Ships that incorporate a reamer can tolerate a high turning angle in the channel, improving their manoeuvrability in ice. However, to prevent damage to the rudder, care must be taken to avoid an excessive turning rate.
These innovations in icebreaking are applied in a wide range of ships and their superior performance relative to conventional designs has been clearly demonstrated. Nonetheless the focus of their development has been ice navigation, and it must be noted that open-water performance is sacrificed to some extent in these designs. The open-water resistance of these types of ships is actually higher than that of conventional designs, and ship behavior in rough seas is conspicuously poor. Ships with spoon bows, for example, suffer from flare slamming due to impulsive wave loads, causing tremendous vibrations that are known to open cracks in the ships' upper structures. Just as in commercial ships, careful consideration of open-water as well as icebreaking performance will have to be taken, and in many cases the more extreme designs will have to be discarded in favor of more conventional approaches.
One idea conceived to provide both icebreaking and open-water capability is the Double-acting Azipod Ship (DAS). The DAS is equipped with an azimuth propulsion unit in its stern. The form of the bow is favorable for open water operations. In icebreaking operations, the ship is turned around and the "stern" goes first. Of course, this requires the stern to have an icebreaking hull form as well. The DAS was developed in Finland on the basis of a wealth of accumulated experience in operations with bow propellers in the Baltic Sea. In spite of potential cares about hazardous interactions between ice and propeller and lack of sufficient full-scale data, the DAS design holds great promise for future NSR vessels. Except for some unusual designs, DAS models incorporate a bridge located near the stern, providing the crew with a better view aft than forward. This feature lends the DAS a great operational advantage in ice. In the Finnish tanker Lunni, the propulsion system was switched to an azimuth design and the stern was remodeled to an icebreaking form, while the icebreaking bow form remained conventional. In summer 1993 the M/T Lunni inaugurated shipping service with a number of trips to the estuary of the Yana River in Eastern Siberia. The experience of the Lunni clearly demonstrates the feasibility of stern-forward operation in ice. Unfortunately it is not yet possible to guarantee the safety of the DAS under harsh conditions such as steep ice ridges. Research will have to continue, using mockups and prototypes, to ensure full feasibility of the DAS design and to learn more about its limitations.
