In World War II movies when a sub is getting depth charged they always seem to change depth or maneuver to get away from being hit. Why didn't they just match the speed of the destroyer and stay under it?
Answered by: Toma Vargazon,Practicing atheist
Yeah, stupid U-Boot captains. If only they thought of that, eh?
Type VII submarine
A type-VII submarine has a top underwater speed of about 8 knots. The destroyer has an economical cruising speed of 19 knots and can make 30 or more in a dash, such as when trying to get over you.
So, yeah. Good luck staying underneath that.
Add to the fact you don’t actually know the exact position or speed of the destroyer. You can make an educated guess based on the hydrophones and you’ll get it somewhat right, then you need to plot that on paper and make adjustments to your course and speed.
By adjustments I mean you need to tell these guys exactly where to turn:
You need to give the exact settings to the two youths in front. No, you don’t get to say “match the destroyer’s speed and course”, they don’t know either of those. You need to tell them the exact settings of the rudder and engines.
There is a maneuver that was highly effective early on in the war, it’s even shown in the Das Boot movie (or miniseries) that provided the command room image above. Wait until the destroyer is within 300 meters of you, execute a 90° turn and pedal to the metal. The destroyer can’t track you that close and there is enough time to get away from the path of depth charges. If the destroyer anticipates the maneuver and it does turn it has 50% chance of turning in the wrong direction anyway. It’s not ideal but it was effective enough to minimize the submarine losses for a while.
Then better SOSUS (Sonar) was developed, alongside with the Hedgehog mortar that allowed the destroyer to engage the submarine while it was still a few hundred meters in front and the expected lifespan of submarines and their crews dropped like a stone.
It would take the destroyers exactly one engagement to deal with the submarine matching their speed and heading, even if it worked perfectly. Just drop the depth charges, they’ll detonate closer to the submarine than the destroyer. You might seriously damage and even sink your ship but a destroyer is cheaper to replace than a submarine and you’ll most likely rescue the crew.