On 19 July "Upholder" left harbour once more, to take part in Operation Substance, which was a supply operation to deliver a much-needed convoy to the island. Wanklyn was moved to guard the south island of Marettimo. Two days later Wanklyn moved to San Vito Lo Capo, on the western tip of Sicily. There, he could cover enemy movements from Palermo. On 24 July he spotted and engaged a transport with a single destroyer escort. He dived and fired three torpedoes at 5,000 yards, one of which struck and his victim, the 5,000-ton "Dandolo" sank. The destroyer dropped 17 depth charges over two hours and attempted a hydrophone hunt, but Wanklyn dived to 150 feet and retired to the north-west. His score now stood at 67,000 tons of enemy shipping sunk.