the sampling rate of DIDAS conflicted with the pulse rate of the sonic transducer, a ground 

 loop developed with the 300-ft transmission lines from the carriage to the recording station, 

 and the signal-to-noise ratio was below tolerable limits. 



TEST PROCEDURE AND RESULTS 



Three separate tests were conducted with two sizes of Rankine ovoids in the MASK 

 facility at the Model Basin. In the first two tests, Ralston 6 and Livingston 7 used a 9.0-ft, 

 7 to 1 ovoid. Ralston measured the surface disturbance both on and off the centerline of 

 travel. Unfortunately he failed to recognize the importance of the transient effect (i.e., 

 length of ovoid travel) on the wave profile and therefore the data were of little use. 

 Livingston, on the other hand, foresaw this problem and was able to eliminate the transient 

 effects of the wave profile and obtain a limited wave profile which moved with the body. 



As a result of Livingston's work, it was discovered that a valid steady-state wake 

 could be established for about one-half of the towed length of run of the body, thereby, 

 significantly reducing the number of test runs needed to obtain a meaningful profile. There- 

 fore, since gravity wave profiles are scaled with Froude number, it was postulated that a 

 similar 4.5-ft Rankine ovoid, half the length of the original ovoid, would give a wake of 

 twice the number of body lengths, in the same length of run. 



The length of all the test runs was limited to 225 ft. The starting acceleration and 

 deceleration distances were about 10 ft each. Therefore, the length of test run, for constant 

 velocity, was about 205 ft. In all but a few tests, the model was allowed to travel the full 

 225 ft. The depth of the water in the basin during the test was 19 ft 11 in. and was kept at 

 this depth within - 1/8 in. During the course of the run, the depth variation of the model due 

 to the sag in the tow cable was no more than - 1.5 in. Figure 6 shows the tow cable catenary 

 for the 4.5-ft ovoid. 



9.0-FOOT, 7 TO 1 RANKINE OVOID 



Livingston towed a 9.0-ft Rankine ovoid at several speeds and at a submergence depth 

 of 3.0 ft and measured the centerline time history of the surface disturbance. Wave heights 

 were measured with a sonic surface wave transducer, and the output of these gages was re- 

 corded on a Series 350 Sanborn Recorder. Despite the 150- to 180-ft runs, he found that only 

 a limited portion of the wave train represented a steady-state condition and that measure- 

 ments at a single point would not give a valid representation of the Kelvin wake for this 

 ovoid. Therefore, it was necessary to construct a wave pattern which moved with the body 

 by taking single wave height measurements from many test runs at finely spaced intervals 

 over the distance. A wave pattern constructed in this manner increased in length as the 

 time from the starting point was increased. 



11 



