How were our submarines detected?

If our captains were lucky or very good, the first indication of one of our submarines near a convoy would be a torpedo exploding against the side of a Japanese ship. The steam torpedoes that we used during WW2 left a trail of bubbles and smoke that pointed back to where the submarine fired them. It wasn’t always difficult to figure out where the submarine had been.

The Japanese did have electronics such as radar and sonar. However, they did not seem to place as much importance on them as we did. Some of their ships did not have radar until late in the war, often about two years after our vessels had it. It also appears that we and the British were making far more upgrades to the designs of our equipment. Still, the Japanese electronics were good enough that they were sometimes able to detect our submarines. They could also detect our radio and radar transmissions. That was a large part of the reason why American submarines used radar sparingly and transmitted few messages.

Another good place to look for submarines was around Japanese convoys. Their aircraft would patrol the routes that their ships used most often. Sometimes they were able to catch our boats on the surface. That would still be useful information even if the enemy plane was not able to attack.