A Study on Effect of Bow Shape on Icebreaking Resistance

Abstract

In the 1st report, it was shown that the icebreaking resistance at low ship speed is proportional to the total crack length independently of the bow shape, and is determined by the tangent of hull stem angle at the hull/ice contact point. The effect of bow shape on the icebreaking resistance is investigated in detail by analyzing the results of extensive model experiment with three different bow forms. The icebreaking resistance is obtained by subtracting the resitance in pre-sawn ice from that in level ice.This paper studies the speed dependency of icebreaking resistance, i.e. the increase of icebreaking resistance with ship speed, comparing it with the vertical motion of ship bow. The major results are as follows : (1) As the ship speed increases, icebreaking resistance increases as a linear function of bow vertical motion. A spoon bow shows the lowest speed dependency, a wedge-shaped bow is the second, and a concave bow is the highest. The icebreaking resistance nondimensionalized by ice flexural strength, ice thickness and tangent of stem angle at the hull/ice contact point increases linearly with the heave amplitude at bow nondimensionalized by ice thickness, independently of the bow shape.(2) Frequency analyses of resistance and bow heave have shown three origins of icebreaking resistance. They are (a) resistance due to hull motion, (b) resistance due to icebreaking at the shoulder, and (c) resistance due to icebreaking at the bow center. The former two components are large in case of the wedge-shaped and concave bows. The third component is large in case of the spoon bow.(3) The rotation of broken ice piece is closely related to ship motion and broken ice piece size and shape. Such ice/ship motion coupled resistance can not be well removed by a pre-sawn ice test.(4) The resistance increase due to broken ice piece rotation is determined by the rotation speed and broken ice piece shape. The rotation speed depends on the hull inclination angle at hull/ice contact point. The broken ice piece shape depends on the bow waterline angle and curvature.(5) Because of the above-mentioned relationship between bow shape and broken ice piece, the wedge-shaped and concave bows produce very large and long ice pieces at the shoulder and the ice piece rotation speed is high, while the spoon bow gives wider but shorter (less slender shaped) ice pieces whose rotation speed is low. This is the reason why the spoon bow shows the lowest speed dependency of ice resistance.