Kort Nozzles, sometime referred to as Ducted Propellers are on the design list. These are essentially propeller shrouds with an airplane wing cross section which increase low speed thruster performance. Tug boats commonly use them and you would think they would be standard on trolling motors.
"Maritime Research Institute Netherlands (MARIN) has been dedicated to furthering maritime understanding and knowledge since 1932." -according to their web site, www.marin.nl, and they are responsible for the extensive testing of nozzle and propeller designs that are documented in the "Principals of Naval Architecture". This is the Bible for ship builders. It is too expensive to buy, but you can get it through inter-library loan if you're one of the mathematical masochistic types.
(1) The Marin Nozzle No. 37 is specifically designed to work best with forward thrust at a small expense for working pretty good when reversing thrust.
(2) The Marin 19A is best for forward speed only. (3) You get a performance increase using any prop, but the Ka-Series was designed for the Kort nozzle and the Ka-4-70 is best suited for uses with the Marin19A. The wide blade tip reduces blade tip cavitation, and the pitch can be adjusted to suite the power.
The advantage of the Kort is derived from Bernouille's principle continuity equation for incompressible flow. Basically some guy figured out that as fluid is sucked into a space that gets smaller, the fluid will start moving faster. So in a Kort nozzle the water accelerates even before it reaches the prop, which makes the props job easier and thus more efficient. Claims for the benefits of Kort nozzles can be wildly exaggerated by those attempting to sell you something, but over all they should extend the battery supply.
(4) (5) I drew up a basic idea for added a Kort nozzle to my Minn Kota trolling motors. (6) (7) Since the pitch of the prop is the greatest factory the determine the amperage load on the motor, I think it best to create a prop that can have it's pitch changed manually. A flat blade is not going to be the most efficient but it could be coated with epoxy resin that could better approximate the profile for a Ka-4-70 prop.