8,273 Oceanic Steam Navigation Co., Ltd. (Ismay, Imrie & Co.), Liverpool 475.9 x 55.2 P/C/R
Harland & Wolff, Ltd., Belfast Yard No. 309 Off No 106581
Torp. and sunk by UC 72, 16 Aug 1917, 135 miles SW ¾ W of Bishop Rock, voy. Cardiff - Montevideo, coal
Source - Starke Register 1897 (World Ship Society)
UC-72 left Zeebrugge on 12 August 1917 for a minelaying operation.
20/08/1917 UC-72 encountered the decoy ship Acton in the Bay of Biscay 46.08N-??.48W.
During the U-boat capain's periscope reconnaisance run in, he actually collided with the Acton. He was evidently satisfied so gave the order to surface. The U-boat was overwhelmed by the decoy's gunfire and sank quickly with no survivors of crew of 31.
UC-72 was the last U-boat to be sunk by a decoy vessel.
Source - U-Boats Destroyed (Paul Kemp ISBN 1-85409-515-3)
Acton (Q.34), credited by German sources with sinking coastal submarine "UC.72" on 20th August 1917 in Bay of Biscay. Some sources still show her loss due to aircraft attack on the 22nd August 1917. Assuming "Acton" did sink "UC.72" this would have been the last confirmed Q-ship sinking.
Just to put a small spanner in the works. If you look at this page on u-boat.net it suggests that UC72 survived the attack by Acton. So it could well be that she was eventually sunk by aircraft on 22 Aug 1917.