NO, you can see your engine revolutions in 2 small dials in the Zentrale and in the Conning tower too, the connign tower are easiest to find, look for 2 identical dials close to eacht other, they're white in the VII an IX boats but red/black in the type II
don't have a screenie for you at the moment
The trick is that at less than 100rpm on the engines you are running silent (where engine noise is concerned anyway) this means setting the speed to slightly less than 2 knots in the speed selection dial, which is the alternate for the engine telegraph.