Questions for getting back to SH5

[Itkovian]

Some more questions.

I finally started my campaign and made my way to the east coast of england for the "coastal waters" mission. After some prowling around and evading a few destroyers, I approached the Firth of Forth and just now made contact with a convoy headed eastwards.

I am in perfect position to move in and lie silently for them to pass at close range, and so I am using the occasion as something of a training ground .

Now I already made one attempt (albeit a not very scientific one), and fully 3 of my 5 torpedoes detonated prematurely (1 hit, the other missed).

Therefore, my questions are:

1- How do I prevent premature detonation? Is that a problem with the magnetic pistol detonators? Should I rely on explosion on contact instead?

2- What method of attack do you prefer to use when faced with a convoy in perfect formation?

What I mean is: it is simple to get a decent solution on a single convoy member (my first torp hit, after all), but how then do you proceed to fire torpedoes at other members of the convoy? Can you obtain several solutions at once? Or do you simply "eyeball it" using the first solution as reference, knowing the rest of the convoy is on the same heading and speed (so you simply need to adjust the range)?

Of course, once I'm done I'll have to evade the destroyers, and my batteries relatively low after diving away from enemy aircraft, so it should be interesting as well. But I simply can't accept not sinking two of those transports, it is such a perfect opportunity.

Thank you.

Itkovian