Could you elaborate this statement, with calculations or test results?
As far as I can see the fastest speed boats all use propellers (without ducts tho).
If your statement is true, the duct drag must bring the efficiency down to jet drive at a given speed. Do you know what kind of speed we are talking about?
We calculated about 1300W power at around 20kmh foiling, with first itterarion of propeller and (rice speed duct) that are not optimized at all.
Will test with a high speed duct to see what speed/efficiency we can get.