Grim vane wheel

The Grim vane wheel is a freely rotating, larger-diameter wheel mounted on the propeller shaft just abaft the propeller, patented by Otto Grim of the Hamburg Model Basin in the early 1970s as an energy-saving device. It is driven by the propeller slipstream: the inner blade radii, inside the race, act as a turbine that extracts swirl and axial kinetic energy normally lost downstream, while the outer radii, beyond the contracted slipstream, work as a propeller and convert that energy into added thrust at no extra shaft power. Reported gains run a few percent. The wheel turns slower than the propeller and unloads gradually outboard, but the slender outer blades are fatigue-sensitive, which limited service uptake.