What is the function of stern planes in submarines?

Stern Planes on a submarine are similar to tail wings on a plane; they serve as trim control for the boat, i.e., allow for more precise depth control.

Fairwater or Bow planes are used primarily to get the boat to an ordered depth'; once there, both planesman (Helmsman and Planesman) use both sets of planes to maintain depth control.

For example, the diving planes might be used to get a boat to 150'; once there, if more precise control is requireed (say moving to 140' or 155') the stern planes would be used.

Stern planes can also be used to help a boat change depth while remaining level; the dive planes change depth, while the stern planes maintain the trim, or level, of the boat as it moves through the different depth changes.