Twisted tail gives an extra kick
Reviving an idea from the 1960s which was actually employed on a number of vessels, DNV GL has issued a white paper and announced that asymmetric sterns are now easier to design and build and can produce fuel savings for some vessel types. 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 optimization, the classification society can now offer ship owners the option of incorporating an asymmetric stern into their new vessels. The shape of the afterbody on a ship with an asymmetric stern imparts a change in the flow to the propeller increasing efficiency and without the fatigue and strength issues that are experienced with some hull appendages and nozzle types. “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 optimization 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-optimized 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 optimization generated a design promising a 3.5% percent 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.” There are a number of patents that have been filed in regard to asymmetric stems and the presence of these has somewhat limited take up of the concept over the years. However, a number of the B183 design 1,012teu container ships built at Stocznia Szczecinska in Poland in the early-mid 1990s were built with asymmetric hulls and several of these – the 1993-built Judith Schulte (now trading as PDZ Mewah) are still in service. All of the patents issued and much of the theoretical work on asymmetric sterns suggest a better efficiency saving on slower bulk carriers and tankers than on container ships. A research project on Stocznia Szczecinska’s B573 44,000dwt product tanker design was carried out where tank tests of a modified asymmetric stern showed an improvement on the standard symmetric form of around 7%.