Simple is better… most of the time. My thoughts of the simplest; yet safe; thruster for an e-foil are 1) direct drive and 2) an optimized ringed propeller.
Some kind of duct or shroud is needed for safety. A well designed duct can increase static thrust but adds drag at speed. A simple safety shroud only adds drag. The drag is not just due to the ring portion of the shroud or duct, but drag is also created by the support struts.
The idea with a direct drive ringed propeller is to simplify the thruster, and also to minimize wetted surface area and drag at speed.
An optimized ringed propeller would probably have blades with a relatively small tip
cord…this allows a minimal safety ring.
The pros for the direct drive ringed propeller are:
- smaller diameter prop for direct drive (probably) = smaller ring, less wetted surface area
- much less wetted surface are than shrouded/ducted propeller, less drag whie at speed.
- cheaper and simpler implementation
The cons for the ringed propeller are:
- greater energy losses, more energy spent spinning water
- less prop guarding, but probably adequate
My gut feeling is the net effect on efficiency will make a well designed, direct drive, ringed propeller comparable to a ducted propeller while operating at higher speeds.
Looking forward to other’s thoughts on this.
ringed propeller with small tips and minimal ring, sorry about the poor sketch.