In particular, because the hull structure around the water line from stem to stern bears the brunt of the ice load, this part of the ship is outfitted with an extra layer of thickness called an ice belt (Figure 4.1-3). Also, the inside of the hull is supported by a lattice of finely spaced frames and beams.
The ice loads affect not only the structure of icebreaking ships but their hull forms as well. A key point in designing the hull form of ice-transiting ships is to design the bow to minimize the icebreaking resistance and to clear the broken ice blocks and pieces from the vicinity of the hull. The resistance of ships in ice arises from either breaking the ice around the ship or pushing it away - a far greater resistance than that of ships in open water. Moreover, the surrounding ice can interfere with the ship's propellers and even threaten the safety of the propulsion system; icebreaking ships have to be designed to minimize the ice-propeller interaction. These considerations were incorporated into the hull form of the Shirase, Antarctic research vessel of Japan (Figures 4.1-2, 4.1-4).
The Shirase shows the distinguishing features of an icebreaker above all in her bow. The bow lines incline markedly forward from the bottom to the water line; the stem angle in particular is as acute as 21°. Ice can be fractured in many different ways, but this distinctive shape fractures the ice around the ship vertically rather than horizontally, since ice is more easily fractured by bending than by compression. Thus the hull form of an icebreaking ship is principally designed to convert her momentum and propulsive force into vertical force, which breaks the ice efficiently through a bending motion. To realize this key icebreaking design criterion, the stem should have an acute angle, as seen in the Shirase. The Shirase also has a forefoot at the bottom of the bow. The role of the forefoot is to prevent the ship from riding up on top of the ice. When ice conditions are too severe for a ship to make substantial headway through continuous icebreaking, the ship must run backward once and then ram full ahead into the ice to break the impasse. This mode of operation is called ramming mode.