[1][2][3][4] The wake-equalising duct is a static flow modifier attached to the hull upstream of the propeller within the wake.
It may be made of a single circular section duct directly ahead of the propeller, with fixed internal and/or external fins to impart rotational changes to the wake flow, or as two semicircular ducts mounted further forwards, one on each side of the hull, aligned to provide the desired flow modification.
These have the effect of increasing the efficiency of the propeller as it is working in a more uniform flow, and less energy is lost to hub vortex.
[6][4] The Becker Mewis Duct is a version based on an eccentric annular nozzle fitted between the hull and the propeller, supported by a number of flow directing radial vanes, each of which is angled to optimise inflow direction.
[6] The Schneekluth Wake Equalising Duct is a version using two semicircular nozzles, one on each side of the hull, centred above the shaft centreline, and angled to provide contra-rotational outflow swirl.