FULL-SCALE TOWED RESISTANCE TRIALS OF THE USCGC MOBILE BAY IN UNIFORM LEVEL ICE

In February of 1986 an extensive series of ice trials was carried out on the USCGC MOBILE BAY in Green Bay. These trials included the first towed resistance tests of an icebreaker ever performed, and marked a new level in the prediction of icebreaker resistance and propulsive performance. The background section of this paper reviews the historical development of techniques for evaluating the resistance and propulsion of icebreakers. It provides parallels to open-water predictions, comparing the current trials with those of GREYHOUND and LUCY ASHTON. The description of trials includes discussions related to the selection of vessels and locations, as well as preparation of test and monitoring instrumentation. The primary emphasis is on the procedures employed to tow MOBILE BAY by a 5-cm (2 in.) steel towline from the USCGC MACKINAW through unbroken level ice. Results are provided for all towed and self- propulsion tests. This section also includes a discussion of the quality of the data collected. During the trials, 17 towed resistance tests and 31 self-propulsion tests were performed. The analysis section of the paper provides insight into the problems associated with predictions of icebreaker performance. This unprecedented dataset has allowed rigorous examination of such critical propulsion parameters as thrust deductions, effective wake, and relative rotative efficiency with some surprising results. Analysis of the resistance data alone is reported, covering the efficacy of nondimensional analysis techniques and existing empirical resistance prediction forms. The data are compared with model test results at various scales and using various ice modeling materials to provide a definitive evaluation of the accuracy of ice model tests. The conclusions and recommendations from the parent study are summarized, with special emphasis on guidance for designers of iceworthy ships.