side). The pressure face is still working more or less as normal. The suction pressure is limited by local water boiling at the low pressure.

    Who ever you were talking to that said the motor would suddenly speed up when the prop cavitates does not understand cavitation.

    Rick W
     

    Attached Files:

  12. Ad Hoc
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    Ad Hoc Naval Architect

    Oh dear, yet again, here we have Rick copying and pasting and as always not understanding what it is he is talking about. Well, he only recently learnt what KM and GM are:
    http://www.boatdesign.net/forums/stability/metacentric-height-free-ship-michlet-15582.html
    so can't expect too much really!..i could list more, but you get the point!

    "...Who ever you were talking to that said the motor would suddenly speed up when the prop cavitates does not understand cavitation. .."

    This is utter nonsense!
    Spoken, yet again, like a someone trying to impress, but doesn't know what they are on about.

    So, what is cavitation? In very simplistic terms it is thus:
    This occurs when the prop is overloaded, ie it is asked to produce too much thrust for the area of its blades. The water on the forward face of the blades literally 'boils'. The fwd face of a prop is the suction (or work done). Hence when this suction pressure falls to zero, ie a vacuum, cavitation begins. So, since no suction, there is no longer any thrust being produced by the prop.

    When there is a breakdown in thrust, what occurs??.....the engine races, why, because there is no longer any resistance/thrust being produced it is free to rotate merely....

    For someone who constantly plugs numbers in to programs and gives endless prop advice, I'm not surprised he doesn't understand cavitation. A program wont tell him the theory, it is just numbers.

    PS
    No doubt i'll get endless neg hits, as always, for pointing out the obvious....'tis a compliment really :)
     

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  13. daiquiri
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    daiquiri Engineering and Design

    Well Rick, you are completely wrong on this one...

    First, it is not true that cavitation affects only the outer portions of the blade. There are several types of cavitation, some of them develop at the blade root, some at central portions of the blade, some at the tips, some at the leading edge, some near the trailing edge, some in the space between blade tips and the hull, some at the hub.

    Second, it is not true that it only affects the suction face. It affects any point of the blade where the pressure drops to nearly vapour pressure of the water at the given temperature. It can develop on the ventral side (pressure side) of the blade too, if the effective angle of attack of the considered foil is sufficiently small to provoke a severe local drop of pressure at some point of the ventral suface. It usually can happen in the proximity of (or immediately behind) the leading edge.

    Third, when cavitation happens, there is a sudden drop in both Kt and Kq prop coefficients, so the torque required to turn the prop becomes smaller than the torque available at the shaft, and the prop revs up.

    If you would like to learn more on this subject, I would suggest you to buy and read this excellent book: "Marine Propellers and Propulsion" by John Carlton.
    All these things are also very-well explained, with sample propeller charts for various cavitation numbers, in the book "Hydrodynamics of High-Speed Marine Vessels" by Odd M. Faltinsen

    I would also invite you not to rely too much on Wikipedia articles or on other non-verified Internet sources for learning about these things. This is a very specialistic area of study and not many reliable data sources are publicly available. So books and verified academic articles are to be preferred instead. The article you have cited gives a very basic information only and doesn't get any deeper into the specific aspects of the cavitation aplied to marine propellers. In general, too many internet articles are just a copy-and-paste work based on some second or third-hand information found at other internet sites. Unfortunately very few info from the original sources (research institutions) is readily or freely available. This is a kind of problem where empirical observations in cavitation tunnels are of fundamental importance as we still don't have a comprehensive and reliable all-round theory to explain and predict every type of cavitation observed (though a remarcable progress in this direction has been made lately by CFD).
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Quite right daiquiri, there are many forms of cavitation. Locations are as I am sure you are fully aware, unlike Rick, of the cavitation bucket, which is dependent upon the blade thickness, camber and AoA.
    The wider the bucket, the greater the range of J.

    As for Wiki, fully concur. That stuff is for kids, not serious academic reference or research.
     

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  15. daiquiri
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    daiquiri Engineering and Design

    Yes, but let alone academics, which requires much more rigour than is probably needed for the purposes of this forum...
    Wikipedia is a fine site if you search for some broad qualitative info. If, for example, you are an ignorant in some field (but you don't want to die ignorant) then Wiki is probably a good source of general info on a particular subject. But as such it is clearly not suitable for engineering purposes, where a detailed insight is important. The Wiki page about cavitation, or about airfoils (for example) clearly shows these limitations.
     
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