One end of each guide cable was anchored to a concrete wall at the end of the basin 

 by a 15-ft piece of chain and a chain fall. The chain fall was used to slacken the cable 

 when not in use, and the 15-ft piece of chain was sufficiently long to allow for adjustment 

 of the guide cable length when towing at different depths. The other end of the cable hung 

 over a pulley and had a steel bucket loaded with lead attached to it to keep the cable under 

 constant tension during tests. In the present experiments, the cable was kept under 750 to 

 900 lb tension and had a sag of about 3 in. over the span of the test region. The Rankine 

 ovoid was ballasted to neutral buoyancy and was attached to the guide cables by a set of 

 stainless steel shoes with a teflon insert for bearings. 



Towline 



The towline was a continuous loop of 1/8-in. aircraft cable. The model had small eye 

 bolts at either end for attachment to the free ends of the cable. A multiple-groove drive 

 wheel and idler pulley system was used to drive the towline and keep slippage minimal when 

 rapidly accelerating and decelerating the model. Measurements during the test indicated 

 that on the average, this slippage was about 3 in. for each run. After four test runs, the idler 

 pulley system had to be slackened and the towline line and model moved back to the reference 

 position. The idler pulley system was also designed to serve as a tensioning device. The 

 tension on the towline was kept at 600 lb at all times. 



Apparatus for Determining Position and Speed of the Model 



The drive wheel of the towline system was designed to turn off 3 ft of towline at each 

 revolution, and a mechanical counter, geared to the drive wheel by a tachometer cable, pro- 

 vided visual monitoring of the model positions at all times. In addition, microswitches were 

 closed at each revolution of the drive wheel by a cam mounted on the counter shaft. One 

 microswitch was not sufficient to provide the accuracy needed, so two more switches were 

 added to provide a switch closure for every foot of travel of the ovoid. The recorded signals 

 from the microswitch and from a timer on the recorder were used to calculate the average 

 velocity of the model during each test run. The recorded signal from the microswitch also 

 provided a record of the distance traveled by the model within - 6 in. so that the positions 

 of the model could be synchronized with the position of the wave transducers. 



An additional, and more accurate, method was used to check the position of the ovoid 

 with respect to the carriage. A microswitch was covered with a waterproof plastic boot and 

 mounted on a probe attached to the carriage. At each passage of the model, an aluminum arm 

 on the model made contact with the flexible arm on the switch to provide a signal to the re- 

 corder. This switch was located 10 ft downstream from the wave probes. 



