prop doing bad

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

  1. bblagonic
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    bblagonic Junior Member

    sorry...forgot to mention two more things:
    gear ratio is 2:1
    and
    max. prop diam. is 17˝
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    bblagonic

    With the basic figures you have posted, the max speed you'll get is around 17~18knots, assuming the 220BHP and 4tonne max displacement, for your hull. The hull sounds like a classic semi-displacement hull, and will get a bit more draggy near its top end and lots of wash. Not to mention may be a bit "tippy" at high speeds, especially when turning. The round bilge causes instability at high speeds.
     
  3. baeckmo
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    baeckmo Hydrodynamics

    In addition to my notes on solidity earlier, it would be interesting to see if a "fake" increase of solidity in Heppler's validation example could reduce the error levels. This might lead back to the specific section of the programme, where an underestimation of the solidity effects is introduced.

    What do you say Rick? Some time when you have the JavaProp up and running, could you recreate the validation propeller and run some calcs with the three blade settings in the validation calc, and see what increase of solidity is needed to reduce the error levels around the eta max advance ratios (if at all possible)??

    The interaction effects between vanes in a rotating lattice has always been a problem for the aerofoil theory of turbomachinery, and I think we have to live with a clever combination of numerical and empirical analysis for a long time yet.

    And, finally, we should be aware of the difference between "propeller design" and "propeller selection"; Bojan started the thread with a selection problem.
     
  4. Guest625101138

    Guest625101138 Previous Member

    Bojan
    I have attached the JavaProp design info for a prop of similar proportions to the AU series you have. The power and thrust figures are too high as JavaProp has no ability to allow for cavitation and a large proportion of the blades are in cavitation.

    I have also included the Cp plot for a 5% foil of similar shape to the AU series. The maximum value of Cp is around 0.33. When you look at the flow vectors at the various radii it shows that the portion of the blades beyond 130mm radius are in various stages of cavitation at 20kts and 1750rpm.

    I use JavaProp as a first pass for prop calculations but I have my own spreadsheet with in-built macros for more comprehensive analysis. I have attached a portion of it that has the key data related to the conditions you are considering. The key values are that the prop will be producing around 8900N thrust, power is 153kW (204HP) and prop efficiency is 60%.

    This data is primarily derived from first principles using basic physics. I adopt this approach to engineering wherever possible as I seek to understand what is happening. It opens a much wider spectrum than just applying empirical relationships.

    Empiricists often have limited understanding of fundamentals and put great faith in the measurement processes that were used to produce the original data. They will always claim discrepancies between an analytical solution and the measurements to be a flaw in the analysis, relying on the bold assumption that the original measurements using instruments of the vintage have no errors.

    Rick W
     

    Attached Files:

  5. bblagonic
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    bblagonic Junior Member

    Can anyone check if I`m going on right way...

    Situation:
    The three blades prop installed 17"x23" runing on 220hp@1800RPM and with B.A.R.0,73 is high into cavitation.
    Further to avoid cavitation I also need a thrust of cca. 9500N for 25kts.

    Results:
    I calculated (assuming the props I can easly get from "Mikado") that following prop would be suitable:

    5 blades
    B.A.R. 1,06
    17"x20"
    skewed (as it is installed behind the strut)
    type: AU

    I know that even B.A.R. 1,06 is not enough to be on 10% back cavitation but this is the great B.A.R. of prop producer I`m dealing with.

    I`m litle bit scared of five blades prop? Is it unusal for a small pleasure craft of these dimensions?

    Could anyone recheck with some software if my engine would run the prop selected ?
    tnx
     
  6. daiquiri
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    daiquiri Engineering and Design

    5-bladed prop is not very usual for boats of that size, but is nothing to be feared of. If you have a heavily loaded prop at risk of cavitation, it is one of solutions.

    I was rather checking your input data and there is something wrong, imho:
    9500 kN at 25 kts gives some 164 HP effective power.
    Having an engine which gives max 220 HP (maybe less), and taking into account the transmission drive losses, it means that you are seeking a prop with some 78% efficiency, which will be impossible to satisfy with commercial props of that size, particularly if you will be running with some cavitation.

    You need to determine more precizely the actual working point. What is the exact boat's weight? Is the actual speed of 20 kts claimed on the previous page reliable? Do you know the engine model and do you have the power curve for that engine?
     
  7. bblagonic
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    bblagonic Junior Member

    that`s what I call brain storming...thank you Daiquiri...

    as Daiquiri made me attention on probably wrong thrust required made following:

    The way I find out the required thrust of 9500N was by entering the actual prop and vessel performance data into the Kt Kq diagrams.
    (With actual prop the boat is performing 20kts but with thrust reduced because of a high cavitation!!!)
    If I put the actual RPM and delivered power into those diagrams, I find out the thrust that this actual prop can make...but without consideration that in a real life the thrust is much lessbecause of a big amount of cavitation!!!
    Finnaly entering the actual data, the diagrams shows that at this speed of vessel and this RPM the prop is delivering 9000N of thrust but this is far away of truth because of cavitation occuring.

