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%
Highly skewed blades
- are used to lower vibration
Prop Boss Cap Fins
Fins of opposite hand to the main blades are mounted on the prop coss cap. These correct the prop hub vortex and recover rotational energy that would otherwise be lost. Fuel savings up to 5% are claimed Contracted Loaded Tip
This is a modern design trend under heavy investigation adn yielding good results. They are screw propellers fitted with end plates at the blade tips. The plates are deigned to give minimum resitance to flow.
Half duct
Sometimes refered to as half kort
One or two nozzles may be fitted just ford of the propeller. There purpose is to steer the flow of water to enter the propeller with minimum shock. Efficiemcy claims are up to 3%. The advantage of this system over the full duct or kort nozzle is that it does not suffer from the same cavitation damage on the inner surface. A simplified version of this is two fins welded to the hull at a slight angle to the shaft centre line
