My test print of the Maritime Research Institute Netherlands (MARIN) No. 37 profile.
The Kort nozzle is based on the one from Tomi Salo put on Thingiverse, I didnt make any alterations to the .dxf file, and a few alterations to the .scad file.
I have sized the nozzle to the same internal dimension as the acrylic tube used as the cowling.
In a Kort nozzle, the inflow velocity is increased, reducing pressure. This lowers thrust and torque of the propeller. At the same time, a circulation occurs, resulting in an inward aimed force, that has a forward component. The duct therefore has a positive thrust. This is normally larger than the thrust reduction of the propeller. The small clearance between the propeller and duct reduces tip vortex, increasing efficiency.
As drag increases with increasing speed, eventually this will become larger than the added thrust. Vessels that normally operate above this speed are therefore normally not fitted with ducts. When towing, tugboats sail with low speed and heavily loaded propellers, and are often fitted with ducts. Bollard pull can increase up to 30% with ducts.
When I have acquired a prop, I will revisit the nozzle sizing with a view to optimising the distance between the prop and the duct.