Among the pioneering users of true diesel–electric transmission was the Swedish Navy with its first submarine, HMS "Hajen" (later renamed "Ub no 1"), launched in 1904 and originally equipped with a semi-diesel engine (a hot-bulb engine primarily meant to be fueled by kerosene), later replaced by a true diesel. From 1909 to 1916, the Swedish Navy launched another seven submarines in three different classes ("2nd" class, "Laxen" class, and "Braxen" class), all using diesel–electric transmission. While Sweden temporarily abandoned diesel–electric transmission as it started to buy submarine designs from abroad in the mid 1910s, the technology was immediately reintroduced when Sweden began to design its own submarines again in the mid 1930s. From that point onwards, diesel–electric transmission has been consistently used for all new classes of Swedish submarines, albeit supplemented by air-independent propulsion (AIP) as provided by Stirling engines beginning with HMS "Näcken" in 1988.