Initial design had the grim wheel mounted on the main propeller boss. Severe problems including entire loss occurred. The more modern approach is to mount the wheel on the rudder horn. This having the added advantage of allowing a dedicated lube oil supply and reducing main prop shaft and stern tube bearing loading
Shown below is a design of unknown origin but has appeared for some time in this site so the source is lost. I am unsure as to its correctness. It is unsual that the propulsive section should be fitted in the vortex produced by the main prop hub.
The Grim wheel is mounted on roller bearing and is therefore free to windmill on the end of the propeller boss. The Grim wheel is of two parts. The outer section acts as a turbine driven by the wake of the main propeller blading. This turns the inner section which is the propulsion section and provides extra thrust.
Increases efficiency by 5%