    To recheck the actual vessel resistance (and thrust required) I made the two following calculations:

    1) I know that this model of boat (and the same weight) performed on sea trials
    vs=26kts
    with
    Pb=200HP engine
    If I assume the
    OPC = 0,6
    the effective power will be
    Pe=132HP
    as
    Pe=R*vs
    leading to R=7255N
    Assuming the thrust deduction factor to be
    (1-w)=0,97
    Thrust will be
    T = R/(1-w) = 7480N

    2) in acknowledgment to Dingo`s generosity I`ve used the excel program based on Savitsky method
    I made a calculation estimating vessel parameters as correct as possible
    The program calculated that for 26kts I need a thrust T= 7100N...this is very close to the thrust calculated in point 1

    What do you think about the methods I`ve used?!
    It is difficult to find out the correct thrust needed when you have a poor propeller installed and there are no specific data of the vessel. I know that these data I can measure from the boat and this is probably the next thing I should do the first time the boat will be out of the water.


    regarding the vessel we are talking about: it is a 15 year old Calafuria(Catarsi)
    LOA=7m
    Lwl= 6,2m
    weight is cca.3500kg
    engine is VM motori 220hp@3600rpm
     
  8. Guest625101138

    Guest625101138 Previous Member

    From this I gather you cannot increase the diameter. Is this correct? Best way to improve efficiency is to increase the diameter.

    Increasing the BAR will reduce the amount of blade in cavitation. So some improvement there.

    You need to increase the pitch if you stay with the same diameter as the motor is near full rpm now at 20kts. The reduction in cavitation does not alter slip much. Why did you select 20" when the existing prop is 23"?

    If you need 9500N to get to 25kts you will not quite make it with the power you have on a 17" diameter prop.

    There is some likelihood that the hull drag is still reducing up to 25kts. So it might be lower than the 8900N I calculated at 20kts. Gives some prospect of actually getting 25kts.

    There is an easy to use Savitsky model here that you can use to cross check Dingo's spreadsheet:
    http://www.electricmotorsport.com/store/ems_ev_parts_batteries_thunder_sky.php
    You will see that the shape of the drag hump is highly dependent on the location of the LCG.

    Rick W
     
  9. daiquiri
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    daiquiri Engineering and Design

    Hold on Rick, before giving numbers. I am still not convinced that the initial data here is all correct.
    I have found this test of the new Calafuria, with very similar dimensions and weight:
    http://www.nautica.it/boatshow/catarsi/7eng.htm
    26 kts max speed has been recorded at 3800 rpm and 3400 kg (half-loaded condition). The prop installed is Mikado ZL 16"x21".

    Then I checked the site of VM Motori's marine division (http://www.vmmotori.it/en/02/03/index.jsp) and discovered that they have 2 engine models giving 220 HP: mod. MD706LH and mod. MR 704LH/704LX, both delivering at 3800 max rpm, which is in line with the above test.

    So Bojan, are you really sure your engine has 3600 rpm max? Did you notice particular exhaust fumes during the boat test run? If the max rpm is 3800, like the engines above, than you are probably currently over-pitched and will need to diminish the prop pitch to let the rpm increase so the engine will deliver more power.
    Next, is the gear ratio exactly 2.0 or a tad less/more...?
     
  10. bblagonic
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    bblagonic Junior Member

    well these data are 100% sure:
    my engine develop max. 220hp @ 3600...this is an older type of engine than those ones on the vm site. I have all the data of motor from VM in original.
    max. prop diameter that could be installed is 17". Bigger would be about 11% of the diameter near the hull. I think it would be too much.

    the half load displacement should be near 3,4 tons...so as from the test on the web

    shortly I will come back with new data. It seems to me now that
    17x24
    BAR1,06,
    5 blades
    would be appropriate.
    I know that I will not look serious giving different prop type each replybut I´m still new in this and I like to understand things!
     
  11. bblagonic
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    bblagonic Junior Member

    further...gear ratio is exact 2,0 :1
    as far as I see...the great deal is to define the exact thrust needed for each speed but do you have some other method to estimate the thrust (resistance) of the actual boat other than above mentioned...
     
  12. Guest625101138

    Guest625101138 Previous Member

    Bojan
    If you have to pay good money for a new prop it is best to sneak up on the best solution by collecting as much information as easily as you can. You have only just started and it is not always easy to get a consistent data set.

    With regard to your proposed new prop. The 5-bladed prop with greater BAR gives an improvement in efficiency.

