On 25 November 1941 "Upholder" left for her eighteenth patrol. Wanklyn positioned the submarine south east of the Messina straits. At 07:30 on the 27 November they sighted a large tanker with destroyers for escort. Wanklyn fired a full salvo at 2,800 yards but underestimated the enemy's speed. This convoy was later intercepted, and the tanker sunk, by Force K. On 29 November "Upholder" sailed into the midst of an Italian naval squadron on a night exercise. The enemy fleet consisted of three cruisers and five destroyers. With the closest vessel only 3,000 yards away Wanklyn continued to calculate the bearing and speed of the enemy, until eventually he ordered the boat to dive. He could not see anything through the periscope in the winter night. By using dead-reckoning, he fired off a salvo at the last ship using his asdic. The target disappeared on a reciprocal bearing and then reappeared right over the top of "Upholder". Wanklyn docked at Malta on the morning of 21 December 1941.