Strängnäs Express was built by the Lindholm Shipyard in Gothenburg and was delivered to Strengnäs Nya Rederi AB in Strängnäs on July 10, 1908. As delivered she had a Lindholm triple expansion steam engine of 660 horsepower (490 kW) and, during trials, attained a speed of 14.5 knots (26.9 km/h; 16.7 mph). She was introduced in 1908 on the route from Strängnäs to Stockholm via Stallarholmen. The ship was sold in 1918 to Ångfartygs AB Drottningholm-Fittja, which in 1925 changed its name to Trafikaktiebolaget Mälaren-Hjelmaren. King Gustav V travelled on the ship from Stallarholmen to Strängnäs in 1923 in connection with the celebration of the 400th anniversary of Gustav Vasa's election as King of Sweden.
After another ownership change, Strängnäs Express was laid up in Stockholm in April 1939. In December 1939 she was purchased by Waxholms Nya Ångfartygs AB, better known as Waxholmsbolaget, who gave her her current name of Storskär in May 1940. The vessel was put into service on routes to the northern archipelago. Among other things, she traveled between Norrtälje and Arholma via
Furusund . A new oil-fired boiler was installed in 1999 at Oskarshamns Varv.