Dynamics of Continuous-Mode Icebreaking by a Polar-Class Hull—Part 1: Mean Response

Abstract

Results are presented from an ice towing-tank study conducted in order to document the relationships between icebreaking pattern, resistance, and hull motions for a Polar-Class icebreaker model hull moving continuously through sheets of level ice. Two sets of experiments were conducted: for one, the hull was free to pitch, heave, and undergo limited roll (free hull); for the other, the hull was restrained from motions (fixed hull). For each condition, the hull was towed at the same range of speeds through urea-ice sheets of similar properties (thickness, strength, etc.). Measurements were made of resistance as well as motions and restraining forces and moments for the free-and fixed-hull conditions, respectively. The temporal records were analyzed to yield values of their mean and standard deviation and spectral densities. In Part 1 of this two-part paper, the experimental equipment and procedures are described and the values of the mean and standard deviation for both conditions are compared and related to observed patterns of icebreaking. It was found that the mean resistance was larger for the free hull than the fixed hull. For both conditions, mean resistance increased with the square of ice-sheet thickness. The relationship between mean resistance and hull speed was linear for the fixed hull, but not for the free hull, which was characterized by a larger and similar slope at low and high speeds, respectively, than for the fixed hull. The speed range at which the slope changes coincides with that at which the motions are maximum. This correlation indicates that the ice-induced hull motions of the free hull are responsible for its increased resistance over that of the fixed hull. Also, two leading prediction methods ("SPLICE " and "Milano ") are evaluated through comparisons with the measurements. Part 2 (to appear in a later issue) will examine spectral densities with regard to the natural frequencies of hull motions and frequencies associated with the icebreaking pattern to determine the dominant cycles of resistance and motions.