The steel mine laying Type UE-1 German U-boat U-77 was laid down at the Hamburg yard of A G Vulcan on 5th March 1915. She was launched on 9th January 1916. She measured 186.3′ x 19.3′ x 15.9′ and her tonnage was 755 displacemen tons (on surface), 832 displacement tons (submerged). Powered by 2 x 900 horse power diesel engines by Benz for surface power and 2 x 800 horse power electric motors for underwater power. Her armament consisted of 2 x 50 cm (1 bow, 1 stern) torpedo tubes and 1 x SK L/30 3.5 inch deck gun. Her main weapon was her 38 mines in mine laying tubes on either side of the submarine. She was commissioned on 10th March 1916 and her assigned skipper was Kapitanleutnant Erich Gunzel.
U-77’s career was to be short. Based at Keil as part of the 1st Half Flotilla she took part in her first sortie accompanied by U-76 soon after her commissioning but both U-boats returned from this first venture into the North Sea with no success. Her second and final sortie under the command of Kapitanleutnant Gunzel with a crew of thirty two men aboard began when she departed Keil on 29th June 1916. She had been ordered to lay her mines between Kinnaird Head and Knock Head – an area the Germans believed was subject to heavy sea traffic heading for the Cromarty Firth. She moved to Heligoland and then departed into the North Sea on 5th July. On 7th July a U-boat was spotted but not identified 50 miles off Kinnaird Head. This may have been the last sighting of U-77 because after this she disappeared with all hands. Her fate has remained a mystery ever since. She is known to have laid some of her mines in the target zone but then vanished. It has been speculated that she encountered a technical problem laying her mines resulting in either inundation of water or perhaps an explosion.
The location of U-77 has been the subject of much speculation over the years and at one point was suspected of being lost near St Abbs Head but has recently been located after extensive research by Kevin Heath and Buchan Divers and a final remote camera exploration of the mark at position 57° 46.283’N, 002° 14.282’W. The mark was last fully surveyed in 1986 when it was reported to be a wreck lying in 99 metres with a least clearance of 92 metres exhibited a probable funnel midships. The ‘probable funnel’ would turn out to be the submarine’s conning tower when the wreck was finally explored by a surface connected ROV camera. The wreck indeed proved to be a submarine exhibiting serious damage to one end of the vessel strongly supporting the theory that she had been lost to an explosion of her own mines during her mine laying operations.