Duration of asdic searches?

[gurudennis]

Originally Posted by Fidd

Interesting, I'd have expected a movement upwards resultant from dc's exploding above, as the aerated water above would be significantly less dense that than the column of water existing previous to detonation: thus causing an uncontrolled rise until the U-boat has exited the area of disturbed water.

I'm no physicist, but my understanding is that while you are correct about the boat diving through the less dense aerated water after an explosion below it, the effects of an explosion above the boat have less to do with aeration and more with displacement.

Let's consider what happens when a depth charge explodes above a U-boat, in phases:

1. The explosion causes a rapid expansion of air away from the epicenter. Water is an incompressible fluid, meaning that the water displaced by the explosion will initially push outward, including downward against the U-boat, causing it to descend.

2. The air bubble collapses, causing an inrush of water into it. At any significant depth, gravity will ensure that most of the water displacing the air will come from above, creating a weaker secondary shockwave as it plunges towards the epicenter of the explosion and then below it under its own inertia, colliding with a weaker upward current from the water below doing the same thing. The U-boat below is likely experiencing it as a further downward push (and a good shake) a second or so after the initial one.

3. If the U-boat is running at speed (not inconceivable under the circumstances) and the explosion was forward of center-mass, the bow is now angled further down than it likely already was.

All these factors put together - I think - may cause an uncontrollable dive that is difficult to arrest because the dive planes lose effectiveness at high speed and downward angle much for the same reason that it is difficult for a plane to pull out of a nosedive.