Moshchny (Russian: Мощный; Finnish: Lavansaari; Swedish: Lövskär, Estonian: Lavassaar) is an island in the Gulf of Finland of the Baltic Sea, located some 120 km west of Saint Petersburg.

During World War II, when Leningrad was almost entirely surrounded and many other islands had been conquered by the Germans and the Finns, Admiral Tributs decided to keep islands of Seiskari and Lavansaari which proved to be very important bases as the war progressed.

On November 18, 1942 three Finnish motor torpedo boats Syöksy, Vinha and Vihuri, as well as a minelaying KM-boat made an assault on the harbour of Lavansaari.

In June the supplementary mine-laying consisted of 244 magnetic seamines, about 3000 anchored depth mines and about 1,630 minesweeping obstacles.

[2] On 23 June 1943 German bombers sank the Soviet motor torpedo boat, MO-171 off the waters of Lavansaari.