II. EXPERIMENTAL TECHNIQUES 



A. Dynamometry 



All experiments were conducted using the hull and propeller shovm 

 in Figure 1 on Carriage II at the David W. Taylor Naval Ship Research 

 and Development Center (DTNSRDC) , using basically the same dynamometry 

 and hardware described by Boswell et al. (1976a, 1976b, 1978). The 

 starboard propeller, on which blade loads were measured, was located in 

 its proper position relative to the model hull but was isolated from 

 the hull and driven from downstream (see Figure 2). This downstream 

 drive system was necessary in order to house instrumentation required to 

 obtain the frequency response characteristics of the system for measur- 

 ing unsteady loading. 



The sensing elements were flexures to which bonded semi-conductor 

 strain-gage bridges were attached. The design of these flexures has 

 been described by Dobay (1971) . Three flexures were necessary to meas- 

 ure all six components of force and moment. Flexure 1 measured F^ and 

 My, Flexure 2 measured Fy and l-tx, and Flexure 3 measured Fz and Mz ; see 

 Figure 3. The flexures were mounted inside a propeller hub specifically 

 designed for these experiments. Only one flexure could be mounted at a 

 time because of space limitations, and this necessitated three duplicate 

 runs for each condition. The flexure calibration procedure was identi- 

 cal to that described by Boswell et al. (1976a, 1976b, 1978). 



The port propeller, on which blade loads were not measured, was 

 driven from inside the model hull as would be the case in a self-propul- 

 sion experiment. The propeller rotational speed, which could be con- 

 trolled independently of the starboard propeller, was measured via a 

 toothed gear pickup and recorded on a digital voltmeter. The time- 

 average thrust and torque were measured for selected runs by a trans- 

 mission dynamometer. 



B. Hull Pitching and Wave Simulation 



The downstream body which housed the drive system was modified 

 from the configuration used by Boswell et al. (1976a, 1976b, 1978) so 

 that it could be operated fully submerged. This was necessary in the 

 present experiment because the large shaft angle necessitated deep sub- 

 mergence, and the operation in waves caused an additional disturbance 

 to the water surface. The modifications included a waterproof housing 

 for the drive motor, waterproof electrical cables and connectors, re- 

 moval of the upper apron which had extended the sides of the boat, and 

 the addition of a nonwaterproof top to the boat. Both the body housing 

 (the drive system was soft mounted to this body) and the model hull were 

 rigidly attached to a pitch-heave oscillator which was driven by a 

 hydraulic cylinder. The pitch-heave oscillator was rigidly mounted on 

 the towing carriage. This arrangement enabled the model hull and the 

 drive system to be dynamically pitched together while maintaining 

 independent support from one another. 



