The maritime transport sector continues to see change in the way energy is sourced and consumed. Energy sources used by large marine vessels have changed considerably over time, most recently from burning coal to heavy fuel oil (HFO) and marine diesel oil (MDO), while smaller boats typically run on engines powered by petroleum fuel or diesel 2-stroke.
Increasing regulation of marine emissions and volatile fuel prices are transforming the way marine vessels are designed and operated. There are many opportunities to improve the efficiency of marine vessels of all sizes, with a range of renewable, hybrid and alternative-fuel options now available. These solutions can often be retrofitted as well as incorporated into new designs.
Keeping the engine and fuel system well maintained will avoid excessive fuel use. Periodic cleaning of fuel injectors and tanks is important in diesel engines. White smoke can be a sign of lean mixture, incorrect valve timing or worn piston rings.
The propeller shaft may also become bent over time, the blades misaligned or the propeller covered with marine growth. A clean, well-maintained hull free of barnacles will reduce drag in the water and cut fuel consumption drastically.
A ship’s speed has a very large impact on fuel consumption, and acceleration requires a dramatic increase in fuel use. In the case of large container ships, fuel consumption increases to the power of three with ship speed.
‘Slow steaming’ (slowing the boat down) by 10% or more can therefore reduce fuel use by at least 27%. Maersk, the world’s largest container shipping line, cut fuel use by 30% simply by slow steaming more often and maintaining constant speed.
Trim refers to the difference between the depths of the hull at the front of the ship compared to the rear. Reducing a ship’s trim improves its balance, allowing it to glide over the water more easily. Many marine vessels are lower at the rear, weighed down with occupants, motor, batteries and fuel.
Passengers or heavier items can often be brought forward to balance the load. Fuel, fresh-water and waste-water tanks can be moved forward relatively easily for a noticeable fuel-efficiency boost. On new ships, positioning the engine rooms further forward significantly improves speed and fuel-consumption performance.
Reduce drag (water and air)
Reducing the amount of drag through the water and air is a cost-effective way to improve fuel efficiency.
This can be achieved by:
- removing or repositioning items on the deck
- installing air deflectors and gap reducers to bulky on-deck items such as containers
- smoothing over welds and rivets and applying low-friction coatings to the hull.
A technique known as ‘air lubrication’ pumps a carpet of bubbles under the boat, reducing drag between the hull and water and cutting fuel use by at least 3%. Minimising the drag of boat attachments is also a good idea. For example, fishing boats can use nets with thinner twine and more efficient trawl configurations.
In general, taking the shortest, most direct course possible is the most fuel-efficient strategy. When running long distances, varying just a few degrees here and there will add many kilometres to the trip. However, over very long distances conditions can vary significantly, and it’s possible to optimise routes and speeds to produce even more fuel savings.
The latest GPS-route planning software continuously updates meteorological information and resistance calculations, and has been shown to reduce fuel consumption by around 4%.
Modern diesel marine engines minimise fuel consumption and optimise performance with precise digitally-controlled fuel injection, instead of less-frequent manual adjustments. Turbochargers can significantly improve diesel engine operation and efficiency of fuel combustion, and can often be retrofitted to existing engines.
Lighting and appliances
Good lighting can improve safety and amenity, while reducing energy costs. LED lights are the best option for most applications, and may be installed with dimming and motion-sensor systems. In addition to lighting, the latest appliances, pumps, and HVAC (especially cooling) equipment can save large amounts of energy.
Capture waste heat
Ships generate substantial waste heat, which can be recovered and used to offset energy use elsewhere on board. Waste heat can be used in applications such as comfort space heating, pre-heating hot water, or clothes drying. More sophisticated systems allow waste heat to be converted into electricity and stored for later use.
Monitoring and tuning
Digital sensors, together with energy management software, is enabling operators to monitor and tune their ship’s energy systems to maximise energy savings.
Most new ship systems include extensive data monitoring and use mathematical modelling to optimise performance and energy efficiency.
Efficiency in port
Improving the efficiency of time spent in port can also deliver savings and reduce local environmental impacts. Reducing the unproductive time typically spent in port can allow for lower speeds at sea without reducing the level of service. Some ports also have facilities to provide shore-side electricity to the ship while docked. This allows the ship’s engines or generators to be turned off, reducing emissions, noise and costs.
Dynamic hull design
Hull design has a major bearing on fuel efficiency. For some boats it’s possible to reshape the hull. Fitting ships’ bows with a bulbous extension below the water line has become a popular retrofit, as it reduces drag enough to cut fuel use by at least 5%. More slender ship designs could cut fuel use by up to 15% at low speeds and up to 25% at high speeds, though payload productivity is also a consideration.
