While Opal and another M-class destroyer escorted a convoy, U-67 fired from near maximum range to keep clear of the escorts. A torpedo struck the Swedish merchant vessel Viking. Opal followed the torpedo track and dropped depth charges, but U-67 escaped unharmed. The submarine could change course after firing, so the wake did not reliably reveal its later position. Before sonar, an escort often had to attack on that limited information.
Notable Point · 30:04-31:01
Following a torpedo track before sonar
Opal's unsuccessful attack on U-67 shows the limits of using a torpedo's wake to locate a submarine.