my own fav strategy, and it works 99% of the time is to get ahead of the convoy (if you are behind it, this is known as an end around attack) and position yourself to within 3k yards of the projected track of the convoy; go down as deep as you can, I usually go down to 300 feet, depending on sub.
Then crawl in at 1/3 until lead escort is approaching. Go silent and wait for lead escort to pass. I then come straight up to periscope depth, scope still down, up scope and then pick two targets, I usually pick one far away (obviously within range!) and fire two tubes. Then find a closer one, wait till I guess other torps are closer to their target and fire two at second target. I usually manage to get torps to hit within 5 secs of each other on the targets. As soon as second firing is under way, get back down deep and change direction towards lead escort, remove silent running when deep so you can reload tubes. Heading towards lead escort means that when he turns and runs in, you pass beneath him before he gets to where he 'thinks' you are. Always works for me. You can always go port or starboard at 90 degrees, obviously up to you. I find there is a bit of a greater chance of being detected if you run off to the side.
new depth charges mod resets depth charges to reflect reality. To kill a sub, a depth change has to be quite close to the target. A sub can be shaken by near misses but they should not be fatal. This mod means that the depth charges can go off as before but do no damage. Which means you have a better chance of evading the attack. The U534, (located on Merseyside) was sunk by a depth charge that went off directly beneath it, (see image) all the charge did was cause stress fractures and buckling in hull, the water did the rest and flooded the sub, the process was such that the entire crew managed to abandon ship before it sank. The depth of the water also has a nullifying effect on the initial explosion, so the closer to the target, the more deadly. The depth charge that hit U534 hit the deck, rolled along and then fell off, as it passed the sub it detonated only a few meter or so below
I notice that destroyers are a bit 'thick' early on but by the time I am in late 43/44 they can gt a bit 'crafty'. The Japs were not thick in reality but neither were sub drivers.
Dont play at a setting too hard, it proves nothing except that you have suicidal tendencies.
I hate lots of images of the U534 and artifacts taken from the sub. If anyone is all that interested its here:
http://www.39-45war.com/uboat534.html