# Fuel coefficient value.

Discussion in 'Propulsion' started by Adeyele, Aug 4, 2012.

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Dear all,

I want to calculate the fuel consumption of a vessel. i have the formular- Displ^0.7*V^3/fuel coefficient. But i dont know wat wil be d value of the fuel coeficient.

The vessel installed power is 300kw

Did any1 knows how to get the fuel coefficient value.

Regards
Mike

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### johneckSenior Member

Fuel consumption is generally given as grams/kW-hr and a reasonable number would be something like 220 g/kW-hr. So to calculate fuel consumption you need to know how much power you will actually be using and for how long. so 300 kW for 10 hrs would take 220*300*10/1000=660 kg of fuel. I think that the displ^0.7*v^3 might be an attempt to determine power required to go a certain speed, but I don't know where it came from or if it is appropriate for your vessel.

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### daiquiriEngineering and Design

That's one odd formula. The 0.7 looks like a single-decimal round-off of the fraction 2/3. If so, then the term Displ^0.7 appears to stand for Displ^(2/3), dimensionally an area.

So the formula apparently is a semplification of a general formula
(1) Power,effective = 0.5*rho V^3 A Cf
which is valid only for low speeds, where wave resistance is negligible. Cf is the friction coefficient and A is the wetted area, substituted by Displ^(2/3).
The engine power will be given by
(2) Power,engine = Power,effective / (prop_efficiency * gear_efficiency)
and the power fed into the engine by the fuel will be given by:
(3) Power,fuel = Power,engine / thermal_efficiency.
Finally, the fuel consumption will be
(4) FC = Power,fuel / fuel energy content

By comparing the formula (4) to the OP's Displ^0.7*V^3/FC (FC= fuel coeffcient) we see that the "fuel coefficient" in the OP's formula is a number which contains all the passages from (1) to (4), which means that it is tightly related to a specific ship and power plant, and is hence impossible to express as one-fits-all number.

Cheers

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