The UB I boats of the Flanders Flotilla were initially limited to patrols in the Hoofden, the southern portion of the North Sea between the United Kingdom and the Netherlands. "UB-4" made the first sortie of the flotilla on 9 April, and "UB-5" departed on her first patrol soon after. On 15 April, from the North Hinder lightship, "UB-5" scored her first success when she torpedoed and sank the British steamer "Ptarmigan". The 784-ton steamer was carrying a general cargo from Rotterdam to London when she went down with the loss of eight crewmen.