Computational fluid dynamics (CFD) allow the details of the flow to be assessed – the colours denote the pressure distribution on the hull (illustration: DNV GL)

A vessel with an asymmetric stern has a twisted aft shape that is designed to account for the differing flow conditions on each side of the propeller. The idea emerged in the 1960s, but due to the difficulty of manually developing the optimal shape, this design failed to make a large impact at the time. DNV GL has revisited the idea and, using high-fidelity computer fluid dynamics (CFD) in combination with parametric formal optimisation, the classification society can now offer ship owners the option of incorporating an asymmetric stern into their new vessels.

“Basically, what we are now able to do is model an aft shape that acts as a propulsion improving device, without the vibration and fatigue strength concerns that come with fins and nozzles,” says Karsten Hochkirch, Head of Department, Fluid Engineering at DNV GL – Maritime. “Using our in-house formal parametric optimisation procedure, we can assess hundreds of options until we find a design that strikes an optimal balance between pre-swirl and resistance, while meeting the design requirements of the customer.”

In a recent project, a 3,000 TEU container ship was tuned to achieve minimum power consumption. Starting from a well-optimised symmetric baseline design, the asymmetric design achieved a propulsion power reduction of more than 3%, a result that was confirmed in tank testing. In another project, the ECO Lines team was asked to find propulsion efficiencies in a 38,000 DWT tanker. The CFD optimisation generated a design promising a 3.5% decrease in propulsion power compared to the symmetrical design.

“This is another instance where advances in computing power and software sophistication are enabling us to unlock efficiencies in ship design. By accurately simulating the performance of these complex hull forms, we are achieving propulsion power improvements of up to 5%, with greater structural robustness,” says Karsten Hochkirch. “And because yards are now able to utilise advanced CAD/CAM techniques and modern, CNC-controlled fabrication methods, they can bring these designs into production much more easily and economically.”

Tags: DNV GL