At 09.20 hours on 24 Oct 1939 the unescorted Tafna (Master Royal Cecil Newlands) received a distress signal from Ledbury that she was gunned and spotted her stationary funnel and masts in a distance of about 10 miles, so Tafna headed five miles to the southeast before she continued on the previous westerly course. Doing so the ship directly crossed the path of U-37 which had headed southwest on the surface after the attack on Ledbury until they had to dive for aircraft arriving in the area. Hartmann decided to attack the ship without a warning after seeing two guns (the ship was armed with one 4in and one 12pdr guns), but it was not possible to get closer to the ship due to an aircraft circling over it and no spread could be fired as only one of the bow torpedo tubes was loaded, so a carefully aimed torpedo was fired from an estimated distance of 2500 meters at 11.19 hours. The commander informed his crew that they had missed when there was no detonation after the calculated running time of 3 minutes, but to the surprise of all the torpedo then struck the ship amidships after covering a distance of 3500 meters. The Tafna was struck on the starboard side just below the funnel while steaming at 9 knots about 100 miles west-southwest of Gibraltar. The explosion must have blown the bottom of the ship out as the bow and stern raised and the middle sagged until the masts came together. Two firemen on watch below were killed. The master and 31 crew members had difficulties to abandon ship as the starboard lifeboat had been damaged and the port boat swamped when it hit the water while the ship still had headway, so they had to leave in an overcrowded little dinghy about 10 minutes after the hit. The ship sank 18 minutes after being torpedoed.

U-37 did not wait for her to sink because they suddenly heard engine noises similar to motor boats from the direction of the torpedoed ship and Hartmann ordered the U-boat to dive to 80 meters and to run silently when he wrongly assumed that Tafna was a decoy ship, which seemed to cooperate closely with motor torpedo boats and aircraft. Unknown to him, the noises they heard were from a London flying boat (202 Sqn RAF) that landed close to the ship to ask if any of the survivors needed medical treatment. The torpedo explosion was heard by the nearby American motor merchant Crown City, which hurriedly picked up the survivors of Menin Ridge and Ledbury and left the empty lifeboats adrift in order to get to the ship in distress as soon as possible, but then just proceeded to Gibraltar to land the survivors after destroyers were observed to arrive. HMS Douglas (D 90) (Cdr R.F.B. Swinley, RN), HMS Keppel (D 84) (Capt F.S.W. de Winton, RN), HMS Vidette (D 48) (Cdr D.R. Brocklebank, RN) and HMS Watchman (D 26) (Cdr V. Hammersley-Heenan, RN) were sent to the area from Gibraltar after the distress signal of Ledbury had been received. At about 13.30 hours, HMS Keppel picked up the survivors of Tafna while HMS Douglas took the London flying boat in tow for Gibraltar as it was unable to take off again. Three of the destroyers then carried out an anti-submarine sweep of the area, attacking U-37 twice with depth charges during the afternoon without damaging her and then returned to Gibraltar the next day.