The most efficient underwater speed?

[Niume]

Whats the most efficient speed while using electric engines underwater?

[ivanov.ruslan]

In my opinion, one can not say exactly which speed is most effective,
it depends on the transition to be done underwater

I usually use ahead one third
For example type IXc, underwater range is 75 nmi with speed 4 knots,as we know
Type VII 80 nmi with the same speed

To intercept a target or avoid a destroyer attack is another

[bstanko6]

The h.sie patch makes 1/3 speed your best bet. You can’t use the periscope over this speed due to vibrations.

[Rhodes]

The h.sie patch makes 1/3 speed your best bet. You can’t use the periscope over this speed due to vibrations.

1/3 speed, with h.sie patch is 3/4 knots and one can use the periscope. It is also the best speed/range ratio (speed) one can have in the game. Ahead standard is over that - 6 kn, and so, the periscope vibrates.
If the question is about maximum range underwater, ahead slow is the one.