@Schulz
The Atlantic should be crossed in 2 rounds, the U-boat need a chance at attacking mid crossing
3 or 4 or 6 turns per year but not 2. I would go with 4 turns/seasons per year.
1941 GCD could have been 6 turns/year, but as we got better, settled for 4 turns/year.
GCD usually lasts 18-24ish turns, but it is for solo play.