Thank you for such positive comments especially about the hours not being very high.
I have had the boat for about 5 years and the engine has always been very good -starting on the button even in winter when very cold.
The engine was like that when i took the boat out of the water
Just before it was lifted back into the water
I started the engine briefly. It started as usual with no problem but I did not run it for long because there was no water cooling
When it was back in the water I tried to start the engine again about 1 and half hours after starting it on land. The engine coughed and spluttered and was reluctant to start. It did eventually fire but on one cylinder only. After perhaps 2 minutes the second cylinder cut in and then after a further 2 minutes the third cylinder cut in. It ran 'rough' for several minutes then decided to work properly and revved up to 2500 RPM
The engine produced black smoke for a bit but this expected as there was plenty of unburnt fuel
in the system.
Then it started to produce clouds of blue smoke when revved over 1700 RPM.
It looked to me as oil engine oil smoke ( petrol car engine experience).
I moored the boat and had to leave it for a few days.
Today I went to the boat and started the engine. I am pleased to say it seemed to run perfectly OK and just as it has been doing for the last 5 to 10 years.
Someone suggested that the injectors must have been a bit ' cruddy' and have now cleared themselves.
I do not know anything else.
I have had comments from club members mostly along the lines that on the road I would have been arrested for pollution!
I have not had a marine/diesel mechanic take a look and at present it would be wasted because the engine seems to be fine.
But it is a mystery and I would like to know what the problem was or is. Although the boat is a catamaran
it only has one engine and I would like to be confident that it is reliable.
Once again many thanks for your comments and your interest. If you have further suggestions please let me know.
What I was and still am thinking is that if I got a second engine I could strip it down and thoroughly check it etc. Then if I needed I could put it in my boat. I could then sort out my old engine and either keep it or sell it on.