How do you guys attack a convoy?

[Rhodes]

It depends! Depends of N factors. The game, gwx or stock, year of patrol, luck, mods that are fully manual or not, surface night attack/ day attack, pitch black night or moon light night!

Mainly now, with the gwx 3.0, until 42/43, night attacks are the best, if I manage to intercept the convoy, a 45º angle aproach or get ahead of the convoy and run a paralel course to it, heading in it direction. The thing is, if the convoy only have a lead escort or plus that, 2 "front side" escorts.
Also, deck wash and pick the most juice targets (tankers, that normaly are in the midlle of the convoy), and try to get a good impact angle (60º-120º; previous middle or late 41).

As many people already said, firing the toperdos at some distance, improves the chances of a surface gateaway! With the type II torpedo it's a lot harder since the 3km range (erroneous I think).

submerge attacks, what you said, is a good way to attack, or try to head to the convoy but near one of it sides and them turn to have a near 90º solution!