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→Kort/Rice Nozzles
When a blade rotates in water, it creates a high-pressure behind the blades and low pressure in front (towards the hull). This differential provides thrust. However, at the edge of the blades, high pressure leaks to the low-pressure side, resulting in loss of thrust.
[[File:KortNozzle.jpg|Thumbthumb|250px|Leftleft|Kort 19A Nozzle – note the smaller exit side – Photo with permission © Rice Propulsion, http://www.ricepropellers.com/]]
In the 1930s, this caught the attention of Ludwig Kort, an aeronautical engineer from Hanover. He developed nozzles (shrouds or ducts) for propellers. They prevented water from escaping from the blade tips. This improved propulsion efficiency at speeds under 10 knots.