Fuel Efficiency / Magnetic Exploders

[Pnzrldr]

Two quick ones. Still in a Type VIIA and would like to know the most fuel efficient surface speed for the U-Boat. Does it vary with weather, crew morale, engineer experience, etc... or is it fixed?

Second - is there a distinct point in the campaign (I'm doing current version TWoS) where the magnetic exploders become substantially more reliable? Or is it simply advisable to max out your torpedo man's experience and use contact heads for everything? Optimal tactics for mag heads? Depth settings? Better at an oblique or perpendicular angle? Would seem oblique would give the mag a longer pass under the metal so greater chance of function, but no idea if something like that is modeled in sim.

Thanks!