Hull design greatly impacts the efficiency of a vessel throughout the range of operating speeds and draft profiles. As well as keeping the vessel afloat, the hull causes wind and water resistance, both of which can seriously impact the amount of propulsive power needed to maintain a given speed. However, hull designs need to balance more than delivering efficiency. They also need to provide the required cargo capacity and possess good seakeeping qualities in a wide range of sea states and weather conditions.
Despite the challenges, some naval architects and ship builders are pursuing efficiency by integrating novel concepts into their designs. Most hull form optimisation strategies involve improving water flow around the forward and aft parts of the underwater hull, and finding ways to reduce resistance around appendages such as rudders, thrusters, propeller blades, and through-hull fittings.
Hull Form Remodelling
In late 2018, NYK Line unveiled an ‘exploratory’ design dubbed the ‘NYK Super Eco Ship 2050’. The pure car and truck carrier (PCTC) utilises a remodelled hull form which decreases water friction and reduces the weight of the superstructure by using lightweight composite materials. The concept includes gyro stabilisers to counter the increased motion associated with lighter ships and employs an air lubrication system to further reduce frictional resistance.
Matching this hull design concept with hydrogen fuel cells and waste heat recovery technology, the vessel replaces the traditional ‘archimedes screw’ propeller with a pair of undulating foils which are modelled on the propulsive methods of dolphins.
Collectively, NYK believes this concept would result in a 70 % reduction in overall energy consumption which is a considerable saving over a conventional 200m LOA vessel of the same type.
Energy Saving Coatings and Devices
Commenting on the importance of energy saving devices (ESDs) over the entire span of the energy transition in April 2023, Lynn Loo said, “I don’t think we talk about energy saving devices enough. They are something we can do today. Energy saving devices improve fuel consumption, and make vessels more energy efficient.”
Energy Saving Devices (ESDs) and specialist coatings can have a significant impact on decarbonisation. The nature of the hull surface is intimately connected with the designed form of underwater hull sections. Generally, smooth surfaces reduce frictional resistance and encourage more laminar water flow, leading to increased efficiency.
The build up of bio fouling such as seaweed and molluscs which grow on the underside of ship’s hulls has a damaging effect, but highly toxic and harmful paints and coatings are no longer in use, requiring new approaches to preventing biofouling build up.
Reducing Frictional Resistance
One such approach might at first seem counterintuitive. The concept is to make the hull surface ‘stickier’ to water using hydrogels. Added to hull coatings, hydrogels trap a microscopic layer of water on the hull surface. In effect, this makes the surface behave like a liquid, which not only deters fouling, but significantly reduces hull friction.
Another approach is to use air to create a boundary layer between the hull surface and the water. Silverstream Technologies’ patented air lubrication system creates a “uniform carpet of microbubbles” which coat the flat bottom surface of ships, reducing fuel consumption and resultant emissions.
In September 2021, Silverstream Technologies announced the single largest order of air lubrication systems in the world when it closed a deal to fit its technology to more than 30 new build containerships being commissioned by MSC. According to Silverstream, MSC should expect to save 1.6m tonnes of CO2 over the lifetime of the fleet, saving an estimated $300 m in fuel costs. The technology was independently verified to save between 5 and 10% in fuel consumption and the first MSC vessel to complete the installation was the 24,000 TEU containership MSC Irena in February of this year.
It is not only important to cut down on the consumption of fossil fuels today, it may become even more important when shipping moves to lower energy density fuels. As Lynn Loo remarked in April 2023, “If you go to ammonia, for example, it’s two and a half times less dense than LNG. So it means you’re either going to be bunkering more frequently or you need a much much larger fuel tank to be able to move that same distance without bunkering. The more energy saving devices you can deploy, the more efficient you are, the less fuel you need, which is good for the environment, but also good for business because you can bunker less frequently or use smaller tanks, creating more space for cargo.”
In the ever-evolving maritime industry, hull design takes centre stage, offering opportunities for enhanced efficiency and sustainability. Innovative approaches, as demonstrated by NYK Line’s ‘NYK Super Eco Ship 2050,’ showcase the potential of novel hull forms and propulsion methods. Additionally, the adoption of energy-saving technologies, such as hydrogels and air lubrication systems, presents promising pathways to reduce friction and fuel consumption. As the industry anticipates a shift towards lower-energy-density fuels, these innovations not only contribute to environmental preservation but also boost operational efficiency, reinforcing the maritime sector’s resilience in a changing global landscape.