To confirm your RPM's, free roam through your boat to the engine gauges [Shift + F2]. Drag your knot needle until it hits the desired RPM. I get very close to the gauge and have found that by dragging in tiny increments I can set it by 10's. If you are below 150 metres, 50 RPM's may not be enough to maintain depth and your boat will begin to sink, slowly at first and then accelerating. You'll have to increase your RPM's in order not to plunge to your doom. I usually set my dive depth to about 150 and only go deeper if I am forced to. At 150 you are usually below the thermal layer [modelled but not indicated, as the Germans did not have a layer gauge.]
__________________
|