Sinking enemy subs

[U777]

I bagged a a British S-Class submarine for a grand total of 767 tons however I never received credit for actually sinking the unit, only added tonnage to my totals. I was puttering along and found it oblivious to my position so I lobed an electric torpedo at it and it sank after awhile. Did the game designers not anticipate such a lucky hit that they never added submarines to the sinking credit database? BTW is there a way to edit tankers so they only list oil, fuels, and other liquids as their cargo?

Patrol 6
U-123, 2nd Flotilla
Left at: March 29, 1941, 17:53
From: Lorient
Mission Orders: Patrol grid AM11

Grid BF 24 Ship sunk! HMS Spearhead (S class), 767 tons
Crew: 34. Crew lost: 14

Grid BF 12 Ship sunk! SS Alwaki (Granville-type Freighter), 4708 tons
Cargo: General Cargo. Crew: 96. Crew lost: 94

Grid AM 54 Ship sunk! SS Trade Wind (Medium Cargo), 4709 tons
Cargo: Machinery. Crew: 43. Crew lost: 6

Grid AM 1 Ship sunk! HMS Nootka (Tribal class), 1850 tons
Crew: 196. Crew lost: 7

Patrol results
Crew losses: 0
Ships sunk: 4
Aircraft destroyed: 0
Patrol tonnage: 12034 tons

EDIT---I see here I was credited for 4 ships, but else where I was only credited for 3 (2 merchants and 1 warship)