What I mean by eyeballing it is I get very close, look at the angle off the bow, and make a rough guess at the target's speed. I then use the torpedo spread angle to lead the target ship, and fire several torpedos at high speed with very slightly varying spread angles to increase the odds of hitting it. With a big enough ship, or if it's a convoy, I fire all my forward tubes.
Trouble with this approach is that accuracy is hit and miss. Sometimes I guess right and nail it, sometimes I guess wrong. Further, if I can't line my sub up so I'm at least roughly perpendicular, the margin for error isn't large enough for me to hit the target even semi reliably with my rough guess.
One trick I try to use when attacking a convoy to increase my odds of getting a hit is to angle myself so when I fire at the leading ships closest to me, any torpedos that miss the target I'm aiming at will be going in the direction of the trailing ships behind it.
Lone merchant ships I usually try to sink with the Deck gun.
Again, while this method works moderately well, it's sloppy and not very effective in the long run, and I'm trying to improve. The thread ya linked may help; but I will need to look through it.
*EDIT* Oh yah, thanks for the help. I apologize. Bit tired and forgot my manners.
As for the latter bit. I'm trying to play on full realism with stock + U-boat missions pack. I thought manual aiming would be the worst thing to get used to, but damn I miss map contact updates. A lot harder to find stuff and predict where it's heading.
Sounds like some of the mods would be hella fun to play and solve some AI problems I've noticed, especially with regard to how the ships behave in harbors (ie, stupidly, and not moving for anything once docked).
Alas, I run the steam version and there seems to be a problem with the u-boat expansion and the stock version corrupting each other's files, requiring you to constantly reverify the game cache, so I'm not sure it'd be a good idea for me to install any modes, considering I already am having problems on that end.
Any case, I was just asking what was normal for patrols for you guys, so I could get a rough estimate. Because, tbh, even with full realism, it seems far easier to sink ships and evade the escorts then it really should be. May be the AI on stock being poor in that regard, but I really have to screw the pooch to get sunk. Most of the times I've been sunk have actually been from planes zooming in before TC drops and buggering my ship with bombs faster than I can say "damn it".
*EDIT 2* God damn, I must be tired. I just read my own OP, and holy crap I am making a mess of punctuation. Especially with the poor comma. I think I'm gonna head to bed and come back to this tommorow. It is 4 in the morning, after all.