For operation in waves, regular head waves were generated by a 

 pneumatic wavemaker (Brownell et al., 1956). The level of the water 

 surface was measured as a function of time by a pulsed ultrasonic probe 

 that was mounted on the carriage; see Figure 4. The output of this 

 probe, which was filtered using a low pass filter to remove the influ- 

 ence of small irregularities in the water surface, yielded the amplitude 

 and frequency of encounter of the wave. 



For operation with forced dynamic pitching of the model hull in 

 waves, a servomechanism was used to ensure that the pitching of the 

 model hull maintained the desired phase relative to the wave at the 

 propeller throughout the experimental run. Figure 4 presents a schema- 

 tic diagram of this servomechanism. In this servomechanism, a servo- 

 control unit subtracts the feedback signal from the hydraulic cylinder, 

 ep, from the signal from the wave height probe, e^, and sends this dif- 

 ference signal, or servo signal eg, to the servo valve. Based on this 

 servo signal, es, the servo valve slightly adjusts the frequency of the 

 hydraulic cylinder so that eg seeks the null signal. When eg is null, 

 ep is in phase with e^; that is, the pitching is in phase with the 

 waves. With this system small corrections to the frequency of the hy- 

 draulic cylinder are made continuously to maintain eg near the null, and 

 thus to maintain the pitching of the model in phase with the waves. The 

 phase of the wave at the propeller plane was varied relative to the 

 phase of the model pitching by moving the wave height probe used in the 

 servomechanism forward or aft a prescribed distance relative to the 

 plane of the propeller. For example, for setting the phase of the 

 pitching, $^, equal to the phase of the wave at the propeller plane, $,, 

 the wave height probe was placed in the propeller plane. For setting 

 *^ - $i|; = 90 degrees, the wave height probe was placed a three-quarters 

 of a wavelength forward of the propeller plane. 



A second wave height probe, which was not used in the servomechanism, 

 remained in the propeller plane for all conditions. The output of this 

 probe was input for the computer and served as a reference for analyz- 

 ing the blade force and moment data as a function of position in the 

 wave cycle. In all cases, the wave height probes were placed suffi- 

 ciently far from the model in the transverse plane so that the model did 

 not disturb the water surface at the points at which the water levels 

 were measured . 



C. Experimental Conditions and Procedures 



Experiments were conducted at several conditions including steady 

 ahead operation in calm water with no ship motions, simulated periodic 

 pitching of the hull in calm water, operation in regular head waves 

 without pitching of the hull, and operation in regular head waves with 

 periodic pitching of the hull. All conditions were run with the model 

 hull rigidly attached to its support, with no freedom to sink or trim, 

 and with essentially equal rotation on the port and starboard propel- 

 lers. 



