Save game solutions, best to worst:
1.
Ticking read-only attributes off, turning Windows indexing off, and using the 4GB patch might solve all your saving problems:
http://www.subsim.com/radioroom/show...=166283&page=3
2.
If not, then no saving near other ships or submerged, or near coast, as StarLion45 said.
3.
If that doesn't help, no saving while at sea. Leave the computer running when you leave the game, or if you have a laptop, use hibernation.