The most efficient underwater speed?
 

Whats the most efficient speed while using electric engines underwater?

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

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.