DESCRIPTION OF MODEL AND EXPERIMENTAL TEST SET-UP 



A 1/4-scale model of the MARK 56 mine was constructed, except for 

 the tail fins, according to drawings provided by the Naval Ordnance 

 Laboratory. The model was designed to accommodate an aircraft-type 

 vertical gyro to measure pitch angle. Photographs of the model are shown 

 in Figures 2 and 3. 



The model was ballasted to the corresponding full-scale weight-in- 

 air, and the center of gravity was adjusted so that the main axis of the 

 model was horizontal while moored beneath the water surface. The inertia 

 of the model in pitch about the center of gravity was determined by 

 oscillating the model in air, using bifilar suspension. The center of 

 gravity was located by using a knife edge and by freely floating the model 

 in water and noting the angle of inclination. Model characteristics such 

 as the inertia, weight, and location of the center of gravity are pre- 

 sented in Figure 4. 



Dynamic characteristics of the model were altered for some tests 

 to provide for a more thorough corroboration of theoretical computation 

 with experimental results. This was accomplished by lengthening the 

 mooring arm 20 inches (7 feet full scale). Characteristics of the altered 

 model are presented in Figure 5. 



In addition to pitch angle, measurements were made of the wave 

 height and heading angle of the buoy relative to the wave direction. The 

 wave height was measured with a resistance type probe. Heading angle was 

 measured by manually rotating the shaft of a calibrated potentiometer to 

 align with the axis of the model. 



The model was moored with soft stainless steel wire 0.031 inches 

 in diameter. Cables from the gyro were looped to minimize interference 

 with the model motions. A sketch of the test set-up is shown in Figure 6. 



TEST RESULTS 



Tests were conducted in waves on the unaltered model at mooring 

 depths of 5 and 10 feet below the undisturbed free surface (20 and 40 feet 

 full scale) . Water depth in the test basin was 20 feet (80 feet full scale) 

 The model was tested through a range of wave heights and for wavelengths 



