Hull Efficiency

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

  1. Amador
    Joined: Sep 2006
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Campinas,S.P.Brazil

    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 ?
     
  2. RANCHI OTTO
    Joined: Apr 2005
    Posts: 1,042
    Likes: 37, Points: 0, Legacy Rep: 535
    Location: TRIESTE (ITALY)

    RANCHI OTTO Naval Architect

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

    Attached Files:

  3. RANCHI OTTO
    Joined: Apr 2005
    Posts: 1,042
    Likes: 37, Points: 0, Legacy Rep: 535
    Location: TRIESTE (ITALY)

    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
     
  4. RANCHI OTTO
    Joined: Apr 2005
    Posts: 1,042
    Likes: 37, Points: 0, Legacy Rep: 535
    Location: TRIESTE (ITALY)

    RANCHI OTTO Naval Architect

    his is the lines plan of the craft..
     

    Attached Files:

  5. jehardiman
    Joined: Aug 2004
    Posts: 3,208
    Likes: 667, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    If you have not desigened your hull efficiency gerater than one, you have not done your best in adapting your prop.
     
  6. RANCHI OTTO
    Joined: Apr 2005
    Posts: 1,042
    Likes: 37, Points: 0, Legacy Rep: 535
    Location: TRIESTE (ITALY)

    RANCHI OTTO Naval Architect

    ....??....:)
     
  7. Amador
    Joined: Sep 2006
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Campinas,S.P.Brazil

    Amador Junior Member

    Dear Ranchi Otto,
    Thank you for your prompt explanation.
    Best Regards
    Amador
     
  8. Amador
    Joined: Sep 2006
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Campinas,S.P.Brazil

    Amador Junior Member

    Dear Jehardiman,
    Thank you for the alert.
    Regards
    Amador
     
  9. jehardiman
    Joined: Aug 2004
    Posts: 3,208
    Likes: 667, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    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.
     
  10. Amador
    Joined: Sep 2006
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Campinas,S.P.Brazil

    Amador Junior Member

    Dear Jehardiman,
    No you were not obtuse.
    Thank you for the explanation.
    Regards
    Amador
     
  11. RANCHI OTTO
    Joined: Apr 2005
    Posts: 1,042
    Likes: 37, Points: 0, Legacy Rep: 535
    Location: TRIESTE (ITALY)

    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....
     
  12. RANCHI OTTO
    Joined: Apr 2005
    Posts: 1,042
    Likes: 37, Points: 0, Legacy Rep: 535
    Location: TRIESTE (ITALY)

    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
     
  13. Amador
    Joined: Sep 2006
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Campinas,S.P.Brazil

    Amador Junior Member

    Thank you
    Amador
     
  14. GumbyTheBorg
    Joined: Nov 2005
    Posts: 9
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Ottawa, Canada

    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
     

  15. Guillermo
    Joined: Mar 2005
    Posts: 3,644
    Likes: 188, Points: 63, Legacy Rep: 2247
    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.