Ships are harder to find when you use realism mods. Still, I can usually bag 3-5 singletons in a patrol. Generally, I stay in waters between major ports, or in waters about 30km outside of major ports (less than 20km away will interfere with ships spawning from that port I understand). I conserve fuel to boost my at sea time.
Also, read the ULTRA messages. Even if you are not near the locations they identify, plot them on your map. Eventually, you will see a pattern to where ships are being seen (Formosa, Luzon, Celebes, and the Peliu to Rabaul run for starters).
Also, remember that shipping patterns shange throughout the war.
In the end, remember that there were virtually no 80,000 ton patrols. Coming back with 15,000 - 20,000 tons was considered an excellent patrol, and would net you major decorations. Many boats came back empty handed for lack of sightings. Thus, I usually keep a good submarine book by the computer for reading in my state room until I'm called to the conn for a fresh radar contact.
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