I think it is a case of being in the right palce at the right time when it comes to making an attack on a task force. Unless you're ahead of them and almost on their track, you'll never get them. Save the gas.
When I get a radio report, I take a look at it to see if they're heading my way and not too far off. If they're more than 100km or so or not even heading my way, I save the fuel for hunting merchies.
I got lucky last patrol out of Bergen in 6/42. I ran into my first task force, which was unreported, with a Southampton cruiser escorted by 3 destroyers. I was in perfect position, right off his track. I had to do very little manuevering to get in position and got him with 3 eels.
The escorts barely had a clue, although I suffered some minor damage to my conning tower and Flak guns from a luck nearby depth charge. Other than that, I escaped after about an hour of hunting by an escort.