Coolant loss

[chal]

I know there are some other threads about this and have checked them, but apologies for any duplication.

Last time I used the boat, the cooling water intake got blocked. I only noticed this when I saw smoke (or maybe just steam, it seemed more like the latter as it disappeared fairly quickly) coming out of the exhaust. No alarm went off. We hadn't been out for long so I assumed the engine hadn't overheated and, as we were pretty close to home, we just headed back. That was a few weeks ago and since then I've been away and haven't had a chance to look at anything. Just done an engine check now and the coolant level in the heat exchanger is pretty low. I don't have any anti-freeze with me so I don't want to fill it to the top but I put maybe half a litre of water in and it didn't fill it enough for the level to be visible. Is it reasonable to assume it has largely boiled out or should I be looking for leaks? The engine is a Volvo MD2020. It's about 20 years old I think but seems in good condition and has been little used. This is my first season of owning the boat and I had the engine professionally serviced in April. No problems reported and it's run without issue since, with the coolant never needing any topping up at all. If there is likely to be a leak, is there any obvious place I should look for it? Many thanks!

Edit: I have checked the bilge and the water in it had a distinctly green tinge so it looks like, one way or another, that's where the coolant ended up.

 

[black mercury]

I would be checking the flexible exhaust if it has run without water. Looking for delamination of the rubber exhaust and signs of melting of any plastic muffler or waterlock if fitted. Also check the condition of the water pump impeller.
Regarding the coolant loss you will have to top up the heat exchanger to just below the top. You can use tap water temporarily for now. Then, if it's holding the level you can start the engine and see what happens, checking all around the engine for leaks. It has gone somewhere, possibly being blown out the expansion csp. Check all around the bilge for signs of coolant. Running that bit hotter could also have revealed a weakness in the heat exchanger. You will have to top up the heat exchanger and try the engine.

 

[chal]

Thanks indeed for the help. I have checked the bilge and the water in it had a distinctly green tinge so it looks like, one way or another, that's where the coolant ended up. Maybe it did get blown out of the expansion cap. There is also a big crack in the rubber exhaust hose so I will have to replace that. Fortunately the muffler is metal and seems undamaged.

 

[Daydream believer]

My vp2020 had a separate expansion tank above the engine level.this has to be filled to half height. An airlock does happen after a change & this actually causes water to be thrown out on old occasions. There are 2 overflow points on mine. One at the separate expansion tank & one on the main header tank. You could put a small container under each pipe to see if any pressure is blowing water back out. This being from the cylinder head leak etc.it could be just the pressure cap knackered after 20 years & the rubber has split if you, or the previous owner has removed it often.
next job is to check hoses to hot water heater, to other parts of the engine- most of these are raw water so not applicable