You get a set amount of renown for reaching the patrol zone (200), staying there (200), and getting home (100).
The renown you get for sinking ships depends on your difficulty setting. The lower your difficulty, the less renown you get. If I've understood the sytem correctly, with 100% difficulty, you would get something like 340 for a large tanker, 60 for a tramp steamer, and 120 for a destroyer. With only 50% difficulty you'd get only 1/2 of the above numbers.
My advice is to save your torpedoes for cargo ships, and avoid shooting destroyers, except maybe in self defense.
First of all, destroyers are difficult to hit with a torpedo, unless you catch them unaware and use electric torpedoes. If they see the wake of a steam torpedo, they are fast to turn and change speed, causing you to miss. And if they know you're there, they often don't come to you in a straight path. And, most importantly, unless you hit them, they will depth charge you, and you usually will be uncomfortably close to the surface on their first pass.
Second, the same torpedo will put a lot more merchant tonnage beneath the waves.
So, usually it is best to just avoid destroyers and other escorts. Any warship larger than 10 000 tons is very much worth sinking, however.
I can think of a couple of exceptions though, where I would consider attacking escorts:
If I find a ligthly escorted (1 or 2 escorts) convoy, I might take the escorts out first, in order to freely destroy the merchants. Of course, this mostly makes sense with a type VII or IX, not a type II.
Also, if I find a single merchant escorted by a single escort in shallow water, I might destroy the escort first to ensure my own escape. More than one escort and shallow water is a big no no for me.
As for torpedo economy, many merchants will often sink with one well placed torpedo. For example, try to aim beneath the first mast, and then wait for 30 minutes. If the ship shows no clear signs of sinking, shoot another one slightly forward of midships (so that both hits are at the same end of the ship).
If the merchant is large and another shot might prove difficult due to escorts,bad weather or high target speed, I find it best to shoot two torpedoes and not risk losing the ship alltogether.
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