van Manen, Oosterveld and Witte 



and measurements of thrust and torque variations of the screws, performed at 

 the N.S.M.B. with the Models I and IV. 



Choice of Propulsion System 



Among the special types of propulsors which have as their object an im- 

 provement in the propulsive quality of the ship, the ducted propeller takes an 

 important place. The ducted propeller is now extensively used in those cases 

 where the ship screw is heavily loaded or where the screw is limited in diame- 

 ter. The possibility of using the ducted propeller for the propulsion of tugs, 

 towboats, and trawlers has been adequately demonstrated in practice in the 

 course of the last thirty years. 



In previous publications of the N.S.M.B. (7-10), the results of systematic 

 experiments with screw series in nozzles were given. These experiments sup- 

 plied design data for propeller-nozzle systems with optimum efficiency. The 

 standard nozzle profile applied by the N.S.M.B. from the viewpoint of efficiency 

 is nozzle No. 19a, having an angle between nose-tail line of nozzle profile and 

 propeller shaft axis a.^ = 9.2°, a camber ratio f/e = 0.069, a maximum thick- 

 ness ratio s/l = 0.15 and a nozzle length-diameter ratio -i/B = 0.5. The ordi- 

 nates of this profile are given in Fig. 5. 



NOZZLE No. 19° 



Fig. 5 - Profile and ordinates 

 of nozzle No. 19A 



244 



