van Manen and Oosterveld 



32,500-DWT tanker. The propelling machinery was supposed to be a steam tur- 

 bine or Diesel engine, each capable of developing 16,000 DHP at 120 rpm. 



The comparison of the stopping abilities of the contrarotating propellers 

 and the conventional screw is based on a stopping maneuver as illustrated in 

 Fig, 18. This maneuver is divided into four phases: 



I. Steam or fuel supply to the engine is shut, the propeller is running slack 

 and the ship speed decreases due to the hull resistance until the propeller rpm 

 is sufficiently low to enable reversing of the engine rotation, 



II, The ship is further slowed down by running the propeller system full 

 astern, until a forward speed of about 6 knots is achieved. At this speed the ship 

 will loose steerability, and tugs will have to render assistance. 



III. The propeller is stopped, and tugs make fast. 



IV. With the propeller slowly turning astern the stopping maneuver is com- 

 pleted. In this phase the ship is steered by tugs. 



For a comparison of the conventional screw and the contrarotating propel- 

 lers, the characteristics for phases I, II, III, and IV of the stopping maneuver 

 were derived from the results of model tests. During these tests the total brak- 

 ing force (hull resistance and propeller force) was measured at different speeds 

 of the model and at different propeller rpm. The speed of the model was kept 

 constant during a test. For the calculation of the head reach it was assumed 

 that the ship's speed changes so slowly during the stopping maneuver that the 

 values of the total braking force, as measured during the stationary tests, were 

 correct. Thus a quasi-steady approach (as described in Refs. 12 and 16) has 

 been used for analyzing the stopping maneuvers. This approach is correct for 

 large ships having relatively low powers installed, so that long stopping times 

 occur (16). To determine the added mass of the ship during the stopping maneu- 

 ver additional dynamic stopping tests were performed. 



A comparison between the stopping abilities of the conventional screw and 

 the contrarotating propellers can be made from the results presented in Figs. 

 19 through 21. Figure 19 shows the head reaches of the turbine tanker to be al- 

 most equal for the conventional screw and the contrarotating propellers. For 

 the Diesel engine tanker (Figs. 20 and 21) the contrarotating propeller reduced 

 the head reach in comparison with the conventional screw. The rpm at which 

 the Diesel engine is reversed affects the head reach of the tanker considerably. 



CONCLUSIONS 



As a result of these investigations the following conclusions can be made: 



• Contrarotating propellers have an open-water efficiency which is slightly 

 higher (about 2 percent) than that of conventional screw propellers; the optimum 

 diameter of contrarotating propellers is less (about 15 percent) than that of con- 

 ventional screws. 



156 



