# Fuel Consumption

Discussion in 'Motorsailers' started by ram68ocean, Nov 22, 2021.

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### ram68oceanJunior Member

What is the process one should make at the preliminary design phase to estimate fuel consumption at service speed with the aim to determine fuel tanks size?
Basically, so that max fuel capacity is enough but not excessively more than what is needed as per requirements.

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### Olavnaval architect

ram68ocean,

you'll need to estimate the required power generated by the propulsion engine(s) at your desired service speed. A quick way if you are at a very early stage of your design would be to consider a vessel similar in shape and size, determine its Admiralty Constant and calculate the power requirement for your project from there. From your SOR you will have a figure of either the required range or endurance at service speed which gives you the needed amount of energy for your propulsion. Then add anything else that might consume fuel (generators/power take-offs for electricity or similar).

A (small) diesel engine will consume something in the vicinity of 200 g/kWh (around 0.24 l/kWh at a density of 830 g/l), but it would be better to check with the actual technical data of some real engines.

In the end you might want to add an additional 10-15% reserve fuel capacity.

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As noted by Olav..

You can only do this once you know the power and thus the engine you have selected for the task at hand, and, knowing the maximum distance of travel from point A to point B, this then fixes the minimum amount of fuel required.

Since you'll ahve the power = engine selection. The engine will have f/c values.
Then knowing your speed and the distance you know the 'time'... this then leads into the volume, in terms of litre/hour.
And add a min 5-10% for unusable in the tank.

If the engine supplier doesn't have f/c values, you can quickly estimate from:

(g/kWh) x MCR/850 = l/h

MCR = power of engine selected.

Olav likes this.
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### ram68oceanJunior Member

Thank you both. I'll do some further work and calculations and post it here.

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### jehardimanSenior Member

Just remember that there are a whole bunch of other tanks besides the bunker tanks in a fuel system (stripping tanks, settling tanks, day tanks, etc). And if using a LSD, there is considerable tankage for consumable lube oil.

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### TANSLSenior Member

I agree with what @jehardiman has indicated and, furthermore, I point out that there may be several other fuel consumers on a boat. So a more concrete answer would need a lot more information.

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