Many thanks for your explanations.
Just to make sure I have understood correctly, in general use, the ballast tanks are either full or empty. The fine regulation of depth is by adjusting the subs buoyancy, and this is done using "regulation tanks".
I have read that a sub maintains depth using two methods, buoyancy control and using the hydroplanes ( dynamic control). Perhaps we should discuss dynamic control later in another post. But I have read somewhere, that precise control using buoyancy alone is very difficult. I.e. say as commander, i wanted the U boat to be stationary at periscope depth, would this be something easily done while stationary.... or would the submarine always be in motion, in order to use the hydroplanes ?

Thanks

Pras