Jet advantages?

Is this guy on the right track using a jet drive or would he be better off

with a prop? Why? I need a jet drive advocates opinion on this.

www.psubs.org/projects/1237684922/r300submersible/

Tom

 

He'd certainly be best off with a prop. The speed is low, the relation between jet (or "slipstream") velocity and forward speed (Vj/Va) for the necessary thrust favours a propeller. He might use a Kort nozzle for protection/ shrouding, but a free prop will have better efficiency, giving longer range for a given set of accu's.

You have to design for a speed range above 30-35 knots in order to get reasonable efficiency from a jet! Below that, it is simply not competitive in efficiency. It is excellent in maneuvering though, but that is a different story, and it has a prize!

Btw, he should use a rudder (even adding a horizontal foil) directly behind the prop as a stator vane to balance the propeller torque.

 

This unit is not performing as a traditional jet ,for two reasons ,firstly the jet exhausts it's thrust under water as opposed to above it and secondly it is not required to "lift " a large volume of water into an intake duct only from one direction.the second of these is the important point as regards efficiency I would suggest that for these reasons it will behave like a Kort nozzle but with the additional advantage of a set of stator blades behind it ,which will reduce torque reaction to maneagable levels, very important in a submarine I am told You also gain the advantage of jet nozzle manouverability in all dimensions in this case .