take a look
http://www.subsim.com/radioroom/showthread.php?t=164901
But if you ask me, realism is fun of Silent Hunter and of course you can't destroy whole Royal Navy in one patrol.
Don't blame yourself If you miss a shot. Even best Silent Hunter player misses at times. Fire salvo (2 or more torpedoes) for greater accuracy.
You can't get more torpedoes with renown. Finish torpedoed enemy ship with deck gun.