As part of a group of six submarines selected for Mediterranean service, UB-43 was broken into railcar sized components and shipped to Pola where she was assembled and launched in early April 1916, and commissioned later in the month.

The U-boat could carry up to 27 tonnes (27 long tons) of diesel fuel, giving her a range of 6,940 nautical miles (12,850 km; 7,990 mi) at 5 knots (9.3 km/h; 5.8 mph) Her electric motors and batteries provided a range of 45 nautical miles (83 km; 52 mi) at 4 knots (7.4 km/h; 4.6 mph) while submerged.

[1] As one of six U-boats selected for service in the Mediterranean while under construction, UB-43 was broken into railcar-sized components and shipped overland to the Austro-Hungarian port of Pola.

While 112 nautical miles (207 km; 129 mi) east of Malta, the British steamer Italiana with her cargo of hay destined for Salonica was torpedoed and sunk.

[12] Three days later, and some 60 nautical miles (110 km; 69 mi) closer to Malta, von Mellenthin sank a pair of British steamers.

[16] On 10 October, the British tanker Elax, carrying fuel oil from Rangoon was sunk off Cape Matapan without casualties.

[17] Three days later, two men were killed when UB-43 torpedoed and sank their ship, the British steamer Welsh Prince, of 4,934 gross register tons (GRT).

[16] Statesman, a 6,153 GRT steamer carrying a general cargo, was first on 3 November; six crewmen were killed when the ship went down 200 nautical miles (370 km; 230 mi) east of Malta.

[22] On 6 November, UB-43 torpedoed the Peninsular and Oriental liner Arabia 112 nautical miles (207 km; 129 mi) off Cape Matapan.

[16] Bretwalda—which had escaped destruction from a mine laid by UC-5 in August 1915—and her cargo of jute were sent down 220 nautical miles (410 km; 250 mi) from Malta.

[16] Author Paul Halpern reports that the majority of the German U-boats in the Mediterranean fleet were undergoing repairs and refits at Pola and Cattaro during January.

[33] On 1 February 1917, Kaiser Wilhelm II personally approved a resumption of unrestricted submarine warfare in order to try to force the British to make peace.

[34] The new rules of engagement specified that no ship was to be left afloat,[35][Note 2] although British reports for several of UB-43's victims suggest that von Mellenthin was already operating in this manner.

[25] Under these new rules of engagement, UB-43 first sank the Greek steamer Miaoulis 130 nautical miles (240 km; 150 mi) from Benghazi on 24 February, while she was carrying cottonseed to London.

[36][37] Two days later, the turret hull steamer Clan Farquhar,[38] carrying cotton and coal for London,[37] was torpedoed and sunk.

On 26 March, the British steamer Ledbury, carrying wheat from Karachi, was sunk 90 nautical miles (170 km; 100 mi) from Benghazi.

[Note 3] On 1 May, Obermüller sank the American-owned (but British-flagged) tanker British Sun carrying a load of fuel oil.

[46] Later in the month, the Greek steamer Dorothy and her cargo of wheat from Karachi were sunk 45 nautical miles (83 km; 52 mi) from Cap D'Armi.

[47] UB-43's final attack of note was upon the protected cruiser HMS Grafton, torpedoed 150 nautical miles (280 km; 170 mi) east of Malta.

Despite the rough condition of the boat, the U-boat was commissioned into the Austro-Hungarian Navy on 30 July 1917 as SM U-43, dropping the B from her former designation.

[52] On 30 November, a leak on U-43 partially flooded the boat and caused her to sink to a depth of 100 metres (330 ft) before she was brought under control and raised to the surface.

[51] During U-43's time under repair, Schlosser was reassigned to command U-14,[53] and Linienschiffsleutnant Eugen Hornyák Edler von Horn was named to take his place aboard U-43 on 18 January 1918.

On 17 March, while returning to Cattaro from patrol, the crew of the Austro-Hungarian destroyer Dinara mistook U-43 for an enemy submarine and rammed her, damaging the diving planes.