I have the same model engine in a 30 footer.
You have a Top Hat 25 don't you? It is certainly pushing her along allright.
General rule is :-
Black smoke = over fuelling or overloading.
Blue smoke = oil bypass.
White smoke = water in the combustion ie head gasket or cracked head.

With these engines, because they have wet cylinder liners, removing the cylinder head just about dictates a full rebuild. Not cheap for Yanmar parts either.

They are about 30 years old but when they are right they will pretty much go forever.

A possibility is that you may have water in your fuel. If that is the case it is an easy fix but one that needs to be done urgently before it buggers your injection pump and injectors.

If there is water in your primary filter bowl you will need to sort that by draining your fuel tank changing filters and bleeding the system. It would also be worthwhile pulling your injectors and taking them to a marine injection shop not an automotive injection shop.

More info about your engine set up would help.

Yes I would check the prop first. When was it last cleaned/prop speeded?

It is in a Compass 29 I was doing a pre-purchase survey on. At half throttle she was doing 5 knots in calm water with little wind.

Inspected the prop from the surface with a inspection camera. Not easy to see a small screen in bright sunlight, but the prop was reasonably clean with just a few small barnacles. Boat was anti-fouled about 10 months ago. Has been used as a live-aboard, and the engine was run occasionally to charge the batteries. Not loaded up for some time, but regularly serviced by the marina.

There is a plastic tube sight gauge for the fuel, and no water seen. Did not check the engine oil after the run. It was a hot day, and the octogenarian owner and I were struggling with dehydration and fatigue!

Owner will ask the marina to have a look today.

Yara said..
It is in a Compass 29 I was doing a pre-purchase survey on.

Is that Orion ?

Looks like smoke to me, not steam, I would do as others have said, check water separator and fuel tank, check prop then oil level and viscosity and if all good then perhaps an Italian tune up. (good hard run to free up any sticky rings and burn off any residual oil in the manifolds) Need to make sure it is smoke and not steam first.

Agree with all the above. If it appears to be caused by gunky stuff on rings/valves etc try FTC in the fuel. Really good stuff.

Lazzz said..

Datawiz said..
I'm certainly no expert, but from the video I don't think you've enough cooling water flow, and the 'smoke' could be steam.
I had a similar symptom a while ago and it turned out to be the primary raw water filter (inlet strainer) extensively blocked with sea grass.
Here's hoping....
regards,
allan

Yeah, doesn't seem to be enough water flowing though - intake filter blocked or the engine needs an acid flush to clean the water passages. might even be the exhaust elbow/mixer blocked.

+1

you did not buy the boat yet right? If so, pay for an engine pre-inspection check.
If you already bought the boat (congrats) my bet is possibly just water in fuel, drain (from filter housing too) and replace fuel.
Run it again, and see how it goes. No need to cry wolf just yet.
But, you should definitely have a pre-filter for water intake and I would recommend replacing the original filter housing with a new water separating filter.