[siber]

Personally, I'll shadow a convoy until I know its (approximate) course and speed. Then, I try to run fast ahead of a convoy to a point well in front of it. Here, I'll submerge and align at an angle of 90 degrees to the assumed course. Here, I wait at 20m with engines stopped and crew on silent running... waiting.

Unless I've been very unlucky, the front escort will pass ahead or behind me. If it comes directly overhead, he'll detect me and I'll have to get as deep as possible and evade - give up on trying to attack the merchants, you need to escape and then start again as the convoy will likely change course while you're being attacked.

Once the escort passes ahead or behind, moving no more than 2 kts, come to periscope depth. Stop engines again. You should see the freighters coming closer, and the front escort weaving away from you. Hold your nerve, as the escort's weave can be confused for them coming to attack. Normally I get the hydrophone operator to follow the nearby escort moving away while watching the merchants.

Move forward slowly, until you can see a line of freighters heading straight for you in a row. This means you're directly ahead of their path of travel. When you see this, check the angle on your scope. If it's 90 deg, you're aligned perfectly. If it's less than 90, order a course change to port the number of degrees they're off 90 deg (eg. if they appear one behind the other at 77 deg, you're 13 degrees out and need to turn port 13 degrees.) If they appear at more than 90 deg, turn starboard the number of degrees they're off 90 deg. This will give you a precise angle to use for calculating AOB.

Now, unless you move forwards (or back) the oncoming freighters will ram and sink you without even planning it. Move forward slowly a few hundred metres (about 5 min at 2kts is good) but remain on silent running at slow speed or that front escort might hear you!

Now, AOB can be calculated very easily: AOB = 90 - angle at which ship appears.
(eg, ship appears at 72 deg, AOB = 18 deg assuming you've aligned to the convoy course as described above.) After about 45 deg AOB, you'll get good range and speed observations. Simply wait until the torpedo computer clicks down to 0 and then fire. Don't forget to open the torpedo doors first!

I've been successful with this method in all but the very worst weather. Note that the minute you are detected, the convoy will zigzag and the AOB method will no longer be accurate. However, if you've a large, close target it'll often give good solutions since you're in the middle of the convoy and the range is low. Also, being in the middle of all those ships, I sometimes get lucky with a torpedo that missed its intended target but hits another further out.

Generally I fire two salvos of 2 fish from the front, and one out the back each attack. However, if you're new at this method, use 3 or even 4 fish in a salvo to ensure hits. I understand this was standard practice in real life.

I can vouch for this method up until about 1942. Due to restricted gaming time and releases of updates/corrupt saves etc, I keep having to restart. I haven't yet been sunk though, but I suppose I haven't made it past 'Happy times' yet...

As you're submerged the whole time, there's no problem with visual detection.