Tater is right. Plus the fact that the Japanese ships are hiding just out of your sonar/radar range. The crews are sitting there laughing at you running in circles looking for them while they eat their fish and rice.
:rotfl: Stop and let the crew fish or swim for ahwile. Change your patrol plans to some place else and you you might get the surprise of you life.
Same happened in 1.4 SHIII. You could go for days out in the Atlantic and not see a single ship. Very frustrating to say the least but you had to change you way of planning your patrol.
But then again, Sh4 is not the same as SHIII in many ways.