Before the warDue do the long finnish coastline, the navy was unproportionally strong when compared to the army and air force. The backbone of the navy was formed by small patrol boats with great firepower, the subs Vetehinen, Vesihiisi, Iku-Turso and Saukko (smaller and designed to operate in lake Ladoga) and the destroyers Väinämöinen and Ilmarinen, both built in the 30’s. The destroyers were rather expensive - the money spent at building them would have bought new guns for the artillery and enough ammo for two years of warfare.
For strategical reasons, the Finnish subs were designed to be compatible with the Estonian subs Kalevi and Lembi. In the 30’s, Finnish defence strategies relied heavily upon cooperation with Estonia. The coastal artillery, which at that time still was part of the Navy forces, could in cooperation with the Estonian batteries at Naissaari and Aegna close the Gulf of Finland and isolate the Soviet navy base at Kronstadt.
Winter warDuring the Winter war, the Navy saw little action due to the fact that the sea was covered with ice quite early that winter. However, the navy did its part by keeping the routes to Finland open and by protecting unarmed vessels. The subs mainly parformed patrol missions is the Baltic sea, and the armoured ship Väinämöinen was anchored in the middle of Aura river to form part of the city of Turku’s anti-aircraft defence. When the ice also in the Baltic sea finally was too much to overcome, the navy vessels stayed in port and the seamen formed battalion Aaltonen.
The coastal artillery played a relatively important part in the war. In the beginning of the war, the Russarö fortress outside Hanko was engaged by the Soviet destroyer Oktjabjarskaja Revolutsja. The destroyer was badly damaged by finnish fire.
Towards the end of the war, the coastal batteries of Mantsi, Järisevä and Koivisto supported the field artillery.
Continuation warDuring the continuation war, the importance of the naval forces grew. Patrol boats performed shorter missions while the subs patrolled bigger areas during longer missions. Finnish mine layers in cooperation with German vessels built minefields in the Gulf of Finland. When the Army reached as far as Lake Onega, a new navy unit was formed; Naval group Ääninen (Onega) which consisted of a gun boat and several smaller vessels. Escort missions were also performed, where naval ships followed and protected unarmed freight- and passenger ships in convoys.