# Propeller shaft and propeller diameter

Discussion in 'Propulsion' started by QUICKNAVAL, Mar 19, 2014.

1. Joined: May 2012
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### QUICKNAVALJunior Member

How can I get the required propeller shaft diameter and propeller diameter, given the Brake power and rpm of a marine engine. (esp. for tugboats, propeller with kort nozzles).

Your comment and opinion is highly appreciated

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### gonzoSenior Member

You need to calculate the torque at the shaft not HP.

3. Joined: May 2012
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### QUICKNAVALJunior Member

Is there any standard for minimum angle of twist of the shaft? The length of the steel shaft is to be 7 meter approximately.

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### TANSLSenior Member

Here is the calculation of propeller shafts according to the rules of Bureau Veritas. The other classification societies use the same formula.
It is not good to have an axis of 7 m long. You must put intermediate supports. How many: I do not know.
The diameter of the propeller is ​​a much more complicated issue that I can not help you.

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5. Joined: May 2012
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### QUICKNAVALJunior Member

Thank you, i noticed that it's the same with RINA rules. For "P" in the formula is it the Brake Power or Shaft Power should be used?

I'm using this formula, in getting the Torque.

Pb = (2)(pi)(T)(n)
where:
Pb = Brake power in kW
T = torque in kN.m
n = rev. per sec.

Then, using this formula for shaft diameter:
Ps = [(pi)(ds)(G)(theta)(n)]/[(32)(Ls)]
where:
ds = shaft diameter
G = shear modulus for steel shaft, 8.35 x 10^7 kN/m^2
theta = mesured angle of twist, deg
n = rev per sec.
Ls = length of shaft

Using this formula to find the shaft diameter have two unknowns the diameter (ds) and the angle of twist of the shaft. I was just confused.

6. Joined: Sep 2011
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### TANSLSenior Member

The rule says. "P Maximum continuous power of the propulsion machinery." I guess it's the brake power in kW.
It has the advantage of being very simple, does not use the engine torque and is internationally accepted.

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### QUICKNAVALJunior Member

OK, thanks again,,,i'll proceed now to propeller diameter cal.

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