Hull Efficiency

[Amador]

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 ?

[RANCHI OTTO]

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

[RANCHI OTTO]

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]

his is the lines plan of the craft..

[jehardiman]

If you have not desigened your hull efficiency gerater than one, you have not done your best in adapting your prop.

[RANCHI OTTO]

....??....

[Amador]

Originally Posted by RANCHI OTTO

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

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

[Amador]

Dear Jehardiman,
Thank you for the alert.
Regards
Amador

[jehardiman]

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.

[Amador]

Originally Posted by jehardiman

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.

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

[RANCHI OTTO]

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]

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

Hydrodynamic isn't an exact science...!

[Amador]

Originally Posted by RANCHI OTTO

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....

Thank you
Amador

[GumbyTheBorg]

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

[Guillermo]

Yes:

http://docs.hydrofoils.org/SAS03.pdf
http://newton.nap.edu/books/0309065372/html/779.html
http://www.rina.org.uk/rfiles/warshi...ents_oct02.pdf
http://newton.nap.edu/catalog/10834.html
http://www.darpa.mil/DARPATech2000/P...ionDragB&W.pdf
http://www.nmri.go.jp/turbulence/gro...crobubbles.pdf
http://www.dsto.defence.gov.au/publi...TO-GD-0290.pdf

Just to mention a few. Google around for 'micro-bubbles drag reduction' or the like.
Cheers.