McClure and Hove 



chosen over the experimental route due to the multiplicity of possible environ- 

 mental conditions, practical difficulties and expense in equipping a model with 

 the six right-angle drive units and automatic controls. The analysis served an- 

 other vital purpose; besides verifying design expectations, it has contributed to 

 design of the control system by providing time constants and values of system 

 constants to be set into the on-board positioning computer. 



Drag and moment coefficients and other platform characteristics were de- 

 termined with the aid of model tests. Four types of tests were conducted: 



1. Resistance and moments in straight-line motion 



2. Resistance and moments in curved maneuvers 



3. Aerodynamic resistance and moments 



4. Propeller performance. 



Resistance and Moments in Straight-Line Motion 



Platform propulsion systems are capable of providing thrust in whatever 

 direction is required to move the platform toward the drill hole. Also combina- 

 tions of propulsive power and wind may move the platform through the water at 

 any angle to the center line. In positioning maneuvers (station-keeping maneu- 

 vers) it is as important to know the moment and side force developed by oblique 

 motion as to know the resistance in the direction of the motion. Principal reli- 

 ance in obtaining these data was placed on model tests which were conducted at 

 two model scales in two different facilities: 1:45 in the David Taylor Model 

 Basin main towing tank and 1:27-1/2 at the University of Michigan Naval Tank 

 (2,3) (Appendix A). 



The model was towed over a range of speeds at headings from zero to 180 

 degrees measured from the fore and aft center line. Figures 2, 3, and 4 give 

 drag force in the direction of motion, side force perpendicular to the direction 

 of motion, and yaw moment about the vertical axis as functions of speed and 

 drift angle measured from the bow. 



These data are scaled directly from the model basin tests and are not cor- 

 rected for roughness and minor appendages. These tests were conducted early 

 in the program and the model incorporated elliptical cross-section diagonal 

 bracing in place of the present circular bracing. The effects of all of these 

 items are incorporated in the equations for the positioning system analysis. 



In addition to the towing tests a composite test was used to estimate plat- 

 form resistance. To eliminate Reynolds number effect, tests were conducted at 

 high Reynolds number in a wind tunnel to determine the resistance of the fully 

 immersed body to which was added the wave-making component of drag meas- 

 ured by towing a cylinder in a water basin. Although a rigorous comparison is 

 not possible, agreement between the two methods is well within the limits of 

 required accuracy and the towing tests which yield the more extensive data are 

 considered entirely satisfactory for design purposes. 



418 