    Going up in pitch will get higher boat speed ONLY IF the motor has the power to achieve it. My calculations show you will need 220HP on the shaft to get to 25kts if the drag is the same at 25kts as I calculated it to be at 20kts. The current prop is absorbing just over 200HP at 20kts.

    As noted earlier there is a reasonable chance that the drag is still coming down the hump. However the fact that the motor does not get to rated speed of 3600rpm makes we wonder about this. It could be an engine problem related to age but 200+HP on the shaft with a 220HP rated engine is quite good.

    One of the measurements you can make to better understand the drag curve is the angle of the trim at various speeds. The trim angle goes hand in hand with the drag. Attached charts show trim and drag at different position of LCG for the same boat weight. Getting the trim angle at various speed should provide a clue about the shape of the drag curve. Do you know the position of the LCG? (These curves do not have the exact boat data but you can do the same thing with your Savitsky spreadsheet. They also are ideal without any appendage drag.)

    If there is severe drag hump then anything you can do to shift the LCG forward reduces the hump. Obviously reducing weight will make it easier to plane providing it does not shift the LCG aft. These things are obvious in an outboard powered dinghy where you can shift weight easily but not quite so obvious in a larger boat.

    So I suggest you try to gather more data before you jump in and buy a new prop.

    Do some measurements of engine speed and trim against boat speed from about 12kts upwards to build the picture. If you can remove some weight and do another run it will provide another useful data set.

    You may also find a prop supplier who can provide tests props rather than buying before trying.

    Rick W
     

    Attached Files:

  13. daiquiri
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    daiquiri Engineering and Design

    I'm affraid Savitsky method is pretty useless in this case, because Catarsi Calafuria is not a prismatic hull. Here are some pictures of the type of boat we are talking about:

    [​IMG]

    [​IMG]

    [​IMG]

    It is a double-ended, round-chine hull. Planing ability has been obtained by applying the additional horizontal surfaces (wedge-type) in the transom area, plus two wide lateral chines. Designed for seakeeping, not for planing efficiency. Kind of boats used by port pilots.

    I would like to hear Bojan confirm or make a correction to this description, because it is fundamental to create the correct initial picture of the problem.
    I'm sorry if it might sound like avoiding to face the numerical solution of the problem, but that's exactly what it is. I have learned in these years not to give numbers before I'm sure that the essence of the problem has been understood well.

    It means that all the resistance figures given in the graphs above need to be multiplied by an unknown, but substantial, correction factor. Could be anything between 1 to 1.5 or even more. So it is not a reliable way to go, imho. Actually, I'm really not sure if any reliable theoretical calculation method exists for this type of planing hull.
    The calculation needs to be performed based on the known test data only, imho. It will be an incremental process - knowing what we have now, let's figure out what we can do to improve it, bit by bit.

    Bojan, could you please post the engine HP vs. RPM curve, if you have it?
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    If the pictures of your boat, are as posted above by daiquiri, then there is no point as he noted using any Savitsky method, or otherwise. The results will be totally meaningless, and to try and use such results, just indications a total lack of understanding of not just what the Savistky program is and its limitations, but basic naval architecture.

    The link to these boats also does not state what condition that those speed which are quoted on the website are taken at...what displacement??. It does not say. So caution is advised..

    Since the figures quoted, implies half load and quotes a value, but the performance speeds quoted do not state catagorically the displacement at the trial speeds given.

    So, even more misleading and consequently confusing data.

    More hard facts before anyone needs to push a button..or make a mark with a pencil!

    Sorry, more leg work, less pushing buttons...for now at least anyway.
     

  15. bblagonic
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    bblagonic Junior Member


    well, I said from beginning that the boat is not hard chine...OK my apologise, I shouldn´t use Savitsky as method.
    I start from beginning. I know the performance of the boat with actual prop (20kts @ 3500rev). Using KtKq diagrams It comes out that with this numbers the prop is producing thrust of 10700N with SHP of 203HP.
    So it is teoretical thrust the prop would produce if there would be everything ideal.
    But in the real world large proporsion of blades are in cavitation (load cav. nr =0,235 and thrust loading coeff. = 0,279). Burrill chart shows point left of 50% line back cav.
    My question now is following:
    Is there a possibility to find out how much thrust I`m loosing because of cavitation of these proportion?!:D

    I`m also wondering about the results that Rick listed. In the first photo the teoretical thrust is cca.10500N (it is close to my calculation using KtKq diagrams which is 10700N) but in the photo showing excel file the thrust is reduced to 8900N. Please explain me why is it so?!

    regarding the shape of the vessel and pictures uploaded. The pictures shows two different boat size. The boat I`m talking about has no big keel as the boat on the last picture even it doesn`t have a stern like arrow like on the first picture. The problem is that I don`t have a picture of the boat bellow water but I will try to make it.

    Bojan
     
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