[sublynx]

A thought or two on this:

1. Somebody once made a statistical test on what is the most efficient search pattern in SH3. If the typical movement of the enemy is east - west, west - east, the grid should be searched with in the same direction. Run from east to west, then head south or north a bit, then run back. The speed should be faster than a single merchant's typical speed. If you go 5 knots you will never catch up on any targets, if you run at 8 knots you might catch those that run at 6 or 7 knots. (However historically the U-boats usually saved fuel and typically moved very slowly in their patrol grids, AFAIK)
2. Regular hydrophone dives help, but depending on the grid there might not be much to find
3. Higher time compression might make you jump right over a contact and not get a report. I usually use 128. However if you are feeling bored you might go to 512 or 1024 - if there is ship near you, there might be a glitch in the running of time, your boat's movement on the navigation map will seem to slow down a bit, the clock seems to slow it's running for awhile. If you take time compression off immediately after a glitch and dive for a hydrophone check you might hear a contact.
4. Higher experience crewmen might spot more targets. I hope someone can confirm this

I no longer think it's a big problem if I don't get contacts. Usually that only means that I'm saving my torpedoes for the best part of SH3 - convoy battles. I usually patrol my grid with a TC of 128 and I feel that I always get a contact report if a convoy gets near me. I think that the game engine almost always gives convoys away with a nice contact report. I think I get at least 90 percent of my convoy contacts from the radio.