ShipInsight Where maritime regulation and technology meet

Decarbonisation in shipping

IMO Decarbonisation roadmap

IMO Decarbonisation roadmap

At MEPC72 in April 2018, the IMO announced an ambitious (some might say impossible) target for decarbonising the shipping industry. In particular, it set out the following programme of ambitions:

  • Carbon intensity of the ship to decline through implementation of further phases
  • of the energy efficiency design index (EEDI) for new ships;
  • Review with the aim to strengthen the energy efficiency design requirements
  • for ships with the percentage improvement for each phase to be determined for
  • each ship type, as appropriate;
  • Carbon intensity of international shipping to decline;
  • To reduce CO2 emissions per transport work, as an average across international
  • shipping, by at least 40% by 2030, pursuing efforts towards 70% by 2050,
  • compared to 2008; and
  • GHG emissions from international shipping to peak and decline;
  • To peak GHG emissions from international shipping as soon as possible and to
  • reduce the total annual GHG emissions by at least 50% by 2050 compared to

2008 whilst pursuing efforts towards phasing them out, as called for in IMO’s Vision, as a point on a pathway of CO2 emissions reduction consistent with the Paris Agreement temperature goals.

As things stand, it is impossible to completely decarbonise shipping as even the cleanest of fuels presently available do produce some CO2 and in some cases, the benefit compared to oil fuels is actually quite negligible. For the short- to mid-term future, these fuels may provide a bridge towards a lower carbon future but not much more. In the long-term, other technologies will need to be developed.

Energy saving measures

Energy saving measures

Today, energy saving has become linked in the minds of many to the slow steaming strategies adopted by some operators – particularly in the container trades. While it is true that some devices such as turbocharger cutouts and concepts such as variable turbine geometry have come about simultaneously with slow steaming, their use can be extended to vessels for other reasons.

The use of batteries as energy storage systems is of growing interest to many shipowners and the number and types of ships employing them is growing. There is no doubt that ships with highly variable energy loads will benefit most but all ship types have periods when energy production exceeds consumption and a battery could be charged from the ship’s own power sources. Using the battery for port entry and exit would definitely reduce emissions and help improve shipping’s image.

Software too has a role to play in reducing fuel use and so cutting emissions. Two types of application in particular are worthy of particular consideration: trim optimisation and weather routeing. Both have been heavily promoted by proponents of e-navigation although the need for such software has been questioned by some who believe that it undermines the knowledge and expertise of ships’ navigating officers.

Coatings used to protect against hull fouling have different environmental impacts. By preventing hull fouling they allow ships to burn less fuel and therefore play a role in reduction of exhaust emissions. However, even though TBT – which was said to have had an adverse environmental effect causing problems for some marine organisms – has been banned from use, some are saying that the copper-based substitutes are also hazardous.

While criticisms are being directed at the replacement anti-fouling products for still having the potential to hard wildlife, the IMO has recognised the role that anti-foulings can have in preventing species transfer. In July 2011, the IMO issued Resolution MEPC.207(62), Guidelines for the control and management of ships’ biofouling to minimise the transfer of invasive aquatic species. Currently the guidelines are purely advisory, although flag states are encouraged to ensure their use on board ships. It is expected that at some future date, the guidelines will become mandatory.

This expectation has increased recently as the IMO has pushed the question up the agenda by way of the GloFouling project, which the IMO says “will drive actions to implement the IMO guidelines for the control and management of ships’ biofouling, which provide a  globally consistent approach on how biofouling should be controlled and managed to minimise the transfer of invasive aquatic species through ships’ hulls. The project will also spur the development of best practices and standards for improved biofouling management in other ocean industries”.

Energy saving devices

Energy saving devices

Measures that are engine-, software- or coatings-related have been complemented by developments in which hull and propeller modifications and appendages have contributed to ship designs that are more efficient than vessels without them.

Some of these measures are relatively inexpensive retrofit options for existing ships and can be incorporated into newbuildings at virtually no extra cost. Others are more expensive but tend to deliver more significant fuel savings and have a short payback period.

Hull appendages

A ship’s hull lines are optimised to both house the intended cargo and enable the ship to move through the water with the least resistance at its intended service speed. Sometimes meeting the two requirements results in something of a compromise and this is especially so if the ship’s design has also required its dimensions to be restricted to allow entry to particular ports.

Often it is possible to reduce the element of compromise that is required by making use of hull appendages that in some way improve the efficiency of the vessel. The bulbous bow is one such appendage and although considered an essential design feature for many years has over the last 15 years or so been rethought.

Today many novel bow forms exist with some of the most well known being the Ulstein X-Bow, Wavepiercing designs developed by VARD and Rolls-Royce Marine (now Kongsberg) Universal Shipbuilders Ax-Bow and Ledge Bow, Groot Ship Design’s Cross Bow and Damen’s Axe Bow.

