Why would U boats even try to torpedo a Fletcher-class destroyer?

Question

Some US Navy veteran — I cannot remember the name — chided Greyhound 2020, because it's unrealistic that U boats would even attempt to torpedo a US Navy WWII destroyer.

But Retired US Navy Admiral James Foggo III did not bring up this rebuke. Foggo said WWII USN destroyers are highly manueverable and speedy. But doesn't this technical fact substantiate that veteran's criticism of unrealism?

Answer 1

Is it unrealistic? No - as @Tetsujin said, it has happened IRL during the war. If you are asking WHY they would attack destroyer, consider two main reasons:

Reducing the threat

It is not hard to notice, that Greyhound is the main threat to the Wolfpack. Sink it, and you can pick the transport ships at your leisure.

Revenge

You've just witnessed an enemy sinking one of your ships, possibly killing your friends. Soldiers are just humans, hence the U-boat captain might want to even the score by sinking the killer.

Answer 2

It may have been rare, but it happened.

Wikipedia lists 5 Destroyers torpedoed by German U-Boats. I don't see a claim of 'unrealistic' can stand up in the face of confirmed proof.

Answer 3

The primary target of the U-Boats were the cargo ships...

Germany's U-Boat fleet existed to interdict the flow of supplies from the US to the UK, and cargo ships were the primary target of U-Boats. Typically, they would conserve the very expensive torpedoes and sink cargo ships by surfacing and firing their deck guns whenever possible. They had torpedoes so that they could shoot at the cargo ships when escorts (or aircraft) were present and surfacing would be far too dangerous. Remember, a U-Boat that sank ten escorts and no cargo ships would have failed in its mission. By contrast, a U-boat that sank multiple cargo ships per patrol without ever sinking an escort would be considered wildly successful.

...but U-Boat crews were people and they valued their lives

With all of the above said, U-Boat crews wanted to get home alive and sometimes that meant using those expensive torpedoes to attack the escorts that were trying to kill them. Torpedo attacks against escorts would be rare, since the U-Boat's first and best option for survival would be to go deep and slip away; however, such attacks definitely happened. In addition, a U-Boat commander might attack an escort to clear the way to kill cargo ships. He might even attack to help a fellow U-Boat being prosecuted by the escort... or to avenge one that was just sunk by an escort. And then, of course, you have accidents: a U-Boat mistakes an escort for a cargo ship, or an escort inadvertently gets in the way of a torpedo aimed at a cargo ship.

The focus was on sinking cargo ships and evading escorts, so attacks on escorts would tend to be rare, but they did occur and it is not unrealistic.