How Efficient Are Ships?

There are many posts around talking about how efficient ships are, many of which are around comparing ships with other modes of transport, but often there is no detail about how these comparisons have come about.

It’s probably useful to first get a feel of context as to why ships are efficient. One reason is that size matters.

Deadweight is a measure of what a ship can carry and includes fuel and water, but not the weight of the structure, machinery or fluids in systems. As can be clearly seen, ships benefit from economies of scale, and are also the slowest form of mass transport in the logistics chain (for more info on ship speeds, see here). Since power and therefore fuel consumption is approximately proportional to speed cubed, slow speeds dramatically reduce fuel consumption. Putting together speed and economies of scale, we get:

We have tried to calculate as far as possible on the same basis, and this means also being quite specific to particular models of vehicles in order to properly represent the technical specifications.

However being the most efficient (and CO2 efficient) mode of transport does not mean shipping should rest on its laurels. So another measure of efficiency is to try to work out how much efficiency has improved over time.

The measure of efficiency that was used in the chart above is calculated either by taking CO2 divided by deadweight x speed for an instantaneous figure, or using the annual CO2 divided by deadweight x distance. Deadweight is a constant, so effectively this is CO2/distance or even fuel consumption per distance.

This allows us to make an analogy with cars. Most cars have readouts of their fuel efficiency, either in miles per gallon or litres per 100 km. Now we have all experienced that when you end up in traffic, calculated fuel efficiency gets worse, while if you are able to travel long distances without stopping, fuel efficiency gets better.

This is the same with ships, the more they stop, the worse their efficiency gets, but there is an additional effect that cars don’t suffer from, and that is biofouling where barnacles, slime and other organisms attach to the hull of the ship to increase the resistance, and therefore the power required to reach and maintain a given speed.