Model tests on ice-rubble size and ship resistance in ice rubble

Abstract

Described here are the results of model tests on resistance to ship-hull motion through a thick layer of ice rubble; layer thickness was 45% of hull draft. The tests were aimed to elucidate the influences of ice-rubble size on ice-rubble resistance. It was found that as ice-rubble size increased so did the resistance encountered by the test hull. However, it was also found that a layer of mush ice produced a greater resistance than did layers comprised of ice blocks. A layer comprised of both mush ice and ice blocks produced a resistance intermediate to layers comprised of either mush ice or ice blocks. Ship resistance generally increased with increasing ship speed. However, the influences of ship speed and ice-rubble size on the contributing resistance processes were found to be somewhat more complex. For example, the frictional resistance experienced by the parallel midbody of the test hull initially decreased with increasing ship speed, when the ice rubble was comprised of ice blocks which were small compared to layer thickness and hull size. The frictional resistance subsequently increased then decreased again with increasing ship speed. When the layer was comprised of relatively large ice blocks, frictional resistance increased to a maximum value then decreased with increasing ship speed. Generally, larger frictional resistance occurred for the layers comprised of larger ice rubble. In accordance with the relative sizes of ice rubble and ship hull, and with hull speed, the variations in frictional resistance can be explained in terms of a layer behaving as either a granular medium or as a viscous fluid. An additional aim of the study was to verify the existence of a thrust due to the ascension of submerged ice at a hull's stern. The existence of a stern thrust had been postulated by Kitazawa and Ettema (1985). The present study indicates that a relatively weak stern thrust may occur, but its magnitude is negligibly small compared to other resistance forces.