Variable-depth sonar reaches beneath masking thermal layers

Why did the Royal Navy develop a large variable-depth sonar?

To search beneath thermal layers where submarines could evade or confuse sonar fixed near the surface.

A variable-depth sonar can be lowered to the part of the water column most useful for the search. That lets the sensor look beneath thermal layers that bend sound and help submarines hide from hull-mounted sets. The concept answered a real acoustic problem rather than merely producing an eccentric oversized system. Its value comes from changing sensor depth as the local ocean conditions change.