Hull Efficiency

Discussion in 'Propulsion' started by Amador, Sep 14, 2006.

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  1. Amador
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    Amador Junior Member

    :!: The Hull Efficiency is given by the formula : Hull Ef. = (1 - t )/ ( 1 - w )
    Where :
    t = thrust deduction
    w = wake fraction
    w is always bigger than ZERO
    t may be = ZERO ( read HydroComp Technical Report number 116, page 2 )
    This means that the hull efficiency may be bigger than ONE ? this is my question.
    Thrust deduction t = (T - Rt) /T , where:
    Rt = the total resistance of the hull when towed.
    T = the total resistance of the self propelled hull
    T > Rt , the propeller action near the stern, creates an augmentation of resistance.
    My second question : is it possible to have t=ZERO, which are the conditions to guarantee t = ZERO ?
     
    Amador, Sep 14, 2006
    #1
  2. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    I send to you a diagram concerning propulsive coefficients...
     

    Attached Files:

    RANCHI OTTO, Sep 14, 2006
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  3. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    Hull efficiency (ETAH) can be absolutely bigger than 1...

    Attached you will find a lines plan of an high speed ferry for coastal navigation.

    The hull has 2 tunnels in the after part and a rounded kick instead a chine.
    I've carried out resistance and selfpropulsion tests in the Vienna Model Basin.
    The ETAH figures were as follows:

    Fv / ETAH
    ----------
    1.5 / 0.973
    1.7 / 0.986
    1.9 / 0.960
    2.1 / 1.004
    2.3 / 1.016
    2.5 / 1.022
    2.7 / 1.033
     
    RANCHI OTTO, Sep 14, 2006
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  4. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    his is the lines plan of the craft..
     

    Attached Files:

    RANCHI OTTO, Sep 14, 2006
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  5. jehardiman
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    jehardiman Senior Member

    If you have not desigened your hull efficiency gerater than one, you have not done your best in adapting your prop.
     
    jehardiman, Sep 14, 2006
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  6. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    ....??....:)
     
    RANCHI OTTO, Sep 14, 2006
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  7. Amador
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    Amador Junior Member

    Dear Ranchi Otto,
    Thank you for your prompt explanation.
    Best Regards
    Amador
     
    Amador, Sep 14, 2006
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  8. Amador
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    Amador Junior Member

    Dear Jehardiman,
    Thank you for the alert.
    Regards
    Amador
     
    Amador, Sep 14, 2006
    #8
  9. jehardiman
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    jehardiman Senior Member

    Sighhhhhh.... I didn't mean to be obtuse. Generally, water velocity slows down in the region of the aft hull. You need to measure this and design a prop for this slower water. So Uw<U therefore T|Uw is igreater than T|U for most props...therefore Tw>T implies (1-t)>>1.
     
    jehardiman, Sep 14, 2006
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  10. Amador
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    Amador Junior Member

    Dear Jehardiman,
    No you were not obtuse.
    Thank you for the explanation.
    Regards
    Amador
     
    Amador, Sep 14, 2006
    #10
  11. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    For help the design of larger propeller for ships, we measure the water velocity at various prop. radius with, instead the stock prop., pivot tubes....
     
    RANCHI OTTO, Sep 14, 2006
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  12. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    the w factor calculated from model tests results has to be corrected with a correlation factor ship/model.

    Hydrodynamic isn't an exact science...!:p
     
    RANCHI OTTO, Sep 14, 2006
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  13. Amador
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    Amador Junior Member

    Thank you
    Amador
     
    Amador, Sep 14, 2006
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  14. GumbyTheBorg
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    GumbyTheBorg Junior Member

    Talking of hull efficiency

    Since we're on the subject, who here has read/done any work relating to the use of micro-bubbles to increase hull efficiency?

    cheers,
    GumbyTheBorg
     
    GumbyTheBorg, Sep 20, 2006
    #14

  16. Guillermo
    Joined: Mar 2005
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    Guillermo Ingeniero Naval

    Yes:

    http://docs.hydrofoils.org/SAS03.pdf
    http://newton.nap.edu/books/0309065372/html/779.html
    http://www.rina.org.uk/rfiles/warship/ws_contents_oct02.pdf
    http://newton.nap.edu/catalog/10834.html
    http://www.darpa.mil/DARPATech2000/Presentations/ato_pdf/6AlbrightFrictionDragB&W.pdf
    http://www.nmri.go.jp/turbulence/group/040615Energy_saving_by_microbubbles.pdf
    http://www.dsto.defence.gov.au/publications/2398/DSTO-GD-0290.pdf

    Just to mention a few. Google around for 'micro-bubbles drag reduction' or the like.
    Cheers.
     
    Guillermo, Sep 20, 2006
    #15
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