Coolant loss in Yanmar 30

[velella15]

The level of coolant in the reservoir of my Yanmar 30 diesel drops markedly during engine use. What are the possible causes - I imagine these could range from bad to disastrous! Couldn't see any red coolant under the engine. Any ideas where to start?

[Amnesia II]

Sorry to hear this. Could be going into oil, or out with raw water. Also check all rubber hoses & clamps....coolant may be leaking, pooling out of sight and drying.

[boatbod]

Most likely is a leak in the heat exchanger that allows coolant into the exhaust.

Next most likely is a bad head gasket, but you'd probably have noticed that by now... Hard starting, white smoke etc.