Just an FYI, i run a Lehman 120 diesel on a small 26 downeast. I would run at 2400 for hours. I would always check the engine before i started it again and always lost coolant. (Never overheated, never got hot)
I thought it was a head gasket. Put an overflow tank on and its been spot on ever since.
Maybe not the same issue, but the tank suggestion is a good one