Calculating propeller efficiency and Cleavers

Discussion in 'Propulsion' started by Joakim, Jan 22, 2007.

  1. Joakim
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    Joakim Senior Member

    I have tried to find some information about the efficiency of Cleaver propellers, but haven't really found much. This graph is the best I have found: http://www.boatdesign.net/forums/attachment.php?attachmentid=9043&d=1157980313

    As far as I know Cleaver is both super-cavitating and semi-submerged. Which curve should I look at? They are used on small raceboats from about 40 kn upwards, thus it must be the most efficient one at those speeds.

    If I compute the open water efficiency of a optimal Wageningen B-screw series propeller at 40 kn and 600 N thrust, the efficiency is ~70% at 10% Burrill back cavitation. At 50 kn and 900 N the efficiency is still ~65%. The Wageningen B-screw series are limited to P/D=1.4, which is less than the propellers used, which would make the efficiency even higher.

    How accurate are the optimal Wageningen B-screw series propeller models at these values? At what Burrill back cavitation level will the efficiency be considerably less than predicted? If I ignore cavitation, I can reach 75% efficiency at 50 kn 900 N, which must be wrong.

    Would Cleaver have a higher efficiency than these or is the benefit only from getting most of the drive off the water and thus reducing drag? Does the propulsor efficiency graph include the drag of the drive or is it just for the propeller? In the 50 kn 900 N case the drag of the drive (using Cleaver) is about 90 N thus 10% of the total drag. Using a totally submerged propeller would probably double the drive drag.

    Joakim
     
  2. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    The open water efficiencies in the graph are obtained in a cavitation tunnel.

    The B Serie Wag. propeller are to be used until speed not exceeding 30/35 knots. With higher speed the efficiencies drop down due to the cavitation and erosion.

    For speeds higher than 35 knots it would be necessary to foreseen Newton Rader or Gawn Burrill screws.

    The efficiencies at this range of speeds are lower than the OPC (OPC=EHPbh/DHP) of waterjets or surface propellers.
     
  3. Joakim
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    Joakim Senior Member

    What values are used in for making these open water efficiency graphs? There must be some assumptions or limits for parameters, since efficiency is certainly not only dependent on speed.

    I guess Newton-Rader is super-cavitating wedge profiled propeller thus at least somewhat similar to Cleaver?

    What profiles are used in normal aluminium propellers of outboards and sterndrives?

    Joakim
     
  4. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    Ref. : Marine Technology October 1977
     

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  5. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    Newton Rader prop's were tested in a cavitation tunnel with model having a diameter of 10" (Vosper cavitation tunnel).

    Pith ratios 1.0 to 2.0 and blades area ratio from 0.5 to 1 with 3 blades.
     
  6. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    For additional informations:

    MT January 1976 and MTApril 1978.

    Prpellers tested in cavitation tunnel but according to Sigma /Cavitation number):

    Wageningen / Gawn Burrill / Newton Rader / SC prop

    The ETA0 obtained at various sigma are "corrected" for real working condition with the losses due to the cavitation.
     
  7. jmcarvajalramos
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    jmcarvajalramos Naval Architect

    Searching Gawn-Burrill

    Hi, how are boys and girls?
    I am lsearching for the Gawn-Burrill Tables in Internet. I must make a Problem for my PhD. Anybody know where I can find. Please.
     
  8. Guest625101138

    Guest625101138 Previous Member

    Normal prop data does not apply to situations where the prop is ventillating and not maintaining flow circulation over the blade. The blades must be fully immersed at all times to avoid this condition.

    I have little knowledge of surface props but my understanding is that they operate more as pumps than foils with circulating flow over the blades. Their advantage at high speed is the reduction in drag from the shaft. It should be possible to analyse them using simple vector anlaysis. I have seen efficiency data around 70% at 60knts.

    Rick W.
     

  9. Guest625101138

    Guest625101138 Previous Member

    Here is the reference on surface prop efficiency:
    http://www.francehelices.fr/pdf/sds.pdf

    I did play around with low power surface piercing props but soon learnt that only the high pressure side of the blade works. So it needs to be vastly bigger than a fully immersed prop at the same rpm - see photo attached. At high power the low pressure side of a conventional prop is cavitating anyhow so you lose nothing by only using the back side of the blades. As noted before you get a gain by eliminating the drag on the shaft.

    Rick W
     

    Attached Files:

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