876 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 7S.10 



Fig. 78. G Fish-Eye View of Arch-Stern Model 



4505-1, Looking Up and Aft, With All 



Appendages in Place 



model are shown in Fig. 67.J of Sec. 67.13; those 

 on the arch-stern model in Fig. 78. G. 



78.10 Open-Water Propeller Tests. It is as- 

 sumed that for the stock propellers hsted in 

 Sec. 78.4, the open-water propeller-test data are 

 available. 



The new model propeller intended to be de- 

 signed and built specially for the selected ABC 

 hull design is to be characterized iir open water, 

 following the standard procedure of the David 

 Taylor Model Basin. 



The predictions indicate that both the special 

 20-ft and the 24-ft (ship size) propellers should 

 be free of cavitation if the bottom of the transom 

 is kept in the water in the one case and the 

 tunnel is kept full of water in the other. There 

 appears to be no need of conducting cavitation 

 tests on a model propeller designed for either type 

 of huU. 



If assistance can be furnished from the Design 

 Office for the purpose, the new propeller is to be 

 measured and checked, and some quantitative 

 values obtained for its surface finish. 



The new model propeller was designed but not 

 built, hence there are no test data for it. 



78. 11 Neutral Rudder Angle and Maneuvering 

 Tests. Tests are to be conducted with the arch- 

 stern model to determine the neutral angle for 



the double rudders and to obtain certain turning 

 characteristics. This model, if it represents the 

 selected design for the ABC hull, is to be self- 

 propelled with the new model propeller; otherwise 

 with the stock propeller. The weight displacement, 

 draft, trim, and the like are to be for the designed- 

 load condition with appendages, as set down in 

 Table 78.c. 



The neutral rudder angle for each rudder is 

 to be determined by a special test, first on the 

 basis of the optimum steering characteristics 

 when running ahead, and then on the basis of 

 good steering and minimum propeller power for 

 driving the ship in straight-ahead motion. Rudder 

 angles for maneuvering tests are to be reckoned 

 from these neutral angles. 



Free-running maneuvering tests are to be 

 carried out in the equivalent of deep water, to 

 determine: 



(a) The complete turrung path through a 180-deg 

 turn, including advance, transfer, and tactical 

 diameter, at an approach speed of between 19.5 

 and 20 kt, model speed between 3.86 and 3.96 kt, 

 with rudders at 35 deg; see item (34) of Table 64.e 



(b) The maximum angle of heel when turning as 

 in (a). 



Because of the interval elapsing between the 

 publication of Volume II and Volume III, the 

 latter covering the subject of maneuvering, none 

 of the tests listed here were made on either model. 



78.12 Controllability Tests in Shallow Water. 

 Controllability tests are to be carried out for the 

 arch-stern model, under the designed-load con- 

 dition described in Table 78. c, -with the model 

 propelling motor and the steering-gear motor 

 arranged for distant manual control. If it so 

 happens that these tests can be conducted in 

 shallow water, when the model is ready, the depth 

 is to correspond to a full-scale depth of 40 ft, 

 model depth 1.78 ft. Otherwise, the tests are to 

 be conducted in the shallowest depth available. 



The model shall then: 



(1) Execute a crash-back maneuver. From a 

 straight approach path at a speed corresponding 

 to 20.5 kt, 4.06 kt for the model, the propeUing 

 motor is to be reversed ■within 3 sec, about 15 sec 

 for the ship, and an astern torque appUed, as 

 nearly as practicable equal to 0.8 of the ahead 

 torque for 20.5 kt. The rate of propeller rotation 

 astern is to be, as nearly as practicable, not in 

 excess of 0.5 of the ahead rate for 20.5 kt. Obser- 



