Hi Tony!

Housed in a retractable dome projected beneeth the ship´s bottom,
but operative only at moderate speeds owing to water noices,
SONAR was employed in 2 ways:

Passive Sonar: Listening for the Uboat´s propeller noices.

Active Sonar: Echo-ranging with a sharp "ping" wich the Uboats´ steel hull
returned; The time giving the range;and The direction givíng the bearing.

The variation in pitch ,known as the doppler effect,told a trained operator,
weather the target was stationary or moving, and its direction and speed.

RN called this apparatus ASDIC
USN called it SONAR.

Since this search method was dependant on the fact that the target
was submerged, it mainly had mounting successes in the later part of the war.
In the first couple of war years when the Allies had no air cover in middle Atlantic,
the Uboats could stay in touch with the convoys in the daytime,
and sneak in to attack on the surface in the dark night hours.
As long as the Uboats were surfaced in the dark,
the asdic or sonar could not locate them!
With increased co-operation with air cover,and the use of surface radar,
Allied asdic/sonar-located Uboat sinkings increased drasticly!

Best regards/Anders.