As well as the various classes of airship that are carrying out patrols for u-boats, 13 April saw the beginning of H12 flying boats based at various stations are using the spider web patrol routes to try and detect u-boats.
Today an SOS call was received from an Italian ship saying she was under attack from a U-boat south of Portland Bill. Surface craft and seaplanes were immediately sent out, and an H12 flying-boat, which had landed at Portland the previous day with minor engine trouble, also went away to search the area. In the pilot’s seat was Lieutenant Commander Waugh, with observer 2nd Class Air Mechanic Charles Stanley Laycock and two other crew members.
The flying-boat’s crew found the U-boat on the surface and, probably because the morning was misty, approached within a quarter of a mile before the submarine commander became aware of the danger which threatened him.
He then prepared to dive, but while his conning tower was still awash, the flying-boat dropped two 100lb bombs which exploded above the submarine. Oil and bubbles came to the surface and, after a further uneventful patrol in the vicinity, the flying-boat’s crew returned to their base reasonably confident that they had disabled or destroyed the U-boat.
About an hour later, however, a destroyer saw the submarine breaking surface again near where she had last been seen and attacked. The destroyer dashed towards her, but she had submerged before the British vessel reached her. Judging the position of their target from her wash, the crew of the destroyer dropped two depth-charges and, following the explosions, oil gushed to the surface and spread over a large area. Patrols were maintained throughout the day and a hydrophone watch kept during the night. Late in the evening, a motor noise heard in the hydrophone of one of the destroyers seemed to imply that the U-boat had got under way again, but the Admiralty, weighing all the evidence, decided that the U-boat had been destroyed and awarded medals to the crew.
The Official history suggests that this was UB39 which sailed on 23 April, but never returned. However German records suggest that ship was lost on 7 May 1917. The wreck of UB39 was also discovered in 2007 east of the Sandettie Bank at position 51.20N, 02.09E – not far from the Belgian Coast and a good distance from Portland Bill and appeared to have been mined.
In fact German records do not indicate any U-boats lost around this time so the ship was either damaged and escaped or released oil to trick the attackers and then escaped.