towing mechanism was supported by submerged towers near the ends of the basin. This per- 

 mitted a 225-ft run of which approximately 20 ft was needed for accelerating and decelerating 

 the model. Wave heights were measured on and off centerline from a carriage suspended from 

 a bridge over the basin. 



RANKINE OVOIDS 



Two 7 to 1 Rankine ovoids which were ballasted and had zero buoyancy were used 

 during the tests series. The 9.0-ft long ovoid was towed at a nominal depth of 3.0 ft, and 

 the 4.5-ft long ovoid was towed at a nominal depth of 1.5 ft. Figure 1 shows the offsets from 

 which the dimensions of the two ovoids can be obtained. 



TOWING RIG 

 Support Tower 



The towers were placed 250 ft apart on the bottom of the basin and were designed witl 

 a three-point support base to ensure against rocking on the slightly uneven floor of the facil- 

 ity; see Figures 3 and 4. The stability of the tower depended upon its gravitational weight 

 because it was not permissible to anchor the towers to the basin floor. Each tower weighed 

 approximately 3000 lb; this was sufficient to counter the overturning moment placed upon it 

 by the cable system. As a safety measure, each tower was weighted with five 1500-lb 

 weights at its base to provide an additional 7500 lb of ballast. 



A crossbar, bolted to the face of each tower, supported the guide cables and towline 

 pulleys. The crossbar could be positioned up and down the tower face in 6-in. increments 

 which gave a range in depth of submergence of 0.5 to 15 ft. The outboard pulleys supporting 

 the guide cables were adjustable in 2-in. increments, spanwise on the crossbar, to accomo- 

 date models up to 24 in. wide. 



The crossbars of each tower were constructed of aluminum; this material was chosen 

 partly for its corrosion-resistant properties and partly to lighten the bars for underwater 

 adjustment purposes. The pulleys that operated underwater were made of stainless steel 

 and rotated on bronze bearings. The structural steel members of the towers were painted 

 with a corrosion-resistant paint. All of this equipment was in good condition after several 

 years of submergence. 



Guide Cables 



The guide cables were 5/32-in. aircraft cable. Aircraft cable had a high tensile 

 strength and could be stretched tightly to obtain a flatter catenary than possible with ordi- 

 nary cable. The cable could also withstand long periods of submergence in water because 

 it was made of a highly corrosion-resistant nickel-steel alloy. 



