The development and application of high efficiency nozzles and rudders. Discussion

Since 1933 when the Kort nozzle was invented, there has been very little change to the duct design. Netherlands Ship Model Basin (NSMB) performed systematic model tests of nozzles in the 1950's and nozzle 19a and 37 emerged as the standard. Established nozzle theory predicts a higher theoretical efficiency than a propeller alone even when the nozzles frictional drag is taken into account. However this was never accomplished with standard nozzle designs. There is a potential to close this gap between theoretical and practical application and to improve the efficiency of almost any type of vessel. Model tests of different nozzle sections show gains at lower speeds but fail to show gains at higher speeds of advance. The nozzle sections drag is many times greater than any standard NACA wing section and this difference has never been explained. In this paper the difference between nozzle theory and the practical application of standard nozzles 19a and 37 is explained as an effect of laminar separations due to the viscous drag on the model scale used during testing. This discovery, subsequent research, and full-scale trials lead to the development of the high efficiency nozzle.