Magnetic Torpedoes

[Slateford-5]

I'm doing OK with the impact torpedoes, 3 patrols and 130,000 tonnes.

So I decide, for a change, to have a go with the magnetic gizmos: no luck. I think I am correct in thinking the idea is the same as for impacts i.e. ideally 90 deg AoB, 1,000 m-ish, hold the boat steady etc. but set the depth for around 1m deeper than the target's keel: everytime I try this, the eel sails happy directly under the target and off into the blue yonder.

So I am wondering what I am doing wrong? When setting the depth, should I factor in how deep my boat already is e.g. if I am at 12m and the target keel is 12m, should I set depth for 13 or 25? Or something else altogether?

Any thoughts