AlanJ

Thank you for all the advice so far. So I was back at the boat today, this is what I found. First check I made was the water level relative to the thermostat. Removing the stat, the water was about about 2 inches below the bottom of the stat. This was lower than it had been. So, replaced the stat and housing and topped up the water level to about 1cm below the top of the header tank on the exhaust manifold (and above the level of the stat). Also checked the venting from the top of the skin tank, that was clear of air. Ran the engine for 30 mins taking regular temperatures with an infrared thermometer. I left the radiator cap off to allow for any "burps" to be seen/released - none were witnessed. Temperature got up to 50 degrees on the surface of the thermostat housing (engine was just idling) Pipes to the calorifier and skin tank both got quite warm (about 35 degrees surface temp). The return from the calorifier slowly moving up from the 18 degrees ambient temperature to about 25 degrees; the skin tank return pipe didn't budge from 18 degrees. The top surface of the skin tank got to the same temp as the feed from the engine with decreasing temp as I worked down the side of the tank. The water in the calorifier did start to warm up as well. I did notice the calorifier is above the engine block uphill slopes on both sides So, I'm confident that it isn't an air lock in the system I am confident that when it does overheat, it truly is overheating (the buzzer didn't go off today when testing but this was after adding coolant to the system) I do think there is a slight leak somewhere in the system - this is going to be the fun bit to find. I have a suspicion about the calorifier as there is residue where this lives. (it is in the engine bay on the Uxter plate?). I have tightened the jubilee clips on these pipes. Given everything is in the engine bay, hopefully it shouldn't be too hard to find. I am concerned that the calorifier could hold an air lock in the circuit but the temperature readings did not suggest this was the case. I think I will look at adding a vent tap to the top feed pipe. So, I think short term it is just a case of keeping topping up the header tank to make sure we don't get so low on coolant again. Obviously, too low is not as low as I thought/expected. Next is to locate the slight leak and remedy. Given my suspicion on the calorifier and it being above the engine, adding a vent here will result in connections being taken off/redone and may well fix the problem

with regards the header tank - it is just the top of the engine with a radiator cap. Apologies if I am using incorrect terminology here - just let me know what I should be calling it.

I am confident it is an overheat but the comment from Tracey rings true also. when I changed the thermostat, taking the housing off, there was no coolant around the stat at that point. I changed the stat and didn’t think too much about it at the time. as it overheated again, I then flushed the system and re-ran the engine having filled the header tank. I wonder if there is a small leak that is causing the level to drop slowly (we ran for about 45 hours before it started playing up again) from memory, the coolant level is about an inch or so below the top of the header.

Interesting idea. Never though of these in this context. Have similar on a couple of radiators at home that catch the air in the system. Given the size of the cap on my skin tank, I don’t know if they will do one that is big enough but makes you think of alternative solutions.

It does slope up to the engine and is pretty much as high as you could go on the side of the skin tank. so air should traverse back to the engine block

I have a big cap on the top of the skin tank and will try to loosen this. the feed pipe at the top of the tank has a tap in it (suggesting the top cap may be hard to remove). I will use these to bleed the skin tank.

Thanks for all the suggestions I am away from the boat now but will be back at the weekend. Will check again for air locks and check with an ir thermometer to see what is happening.

Morning Thanks for the additional comments. I did replace the thermostat after the "steam" incident. I tested the new stat before putting in to ensure it worked correctly I also tested the old one that I took out and it worked correctly, it was not "sticky" I haven't had steam since - more because we stopped the engine before getting to that point. I am happy to add a gauge as Higgs says, a buzzer tells me when it is going wrong, a gauge will allow seeing the problem develop. When the buzzer when, the pipes to the calorifier and skin tanks were relatively cold (could easily be held) On restarting the engine and when the buzzer did not sound, these pipes quickly got too hot to hold.

Sorry, missed this when viewing on my phone Boat is about 30 years old. Engine is a Beta 38 - not sure of age skin tank which normally does get warm. I thought all air was out of the system

I would need to add the wire through to the gauge

I don’t have a temp gauge, just a buzzer/light. the sender on the thermostat housing has two spades on the back of it. One is connected to the control panel, the other is not connected, so I could buy and attach a gauge to it (have been thinking about this if anyone has a recommendation for a garage for a beta 38)

It happened again this morning. Cleared itself before we could clear the lock and get to the bank. Engine had been running about 10 mins. We continued for another 5 hours without the alarm sounding. fan belts all seem fine. will try running without the cap on the header tank and see what happens. Obviously not cruising when I do it!

Fair question. before I replaced the thermostat, the time it happened, we had steam coming from the engine. another symptom was that the pipes to the calorifier and skin tank were not hot

Evening all after some thoughts of where to head with a problem we are having. this morning, we pulled away from our overnight mooring at tick over past moored boats. After 5 minutes, the engine overheating light/buzzer come on. we paused to sort out drinking water and toilet and turned off the engine. after 10 mins, we decided to try the engine again to pull away from the service point. The buzzer didn’t restart, we left it running for 15 mins without issue, so carried on our way. stopping for lunch and some shopping, when restarting, the same symptoms, warning buzzer after a few minutes, turn off the engine and wait 10 mins then it ran fine. we’ve been out for a week working round the four counties ring without any other issue. engine water level was fine on the daily check. There is water in the bilge, but it looks canal water dripping through the stern tube. We were having this issue before, I changed the thermostat and flushed the engine block through with fresh water. The problem went away. Any suggestions of what might be the next item for testing?

I would suggest making a "temporary" mooring pin as others have suggested. A few good solid branches a couple of feel long and a penknife should see the job. Make a point on one end of each branch and a notch near the other to hold the rope. Get them as far down as you can and I'd say you'll probably need at least 2 to be sure of holding it. Edit to add: If you have a searcher magnet (or similar) go and have a dip, you might find some pins!