# Hull Efficiency

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

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:!: 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 ?

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### RANCHI OTTONaval Architect

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

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### RANCHI OTTONaval 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

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### RANCHI OTTONaval Architect

his is the lines plan of the craft..

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### jehardimanSenior Member

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### RANCHI OTTONaval Architect

....??....

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Dear Ranchi Otto,
Thank you for your prompt explanation.
Best Regards

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Dear Jehardiman,
Regards

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### jehardimanSenior 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.

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Dear Jehardiman,
No you were not obtuse.
Thank you for the explanation.
Regards

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### RANCHI OTTONaval 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....

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### RANCHI OTTONaval 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...!

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Thank you

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### GumbyTheBorgJunior 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

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### GuillermoIngeniero Naval

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