In GW the convoys are not running in a straight line, they make course changes all the time but they head in a general direction.
When planning your intercept use the speedchart to draw a distance-circle to indicate the possible area the convoy covers
in the time you need to go to the intercept point.
I make the circle about 2/3 of the distance the convoy could make in a straight line.
Once you get there go to 30m and use the sonar.
If you find nothing you have to guess a bit where the convoy could be and try to make
a new intercept.
In two of tree times, i'm now able to make an intercept.
The hunt is far more interresting this way but sometime i curse like hell when i lie
ahead of the convoy in a perfect position and the convoy changes course
As for not to find anything in your assigned grid, thats not specific to GW.
You get it in a plain installation too.
After spending the 24 hours in the patrol grid go to a better hunting ground like BF13, BF15
All in all, i find GW much better than the stock game.
Regards
Dieter