here is a mod which gives you the ability to get the type XVIII in the beginning of the campaign.
http://www.subsim.com/radioroom/showthread.php?t=132224 I have tried the XVIII and it is a lot of fun and you can outrun and outmaneuver the destroyers plus you get 6 fore tubes and a greater diving depth but you don't get a deck gun. Also there is a snorkel mod for the type IX and XVIII.
http://www.subsim.com/radioroom/show...hlight=snorkel