Silent running, Stealth vs Detection

[Shadowblade]

Hi,

I am on harbour raid mission, target location: Southend, Oct 28, 1939. When I am approaching to Southend, my VIIB is always detected by patrolling ASW trawler and bombed with its ash cans. VIIB is submerged and speed is decreased to slow before entering range of enemy's hydrophones. VIIB was detected when moving submerged with speed of 2 knots (default slow speed), course: NW, so I've decreased speed to 1 knot, course: W (greater distance from patrolling ship) and made new test. No change - I was detected again. ASW trawler always changed its course from its patrol trace on south-west, turned to my position on north-east and started its bombing run, so I was forced to start evasive maneuvres.

Is silent running too loud here? What is causing my detection ?

Secondary question: when is speed 2 knots safe and when is needed to cut speed to 1 knot ?

My SH3 is GWX 2.1, during approaching I was also using time compression (speed 1-2 knots takes pretty much time)