Most of the traditional icebreakers have had good capability to run astern in ice even if the vessels are not designed for that. The development during the last 16 years has made it possible that running astern could be considered as the main way of operation in heavy ice conditions. The key to this development has been the use of azimuthing podded propulsion, which gives the vessel the benefits of both electric propulsion and excellent maneuverability. During these years there has been a number of vessels designed and built according to this operation principle. A lot of operation experience has been gained during dedicated full-scale ice trials and during practical operation of the ships. The possible propulsion devices today include the direct electric podded drives and mechanical azimuth drives.
This paper concentrates on the evolution of the concept, description of realized projects and operational experience of the double acting principle gained so far including information of the capability of the vessels in different ice conditions.
First the development of the Azipod electric azimuth propulsion took all the attention in the beginning of the 1990ies . However already the possibility of running astern in difficult ice conditions could be seen. Several projects were drafted and vessel concepts developed but finally the Neste Oil's 106000 DWT tanker project built by Sumitomo in Japan in 2003 based on the concept developed by Aker Arctic's predecessor Kvaerner Masa-Yards Arctic Technology, [3, 4]. This project showed without any doubt that a vessel can be good both in open water and ice .
In general it must be said that level ice breaking is only a small fraction of the total sailing time. The reason, why level ice is so often spoken about is that level ice thickness measurement is easy to perform and ship performance in level ice is easy to measure.