Manoeuvrability and Propulsion of Very Large Tankers 



requires about 6 percent less SHP in the loaded and 9 percent less SHP in the 

 light condition. The application of an extreme bulbous bow for this type of ship 

 will possibly lead to a 15-percent SHP reduction in the light and no reduction in 

 the loaded condition. The time this type of ship will sail loaded and in ballast, 

 will be about equal, so the improvements in SHP with the Hogner stern and noz- 

 zle fitted and with an extreme bulbous bow are of the same magnitude. At this 

 moment we do not have experience with respect to the resistance and the re- 

 quired shaft horsepower of ships with both an extreme bulbous bow and a Hogner 

 stern with nozzle fitted. 



We agree with Mr. Strom-Tejsen that the jet system according to the paper 

 would require excessive power in order to produce at service speed forces 

 comparable to those obtained by a conventional rudder, due to the relatively 

 small nozzle diameters. The comparison between the EHP of the model with 

 the Hogner-type and the conventional stern has been based on experiments with- 

 out jet tunnels installed in the model. The increase in resistance due to the 

 tunnel openings (for these small tunnel diameters less than 0.5-1.0 percent) was 

 not taken into account. 



We agree with Mr. Bindel that special attention must be paid to the problem 

 of course stability. Some remarks can be made already. Firstly, we are aware 

 that the Hogner stern will not affect favorably the course stability. On the other 

 hand we expect that the nozzle itself will form a positive element with respect 

 to course stability. Finally the tail piece will be an addition and forms a means 

 for further affecting (by increasing the lateral area) the course stability of the 

 tanker. 



In regard to Mr. Bjorheden's questions, it must be remarked that for the 

 determination of the propeller, hull, and relative rotative efficiencies, the thrust 

 of both screw and nozzle must be measured. Only the thrust of the screw was 

 measured here. 



The data given by Mr. Bjorheden with respect to the reduction in tug assist- 

 ance costs due to the application of bow thrusters and his remarks with respect 

 to the stopping characteristics of large vessels are very interesting. Recently, 

 investigations were performed at the N.S.M.B. with respect to the stopping 

 abilities of a 100,000-TDW single-screw turbine tanker with an output of 28,000 

 HP at 85 RPM and equipped with different propeller types. The stopping abili- 

 ties have been derived from model test results by means of a qua si -stationary 

 method. Stopping times of about 32, 28, 23, and 18 min from full speed of 16 

 knots were found for, respectively, a conventional screw, a screw and nozzle 

 system, a contra-rotating propeller system, and a controllable -pitch propeller. 

 The head reaches were, respectively, 5100, 4800, 4450, and 2900 meters. For 

 an extensive discussion of this matter, we refer to the original paper of Hooft 

 and van Manen, to be read at the Spring Meeting 1967 of the RINA. From these 

 results it can be seen that the stopping characteristics of the ducted propeller 

 are better than those of the conventional screw. 



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