If the ship rides up too high onto the ice during ramming, the ship's stability may be threatened and she may be forced astern; the forefoot is thus useful to prevent a dangerous ride-up, particularly in ramming mode.
The Shirase has a rise-of-floor at her bottom. Rise-of-floor is a common hydrodynamic feature in the design of conventional freighters. The rise-of-floor of the Shirase works to push the broken ice blocks away from the keel and toward the gunwales, preventing unfavorable interactions between the propeller and ice pieces. The propulsion and steering systems, especially the propeller, shafting and a rudder, are often damaged by interaction with the ice blocks, sometimes causing serious accidents. Another device seen in the Shirase is the ice shoe, a triangular projection located in the stern above the rudder. It protects the rudder and propeller from the impact of ice fragments when the ship moves backward in ramming mode. This appendage is also unique to icebreaking ships.
In the preceding section we have used the Shirase as an example of an icebreaking ship to summarize her main features. However, to carry scientists, crew and cargo to the Antarctic efficiently and in appropriate conditions, the Shirase had to provide acceptable performance in open water as well as sufficient icebreaking capability. The Shirase was thus designed to fulfill both of these very different requirements adequately. Before she arrives in the Antarctic Ocean, the Shirase has to cross the trade-winds zone and endure the rigors of rough seas. Similarly, the NSR includes some regions of rough seas in the North Atlantic Ocean, so the overall performance of the NSR ships will have to be carefully considered on this point. Also, a number of ship designs incorporating high performance in ice-covered waters have recently been developed and demonstrated in Europe, where ships must often enter ice-covered waters as soon as they leave port. These novel designs for icebreaking ships are discussed in greater detail in section 4.1.3.
Propulsion systems
Navigation in ice-covered waters demands ships equipped with the following features. First, propulsion systems must be capable of delivering high power output in the low-speed range. In ice-covered waters, ships must quite frequently travel at low speeds under high resistance, so the propellers are working almost in bollard conditions. The second basic requirement is the capability to change operating mode rapidly, especially the ability to switch rapidly between moving full ahead and full astern. Ships operating in ice-infested waters are subject to an extreme range of operating conditions, and are frequently required to respond rapidly. The ramming mode described above is a particular case in point, where the ship must possess both sufficient astern power and a system capable of rapid switching between full ahead and full astern. Third, the propulsion system must be able to withstand the ice block impact. Protection of propulsion systems from the damage, including propellers, shafts and propulsion machinery, is one of the most vital safety issues for the ships operating in ice.
