How were ship kills accounted in WWII?

Question

In the movie Greyhound, at the end, the fleet requests an oral report of the incidents in which the captain says "My escort group sank three others...".
But in the movie it is seen that Greyhound sank at least 2 subs and helped in the sinking of another two (marked one with fire for a plane to drop bombs and finishing another on the surface that was previously forced to emerge due to a depth charge from another cruiser).
Is this an error on the script? How were complete and partial kills accounted in WWII?

R Leonard · Answer

As far as sinking submarines wwas concerned, from the USN perspective may I suggest a look at https://www.ibiblio.org/hyperwar/USN/rep/ASW-51/ASW-PART-II.html which discusses antisubmarine measures and effectiveness.  For ascribing some sort of credit, you will find that reports of attacks on submarines by surface craft were evaluated as:
A. Known sunk;
B. Probably sunk;
C. Probably damaged, possibly sunk;
D. Probably damaged;
E. Probably slightly damaged;
F. Insufficient evidence of damage;
G. No damage;
H. Presence of submarine uncertain;
I. Target attacked not a submarine; and
J. Insufficient evidence to assess.
In practice, this list was truncated into groups thusly:
Sunk A, B;
Damaged C, D, E;
Undamaged F, G; and
Non-submarine H, I, J
Based upon after action reports, every attack was evaluated and rated as one of the above.
Interestingly, as this document points out, in its appendix one can find that the A-B class evaluations correlated well with actual U-Boat losses, and, even, in fact, in the half of the war tended to be a little conservative in estimates of actual German losses compared to the actual losses.
Of course, one should not mistakes movies, made for entertainment and profit purposes, with history.

How were ship kills accounted in WWII?

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