My five cents about Type XVIII U-boat endurance: I never tested it in the game but I have recently checked her real data on the Internet. Here you are:
- surface range on diesels: 5200Mm@12kts
- underwater range using batteries: 45nm@4kts
- underwater range using Walter turbine: 202nm@24kts
Also for comparison you have the following data:
Type XXI:
- surface range on diesels: 11150nm@12kts
- underwater range using batteries: 340nm@5kts
Type IXC:
- surface range on diesels: 11000nm@12kts
- underwater range using batteries: 63nm@4kts
Type VIIC:
- surface range on diesels: 6500nm@12kts
- underwater range using batteries: 80nm@4kts
As you can see it is obvious that Type XVIII U-boat was not designed as ocean-going submarine! It was simply direct Type VII boat successor and thus developed solely with ATO in mind. This sub should have shattered Allied convoys around Britain with impunity thanks to her amazing submerged speed.
So I think your blaming on Type XVIII U-boat is unfounded! This sub was good in concept (aside of unreliable propulsion but I assume this was successfully solved in our fictional game reality) and would be very deadly. One thing UBT developers missed is wrong TMO they assigned this U-boat. In short, huge Pacific distances exclude medium range subs as effective tool of warfare. Note US fleet subs were very large boats as for Atlantic standards. They were in 1500t/2500t displacement class, not even produced by European countries during war. That is why you still meet all these troubles with limited range navigating Type XVIII boat...but this is not Type's XVIII fault per se!
Simply Type XVIII should belong to SH3 and Type XXI sub should go into UBT...
