test questions

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blackjack1111
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test questions

Postby blackjack1111 » Fri May 14, 2010 9:15 am

Does anyone know the formulas for propeller percentage of slip and specific fuel consumption.Thanks

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Big Pete
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Re: test questions

Postby Big Pete » Fri May 14, 2010 1:53 pm

% Slip only applies to fixed pitch propellers.
It is the difference between the distance actually travelled by the ship and the theoretical distance it should have travelled (calculated by multiplying the pitch of the propeller by the number of propeller revolutions), divided by the theoretical distance.

Specific fuel consumptio??
Do you mean Brake Specific Fuel Consumption (BSFC) ? or an other specific fuel consumption?

BSFC is simply the mass of fuel burnt in a period of time,divided by the Brake power developed by an engine.

BP
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JollyJack
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Re: test questions

Postby JollyJack » Mon May 17, 2010 7:07 am

ref Reeds Motor Engineering Knowledge for Marine Engineers, vol 12, chapter 1, page 3

Specific Fuel Consumption
Fuel consumption per unit energy at the cylider or output shaft, kg/kWh (or kg/kWs). 0.21 kg/kWh would be normal on a shaft energy basis for the engine only and 0.235 kg?kWh all purpose (i.e. including auxiliaries)

As BP says, slip is calculated by dividing the actual distance run by the theoretical distance, obtained by multiplying the prop pitch by the number of revolutions in a given time. Slip is not relevant to controllable pitch propellors.
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Big Pete
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Re: test questions

Postby Big Pete » Tue May 18, 2010 1:13 pm

I think there might be a slight problem with Jolly Jack's formula for slip.
According to him, if the theoretical distance steamed is the same as the actual distance steamed, you would have 100% slip. But in fact you would have 100% traction.

BP
It is always better to ask a stupid question than to do a stupid thing.

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JollyJack
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Re: test questions

Postby JollyJack » Wed May 19, 2010 4:09 am

Pete, to clarify that, if you steamed 200 miles, but revs x prop pitch indicates 300 miles, the formula is distance steamed (200) divided by distance indicated (300) or 200/300 = 0.666, which means the prop is doing 66% work. Therefore, the slip is 33%. Theoretical distance and actual distance are never the same.
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