 Complete the following phrase:  Both U-1-class submarines were launched in 1909. An experimental design, the submarines included unique features such as a diving chamber and wheels for traveling along the seabed. Extensive sea trials were conducted in 1909 and 1910 to test these features as well as other components of the boats, including the diving tanks and engines for each boat. Safety and efficiency problems related to the gasoline engines of both submarines led to the Navy to purchase new propulsion systems prior to World War I. The design of the U-1 class has been described by naval historians as a failure, being rendered obsolete by the time both submarines were commissioned into the Austro-Hungarian Navy in 1911. Despite this, tests of their design provided information that the Navy used to construct subsequent submarines. Both submarines of the U-1 class served as training boats through 1914, though they were mobilized briefly during the 