You have no idea how many times I've shadowed a convoy only for it to dissapear on load

. Saving such large data changes can be made easy by compacting them in layers, depending on distance from the sub and saving them at different intervals. Anything on the ship gets saved immediately, like orders or crew intentions ("I'm going to the loo"), anything in 10 miles radius gets saved every minute and so on. Since we get multi-core support changing a few flags for units can't be that system intensive.