Nothing at all.
Think about it. A sub is like 5 IC for 90 days. Will it do anywhere near that damage to British industry? I doubt it.
are you serious?
yes it will do more damage than the expenses if they are used correctly in hoi2, ive often had 2-3 times the ROI for my subs when they are used with air cover and in nicely sized wolfpacks.
1 submarine alone isn't going to break a convoy especially if it is escorted but a wolfpack of 12+ subs are going to wreck havoc on the average convoy
the historical ROI was at around 10 times their expenditure for germany during the war and that is just a conservative estimate, it was around 40 times for the US subs in the pacific due to the japs lack of anti submarine equipment and training.
http://www.navy.mil/navydata/cno/n87/history/wwii-campaigns.html
you might also want to read "the U-boat War" by david westwood
isbn 1-932033-43-2
it deals only the the atlantic war, for the pacific side you have to look elsewhere.
1 submarine model 3 (long range sub 38) is 3 IC for 160 days minus production gearing and modifiers (3*160=480)
10 convoy ships are 4 IC for 75 days (4*75=300)
10 escorts are 4 ic for 80 days (4*80=320)
1 model 3 DD is 3 IC for 140 days (3*140=420)
my own experience:
the british use around 1000 convoy ships to fully supply all theaters outside UK and recive all resources from their colonies, their convoy system tend to start breaking down when they reach 500 convoy ships depending on how many troops they supply around the world.
since a submarine cost around 480 ic/days they cost the same as 16 convoys ships so by the most basic calculation a submarine must sink more than 16 convoy ships right? wrong, since without escorts convoy ships are just dead meat to a sub a sustains virtually no losses and would be able to rack up more than 16 kills at leisure.
(480/300 = 1.6*10 = 16 convoy ships)
to prevent a submarine from getting some very happy days in the atlantic you need escorts to escort your convoys and their valuable cargo since they wont bring the goods to their goal if they are on the bottom of the ocean.
my experience usually says 1 escort for every 5 convoy ships and you are ensured that any attacker would be bloodied atleast but for arguments sake lets put the escort to convoy ship ratio at 1 to 10.
to make sure that the previous mentioned 16 convoys ships make it safely to port we need atleast 1.6 escorts, if we round this up to 2 escorts this will cost us an additional 64 IC/days to a combined cost of (16 convoys ships - the cost of the escorts = 13.8 convoys ships)
13.8 convoys ships might be a tad more managerble for your average submarine captain but our little calculation does not stop here for since every submarine force that encounters an escorted convoy risks damage and destruction from the escorts this is not very likely to happen with only 2 escorts...
what every submarine captain and commander fears are the destroyer hunting groups that prowled the atlantic, these are the usual bane for any small pack of subs. so in order to hunt and destroy with just a resonable degree of success there need to be several groups patrolling choke points (the greenland-Iceland-Faroes-Scarpa Flow gap and the english Channel) and the convoy sealanes.
if you build just 1 DD for this duty for every 5 enemy subs out there you will probably not win the war in the atlantic (especially against a good and dedicated opponent) but 1/5th of a DD costs 84 IC/days (420/5=84) and if we add the cost to our little convoys to subs calculation we end up with 480-64-84=332 IC/days/300*10= 11.06 Convoy ships per submarine
bear in mind that this is not an accurate or very good way to calculate the cost but it indicates to a degree the cost benefit of submarines
11 ships per submarine, is this possible to achive as a german uboot admiral?
that depends entirely on what resources the allied puts into hunting for those subs, since the atlantic is big it takes a lot of submarine groups to find the convoys but it also takes a lot of escorts and destroyers to prevent the subs from racking up their kills and ive not included air cover aka naval bombers into this calculation as both sides could and should have them.
now when i play germany in HOI 2 and decide on doing subwarfare in the atlantic against UK, the end result is usually a crippled UK by 43 since i use wolfpacks of 12-18 subs which just so happens to be able to kick the living shit out of any convoy they happen uppon and any DD hunting group they cannot defeat they have plenty of time to run away from in most cases... in order to take on a wolfpack of around 12 subs you need atleast 6 DD's (which is on the low end of the scale) the cost of 6 DD's is 420*6= 2520 IC/days which is very little compared to the 12*480=5760 IC/days for the subs... but when you start to do the rest of the math for the subs you probably end up at a really nice cost benefit for every submarine... i do atleast
