DNV GL has added asymmetric stern to its ECO Lines hull optimization service. The classification society uses high-fidelity computer fluid dynamics (CDF) in combination with parametric optimization to show the effect of an asymmetric stern.
A vessel with an asymmetric stern has a[ds_preview] 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.
»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. »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.«
Field trials with tanker and container vessel
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 DNV GL stated. 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 % % decrease in propulsion power compared to the symmetrical design the classification society informs.
