[Rockin Robbins]

The American subs were vastly superior to the German subs. They not only were much faster on the surface but had a little device called radar. This allowed a search radius more than twice visual range.

Now, the number of contacts you raise is directly proportional to the number of square miles of ocean surface you search in a day. Cutting a swath 20 miles wide and 216 miles long (the distance traveled in a day at 9 knots) searches 4320 square miles of ocean surface. There is no way to get more than a quarter of that by taking a dip once in awhile and listening.