Greener Shipping: Improving Efficiency

No matter which propulsion package your vessel has, using it as efficiently as possible is a surefire way to save on both emissions and fuel. So how do you actually know whether you are propelling your vessel efficiently? That can be a hard question to answer, since it involves a lot of variables. Also, the most efficient propulsion settings are not a static thing – it all depends heavily on vessel speed, draft and trim.

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Greener shipping

On the whole, shipping is not that big a contributor to global warming today – about three percent of the total CO2 emissions seen today come from the shipping sector. Not so in the coming years, however. As green tech improves efficiency in other areas, by shifting from gas fuelled cars to electric for instance, shipping will be responsible for a much larger chunk of the total emissions. That is of course, unless something significant is done to change that.

Fortunately, something is being done in a lot of different areas to lower emissions. Generally speaking, emissions come in two flavors:

  • Immediately polluting emissions, such as SOx, NOx and super fine particles. These are substances that have a direct and immediate impact on health and environment. SOx causes acid rain, NOx is carcinogenic as are the super fine particles that also cause respiratory issues.
  • Clean combustion products, primarily CO2. CO2 is a greenhouse gas and it is by far the largest contributor to the global warming we are currently experiencing.

Part easy, part hard.

The immediately polluting emissions can be removed from exhaust gases using scrubber technology. Basically, scrubbing is about chemically binding harmful gases as salts that can be either stored on board and processed later or flushed. The technology to do this is well tested and installed on many vessels today.

It is also possible to select a fuel that has a low sulphur content to begin with. These fuel types are considerably more expensive than heavy fuel – typically about 50 percent or so. Environmentally speaking, this is the best option as the harmful substances such as sulphur are removed and handled by the refinery.

Removing CO2 from emissions is not so easy, however. Firstly, the amount of CO2 generated by fuel combustion is quite large, since every kilo of fuel burned produces around three kilos of CO2. Using a reasonably clean fuel (i.e. low sulphur), this means combustion generates 50-100 times more CO2 than SO2. The volume alone makes scrubbing impractical and the technology to do so is not ready.

For new vessels, there are several options, depending on operating conditions, for reducing CO2 emissions. One very important component to consider is the engine, naturally. Traditional wisdom dictates you should select an engine that is efficient at the typical load the vessel will experience. This is good advice, but even so, there are still a lot of options.

  • Modern diesel engines are very efficient, even if the basic technology is old. It is possible to achieve very low consumption with a new engine – even as low as 160 g/kWh. Compare this to a petrol car engine that commonly uses 400 g/kWh. The improvements to diesel engines are incremental in nature, however. There are no revolutionary leaps and bounds here, but the results are still very impressive and the price level of classic diesel engines is typically attractive.
  • Selecting a dual fuel engine makes it possible for a vessel to run on natural gas (LNG) which results in lower CO2 emissions, compared to heavy fuel oil. That translates to a lowering of CO2 emissions by about a fifth, but natural gas does have some limitations that make it unsuitable for long haul vessels. Specifically, LNG takes up about twice the volume of heavy fuel and bunkering of LNG is not widely available. Operating LNG equipment is also more demanding and requires training.
  • Electrical propulsion packages are also growing in number on ferries and other vessels that are able to reliably charge battery packages whenever necessary. Batteries take up space and the energy density of batteries is nearing the limits of what is possible. At the same time, the energy density of even the best batteries is about 100 times lower than that of diesel for instance – they take up a lot of space. That makes batteries a solution for short distances only.
  • In the long run, nuclear propulsion might even be a possibility. That would be a zero emission solution but one that comes with a completely different set of challenges. The technology is available and well tested in military applications, but there are virtually no civilian nuclear vessels around. This is largely due to public opinion but also because implementing nuclear propulsion would mean retraining technical staff on a very large scale to handle this new technology.


Doing a sea-trial is a good starting point for figuring out what you might expect to gain from adding an optimization product to your propulsion package. Doing just that is one of the services we offer.

The goal of a sea-trial like the ones we do is to systematically try out different combinations of engine RPM and propeller pitch to see how much speed and power you get and how much fuel you burn getting it. Once done, you will probably have a number of RPM / pitch combinations that yield a certain speed. Among these, one will be the best fuelwise and a collection of these best points across a range of speeds is what we want.

This collection is directly comparable to the fuel consumption you get across the same range of speeds, using the existing power handle on the bridge. This comparison is quite often very surprising, since the amount of fuel you can save tends to be significant.

Power generation

All vessels have diesel generators for producing electricity but many have shaft generators (SG) too. These are clever devices that piggy back on the main engine to produce power more efficiently than the diesel generators are able to. Since most devices need electrical power (AC) at a specific frequency, this limits the engine and the shaft generator to run at just one specific speed.

If you want to optimize propulsion efficiency, this limit needs to go. To free the engine from the restrictions imposed by an SG, you have two options:

  • One is to simply turn off the SG and use the diesel generators for power generation. This may be a good option in some cases where running the generators is not disproportionately expensive. It is also likely to be what you want to do for a sea-trial.
  • The other is to add a variable frequency drive (VFD) to the power generation setup. This kind of device basically accepts power from the shaft generator within a large frequency range as input and provides power at a stable frequency as output. These are typically very efficient and low maintenance devices, but not cheap.

Figuring out which one of the two solutions above you should select is a matter of considering:

  • How long do you plan to operate the vessel?
  • What is the operating expenditure on the diesel generators?
  • What is the cost of a suitable VFD solution?
  • Is there any legislation or regulations in place (or coming) that points in a particular direction?
  • How much can you save by optimzing your propulsion package?
  • Is there any kind of financing available that lets you pay as you save?

Frugal Technologies delivers reporting on all of the above when we do the sea-trial on a vessel. We do this to figure out whether Frugal Propulsion is a good option for your vessel.