
From the beginning it was clear that submarine flotilla was needed as the offensive part of a modern navy. Finns knew well that the best submarine designers were in Germany.

On the 30 November 1939 the Soviet Union attacked Finland. Vesikko was in service for one month, patrolling the Gulf of Finland. On 29 December Vesikko was docked for the winter.

The armistice between Finland and the Soviet Union got into effect on 4 September 1944. Finland declared war against Germany but submarines were never used against the new enemy. The boats were ordered back to their naval bases.