Most of these have no bulb but even where the bulb is retained, changes to its shape and size can offer significant savings potential. After adopting slow steaming strategies, several container ship operators had existing bulbous bows on their vessels replaced by a new configuration better suited to the slower speed of their ships.

Smaller appendages are to be found at the aft end of the vessel and are more common on ships with a high block coefficient such as bulk carriers and tankers than on container ships.

This is because on wider vessels, the water flow to the propeller can benefit from being modified so as to allow the propeller to impart greater thrust to the vessel.

There is one appendage that has been developed by Japanese builder Naikai Zosen Corp that is placed forward on the hull. The device is known as STEP (Spray TEaring Plate) and consists of a pair of plates around 5m in length, one attached to either side of the bow above the waterline.

At first sight in calm water conditions the device would seem to offer little in the way of energy saving but it is designed to operate by reducing the resistance caused by the bow wave that builds in heavy seas above the waterline in medium and high speed ships including car carriers and container vessels. In tank tests, the device was estimated to decrease the wave resistance by about 18% under heavy wave conditions and to reduce the energy consumption by about 2% under head waves at Beaufort scale 6.

Concentrating the flow of water to the propeller on slow, wide-bodied vessels is widely recognised as having a beneficial effect on a ship’s efficiency although the majority of ships of this type have had no modifications aimed at improving their performance. The possible modifications available fall into three categories of appendages fitted upstream of the

propeller. The categories are; fins or spoilers, ducts and pre-swirl stators. The devices can be employed alone or in combination and may even be complemented by more devices downstream of the propeller and optimised rudder/propeller systems. Different builders have developed proprietary versions of one or more of the devices and in Japanese and some South Korean yards they will be offered as part of the package when selling standard designs.

Fins and Spoilers

Fins or spoilers are superficially similar to a bilge keel but much smaller and located strategically to concentrate and direct the wake flow to the propeller. As well as improving efficiency, these devices can also contribute to a reduction in propeller cavitation which produces other benefits including more comfort and less noise. Several shipbuilders have developed designs that feature fins of this type and have given them proprietary names such as Sanoyas Tandem Fin, Onomichi Parallel Fin, Namura Flow Control Fin and Oshima Flipper Fin to name but a few. Although all those mentioned are Japanese, South Korean builders also employ similar devices. Some builders have adopted very similar systems while others have unique patented devices.

Regardless of type, energy savings between 1% and 4% are claimed. For EEDI purposes, the fuel saving effect can be measured either by comparing identical hulls with and without the devices during sea trial or by model testing or CFD.


Ducts that equalise the wake and concentrate it more than fins can deliver significant savings alone and can further enhance the small benefits that fins can confer. As with fins, ducts come in many forms from simple LJ bracket types to more complex versions such as Becker’s Mewis Duct.

In some cases, the duct will be formed by structures either side of the skeg but some are fully circular structures placed just forward of the propeller. In the latter case they should not be confused with a nozzle propeller in which the propeller is located inside the nozzle. Ducts are said to be more effective on wide-bodied slow-moving vessels and savings have been claimed in some instances to be in double percentage figures.

Many builders of bulk carriers and tankers have standard designs that feature some form of duct. In recent years, Becker Marine has promoted its Mewis Duct as a retrofit ESD and has managed a considerable number of sales. The device is also suitable for newbuildings and these now make up the bulk of sales. Since so many have now been retrofitted to vessels, the company claims that statistics of ‘before and after’ consumption figures can now prove an average fuel saving of 6% rising to 8% when combined with an appropriate design of Becker rudder.

The basic concept has been developed further into the Becker Twisted Fin, designed for use on vessels with finer lines than the bulkers and tankers the Mewis Duct was developed for. In the Twisted Fin, the nozzle ring is considerably smaller than the Becker Mewis Duct and has a slimmer profile. The fins on the inside of the nozzle ring extend outwards beyond the nozzle. To prevent the formation of a swirl with cavitation at the ends of the fins, Becker has developed special end caps for the fins similar to the winglets familiar from modern aircraft wings.

Pre-swirl stator

The final type of ESD fitted forward of the propeller is a pre-swirl stator. This generates a swirling flow opposite to the rotation of the propeller and creating an additional blade loading through which the delivered thrust per unit of power is raised. A pre-swirl stator can result in fuel savings of around 4%-6%.

The device has the appearance of a small fixed multi-bladed propeller mounted directly forward of the propeller itself. The number and size of blades varies between designers and shipyards and may be described using a proprietary name. The fins inside the Mewis Duct act as a type of pre-swirl stator. It is usually possible for a pre-swirl stator to be fitted as a retrofit on most vessels.

Copy link
Powered by Social Snap