Asymmetric stern

An asymmetric stern is an afterbody with deliberate port-starboard asymmetry, shaped to pre-swirl the inflow into a single-screw propeller against the direction of propeller rotation. By twisting the aftbody sections so the wake field already rotates opposite to the blades, it recovers part of the rotational energy the propeller would otherwise shed, raising propulsive efficiency by a few percent at design speed. The concept, attributed to Nonnecke in the 1980s, trades a small build complication for a measurable fuel saving and is one of the hull-integrated energy-saving devices alongside ducts and pre-swirl fins. The optimum asymmetry is tuned to the rotation sense, so a left-hand and right-hand propeller require mirror-image sterns.