Klas Horn was originally the Kazarski-class torpedo gunboat Posadnik of the Imperial Russian Navy.

One of three built for the Russian Baltic Fleet, Posednik was used as a despatch vessel during World War I.

During the Russian Revolution, Finland declared independence from the Russian Empire and the ship, which lay abandoned in Finnish waters, was taken over by the newly formed Finnish Navy and renamed Klas Horn, after a 16th-century admiral.

[1] They were propelled by one shaft powered by a vertical triple expansion engine fed steam by two Ioco boilers rated at 3,500 indicated horsepower (2,600 kW).

The basis for the new nation's navy were a series of former Russian vessels abandoned during the revolution in Finnish waters.