# What is Shaft power formular

Discussion in 'Boat Design' started by Adeyele, Aug 30, 2012.

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Dear all, pls can somebody help me out with a simplified formular for "Propeller Shaft Power".

Regards
Mike

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### rasorincSenior Member

Not sure if this is what your looking for but shaft power is power at the prop. If you know the rated horse power then multiply by 7 to get shaft horse power.

3. Joined: Jun 2012
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Yes! i mean shaft power formular or delivered power formular. i have a formular here that states: Shaft power =torque x 2pien. but am in thought of it. the torque=(Rho)Va^2 D^3 X Fq (ND/Va)

in dis form. i dnt knw.
D=prop diameter.

Any hlp is welcomed.

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### jehardimanSenior Member

Shaft Hp (shp) = QN/550 where Q is shaft torque in ft-lbs; N = rps (1/sec); 550 is the HP constant.

Effective Hp (ehp) = T*Va/550 where T is the thrust in lbs, Va is the speed of advance in ft/sec.

Q = Kq rho N^2 D^5 where Kq is the torque coefficient from the prop curves at the Advance Coefficient (J = Va/ND)

T = Kt rho N^2 D^4 where Kt is the thrust coefficient from the prop curves at the Advance Coefficient (J)

Last edited: Aug 31, 2012
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### gonzoSenior Member

The power gets calculated the same whether it is at the propeller shaft, a crankshaft or any other. It is a function of torque and RPMs. Outboards are rated at shaft HP. If you have an inboard, the rule of thumb is that you have a 3% loss in the gears.

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### Leo LazauskasSenior Member

You could also have a look at the excellent and improving ITTC dictionary.
http://ittcwiki.org/

There is a lot in the section on propellers.
http://www.ittcwiki.org/doku.php/structured_dictionary:propeller

Last edited: Aug 31, 2012

7. Joined: Aug 2004
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### jehardimanSenior Member

Damme Leo;

Now you made me notice a typo...last line should be...

T = Kt rho N^2 D^4 where Kt is the thrust coefficient from the prop curves at the Advance Coefficient (J)

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