The excessive weight of many ships can also be a cause for poor fuel efficiency. Because of the wide availability of cheap (and often dirty) fuel, the design of large marine vessels has not traditionally focused on weight as a construction criterion.
Most cargo ships, for example, are made of heavy steel rather than lighter materials. Light-weighting principles can be applied in the design of new ships, as well as in the retrofit of existing vessels. The use of fibre-reinforced composite materials in construction is gaining favour.
LNG and biogas
Globally, the shipping sector is looking to liquefied natural gas (LNG) as a transitional fuel for a low-emissions future. The environmental advantages of LNG over traditional marine fuel products include virtually no nitrogen and sulphur oxides in addition to its lower emissions of CO2.
Some LNG ships claim a reduction in CO2 emissions of 15%, though that depends on keeping leakage of methane (the main component of LNG) to a minimum in ships and bunkers. Just like natural gas, biogas can also be liquefied and used as a transport fuel.
Biofuels are another alternative for reducing fossil fuel use in the marine sector. However, experience with their use in the sector is still very limited. The most applicable biofuel types include biodiesel (including hydrogenated vegetable oil), renewable diesel, and pyrolysis oil.
Biodiesel is currently the most cost-effective of these options but, unlike renewable diesel, usually requires blending with conventional diesel. Ethanol can be used in some suitably designed marine engines, however a high degree of caution is necessary as the use of ethanol or ethanol blends (including E10) may be detrimental to some engines.
See the Renewables page for more information on biogas and other renewable forms of energy.
Batteries and hybrids
Batteries have long been used to power smaller boats, but also offer a hybrid solution for medium-sized ships. The low loss hybrid (LLH) system uses different power sources in combination with batteries to operate the ship close to maximum efficiency. The system can reduce transient engine loads that cause increased fuel consumption and emissions.
Around the world, many ferry companies are looking to trial and soon adopt 100% electric powertrains. As ferries often serve the same service route, it is easy to plan for the required range and charging strategy.
Ferry builders in Europe have demonstrated large scale application of the technology. This often incorporates the latest in storage technology such as super-capacitors that can be recharged portside in minutes.
Hydrogen fuel cells
The use of hydrogen fuel cells to power boats has been successfully demonstrated, but cost remains a barrier to broader uptake. In the longer term, hydrogen fuel cells are expected to be economically suited to long-distance freight due to the inadequacy of solar PV and battery-storage solutions for such operations.
As fuel cells avoid the problem of downtime when charging, the technology may also prove cost-effective for other forms of constant-use marine vessels, such as passenger ferries.
The use of sails has undergone something of a renewal over the last decade. Examples in commercial use include fixed-wing sails mounted on rotating masts, and kite sails mounted from the bow (front) of the ship. Fixed sails can be combined with solar PV panels to provide useful amounts of power for vessels such as harbour ferries. Sails can be either retrofitted to some existing assets or incorporated into new design.
Designers around the globe have been working hard to improve the efficiency of propellers. Hyundai Heavy Industries (HHI) has used an energy-saving device called Hi-FIN attached at the hub of the ship propeller, which generates swirls that offset the swirls generated by the propeller, improving fuel efficiency by 2.5%.
Another example is the Becker Mewis Duct, a cylindrical channel with integrated fins that sits in front of the propeller and guides the ship’s wake current towards the propeller, streamlining flow and increasing the energy-efficiency of the propulsion system.
Flettner rotors are a motorised cylindrical ‘sail’ used to capture power from the wind. When wind hits the rotor a phenomenon known as the Magnus effect is created, resulting in a vertical force that propels the ship. This technology has been in use for 100 years, but has seen increasing interest recently due to energy-efficiency concerns.
Internet of Things (IoT) optimisation
IoT is beginning to have an impact in the design and operation of marine vessels. Austal, the Australian defence and fast ferry shipbuilder, has developed an IoT system aimed at reducing fuel consumption and improving ride comfort. The system uses about 100 different data points to predict fuel consumption and inform the crew to adjust settings.
Energy Efficiency Design Index (EEDI)
The EEDI for new ships is a regulatory framework promoting better fuel efficiency, developed by the International Maritime Organization (IMO). The EEDI is a non-prescriptive, performance-based mechanism that leaves the choice of technologies to use in a specific ship build up to the engineers and designers.
EEDI requires a minimum energy-efficiency level per capacity mile for different ship types. The stringency level is tightened incrementally every five years, thereby stimulating continued innovation and technical development.
Measures to reduce greenhouse gas emissions from international shipping Australian Maritime Safety Authority, Australian Government
Renewable energy options for shipping International Renewable Energy Agency
Energy Efficiency Design Index (EEDI) for ships International Maritime Organisation
Sydney Harbour cruise ship emissions Australian Maritime Safety Authority, Australian Government