The A class was the Royal Navy's first class of British-designed submarines. Thirteen were built by Vickers at Barrow-in-Furness between 1902 and 1905 as an improvement on the US Plunger class. While there was considerable variation amongst the boats of the class, they were around 100 ft (30 m) long and displaced around 200 tons when submerged. All were propelled underwater by battery-powered electric motors and on the surface by shaft-drive Wolseley petrol engines of 400 bhp (A1), 450 bhp (A2-4) or 600 bhp (A5-12). A13 had an experimental Vickers diesel engine, which proved to be unreliable.