At 00.28 hours on 13 May 1942, U-128 fired a salvo of four torpedoes at the convoy SL-109 about 300 miles northwest of the Cape Verde Islands and observed one detonation after 3 minutes 25 seconds accompanied by a pillar of smoke and then another explosion. Heyse thought that he had sunk one ship and damaged another. In fact, only the Denpark (Master John McCreadie) was hit and sunk. The master, 15 crew members and five gunners were lost. 22 crew members and three gunners were picked up by the Danish merchant Nordlys and British merchant City of Windsor and landed at Clyde.