I rotate the same two saves at sea, overwriting each time. So I'll save a game called TDK1 and then about five minutes later save TDK2. Five minutes later TDK1 again and so on. I rotate and save frequently. I have never had a corrupt save in either SH3 or SH4 doing it this way. The one time I had a power outage, I was able to reload my last save and I only lost a few minutes of game time. I also save in the office prior to a mission.