How Many Miles per Gallon!?

How Many Miles per Gallon!?

Fuel is the biggest expenditure on-board the majority of ships, so just how much does it cost to fill up and how far does that get us?

There are a few differences from filling your car and filling a cruise ship.  Firstly we don’t work in litres, in fact we don’t work in any volume.  The volume of fuel can vary slightly depending on the temperature of the fuel and as the quantiles involved are so large a small percentage can be tens of thousands of pounds, to combat this we order fuel by weight, in tonnes which is 1000Kg.

We burn two grades of fuel on-board, one which everyone is familiar which we call marine gas oil or MGO.  This is a low sulphur fuel and very similar to the diesel we burn in our cars.  The other fuel is called Heavy Fuel Oil and it is a residual oil.  It is called a residual oil as it is what’s left after the refining process, once the lighter fractions like petrol and diesel and kerosene have been boiled off. At room temperature it has viscosity similar to treacle.  This fuel isn’t as clean as diesel and tends to have a higher sulphur content (max 3.5%) and also more impurities.  As you may have guessed this fuel requires a lot of pre-treatment before suitable for injection/combustion in the engine, this includes centrifuging and heating to +110°C.

Certain areas of the world have been designated emission control areas, these areas consist of the North Sea and English Channel, North American Waters, United States Caribbean Sea area and the Baltic Sea.  Here we cannot burn a fuel which has a sulphur content of more than 0.1% sulphur so this rules heavy fuel oil out of the equation unless exhaust cleaning system are employed but this is a different topic altogether.  Ships will have different fuel tanks for the different grades of fuel and will change over while sailing as they enter and exit emission control areas.

The reason they change over is simple, firstly within the Emission Control Areas they must be on MGO as it’s the law.  Outside these areas the cruise ships can burn what they like but at the time of writing the average price of MGO was $638 per metric tonne and average price of HFO was $382 per metric tonne, this makes HFO the obvious choice.

Normally when the ship is bunkering fuel it is done so from a Barge. The biggest cruise ships today have a capacity over 5000m3 and can bunker the fuel on at a rate of around 500m3 per hour.  Ships normally bunker at their homeport once or twice a fortnight.  The amount of fuel ordered will of course depend on fuel quantity on-board and planned consumption in the near future but we are talking about millions of pounds each time.

Is it value for money?

The fuel consumption of each cruise ship varies depending on the size, shape of the hull and propulsion system on-board.  I worked out a rough estimate of one of the largest ships I’ve worked on and we would achieve roughly 20 metres per gallon, but to be fair though we are talking about propelling almost 100,000 tonnes of steel, passengers, and supplies through the water.

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