Double Acting Ships vs. Ice Bow

Since early 1990ies the emphasis in icebreaking vessel development was to create the ultimate bow. This rivalry lead in to designs that were extremely good in one ice condition, namely in level ice, but could be catastrophic in for instance ice ridges. In the late 1980ies the development on another path started in Finland by Wartsila Marine (today Aker Yards) and ABB to develop a new podded propulsion device for icebreaking ships. At first it was discovered that the new device, AZIPOD, enhanced the icebreaking characteristics of the prototype installation, MS Seili, dramatically and first indications of possibility to operate running astern in controlled way, even at speeds 5~10 knots, was noticed. Later after the next installations, MT Lunni and MT Uikku, it was confirmed that in certain extreme ice conditions, like heavy ridges, the vessel could proceed through without ramming but creeping slowly. This design allows the vessel to be optimized for different operation ways for the bow and stern. The extreme case would be an icebreaking stern and openwater bow. The paper describes the performance of different Double Acting Designs compared with conventional types of icebreaking bows.