Some Ideas on Sea Ice Climate and Navigation in the Baltic Sea 
69 
The political decision - based on 
strong socio-economic require 
ments - to further increase and 
modernize the state icebreaker 
fleets in order to guarantee an 
unbroken navigation season for 
the whole ice winter, was per 
formed from the late sixties to the 
season 1976/77, when new 
powerful icebreaker classes were 
introduced. The number of 
icebreakers was raised by 6 to 18, 
the total capacity was more than 
doubled to 231,000 HP (169,000 
kW). The last winter with an 
interrupted navigation season for 
the main harbours in the northern 
Gulf of Bothnia was mostly 
1969/70. This development was 
somewhat favoured by milder 
winters in the first half of the 
seventies. 
The requirements for improving 
the icebreakers with respect to the 
modern development of the sea traffic (much more and bigger vessels) continued, and today 
the ‘old-fashioned’ ice-breakers are partly replaced by very powerful multi-purpose vessels (see 
Figure 2). In 2005 the icebreaking fleet available in Finland and Sweden consists of 19 units 
with in total about 325,000 HP (240,000 kW). 
Ice Navigation 
Navigation in ice covered areas does influence the character and behaviour of the ice cover 
itself. The intensity is of course depending on the number of the vessels and the frequency of 
passages. In polar regions with vast ice covered areas and only single ship movements the 
affect is negligible - as in former days in the Baltic Sea, too. However, today smaller sea areas 
as the Gulf of St. Lawrence and mainly the Baltic Sea with many thousand of port calls during 
each season, the navigation is affecting the ice cover considerably, even if the traffic is mostly 
restricted to special shipping lanes. However, these are fixed in position only in the coastal fast 
ice.