Hybrid drive, JetProp

efoils – Electric Hydrofoil Surfboards
#1

Here is a concept for a hybrid jet/propeller drive. This concept avoids mechanical gear reduction while allowing a larger propeller for better efficiency.

The motor spins a pump.
The water then flows through a turbine rotating the opposite direction.
The turbine and propeller are a single integrated part
The turbine/propeller will be designed to decrease rotation speed of the turbine and increasing torque; hydraulic gear reduction.
The turbine/propeller will spin freely during unpowered foiling

jetprop
jetprop1920×2560 183 KB

#2

My bet is that the overall compressor+turbine efficency is way lower than 80% ( 90% in both compressor and turbine!)
A folding prop + gearbox is simpler and more efficient both powered and unpowered

#3

You are probably correct. Part of the idea was to create something that could easily be 3D printed for the home builder.

Some of the (hoped) efficiency gains are:

The flow through the pump/turbine contributes towards thrust
The counter-rotating pump and turbine helps minimize wasted energy by straightening the jet portion of the flow.

Some other small advantages:

Corrosion resistant construction
No need to seal the drive train other than the motor
Fairly simple construction for a DIY builder

The big hurdle would be designing and refining the combination of pump/turbine/propeller to make it work acceptably well.

A folding prop could be integrated into this concept.