At 02.01 hours on 25 Aug 1942, U-438 fired its stern torpedo at a ship in convoy ONS-122 and observed a hit below the bridge, then turned around and fired one bow torpedo at another ship at 02.05 hours. It missed but a detonation was observed after about 4 minutes and Franzius thought that it had hit a ship beyond, but probably misinterpreted another explosion at that time as torpedo hit. The Trolla (Master Ole Grande) in station #71 was hit on the starboard side in the engine room by a torpedo and sank by the stern within one minute with only the bow remaining visible above the surface. The master and four crew members were lost. The survivors (nine of them injured) abandoned ship in one lifeboat and a raft and were spotted after 30 minutes by HNoMS Eglantine (K 197), but the corvette had to leave them behind when they located a U-boat by radar. Thick fog set in and the Stockport (Master Thomas Ernest Fea, OBE) was unable to find them, but by chance HNoMS Potentilla (K 214) (LtCdr C.A. Monsen, RNoN) spotted the survivors while searching for a U-boat about 05.00 hours. While the corvette stopped to pick them up, an approaching radar contact was obtained and the commander urged his men to hurry up, taking the survivors all aboard within 10 minutes and engaging the enemy at 05.21 hours.

On 26 August, a Surgeon Lieutenant was transferred from HNoMS Acanthus (K 01) to HNoMS Potentilla (K 214) to take care of four badly injured survivors from Trolla and he reported that they must be in hospital within 36 hours if they were to survive. Unfortunately the corvette had to remain with convoy until reinforcements arrived nine hours later and then immediately left for St. Johns, arriving on 27 August. The patients were taken to a hospital where the chief engineer died of wounds on 28 August.