Why a twisted tail could be an energy saver

A twisted stern shape on the model of a Handymax Bulker design

MAY 10, 2017 — 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, the design failed to make a large impact at the time.

Classification society DNV GL has revisited the idea and, using high-fidelity computer fluid dynamics (CFD) in combination with parametric formal optimization, can now offer shipowners the option of incorporating an asymmetric stern into their new vessels.

And from a shipbuilding standpoint, CAD/CAM techniques and CNC controlled fabrication could make these sterns more buildable.

“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 Hochkirch. “And because shipyards are now able to utilize advanced CAD/CAM techniques and modern, CNC-controlled fabrication methods, they can bring these designs into production much more easily and economically.”

DNV GL has published a white paper on optimization of ships with asymmetric afterbodies. You can download it HERE

Computational fluid dynamics (CFD) allows assessment of the details of the flow. The colors denote the pressure distribution on the hull

A bulker / tanker hull with asymmetric aftbody is tested in HSVA’s towing tank to confirm the result of the optimization