The Yanmar YSM8 is the little old single cylinder diesel dating back to circa 1978. This is the horizontal cylinder engine.
I vaguely remember people criticizing the horizontal version. I think Volvo also had something similar.

Anybody with experience of the YSM8 engine willing to comment?

I used to own a Compass 28 with a YSE 8 that had the single horizontal cylinder. The yacht was very under powered and the motor had a lot of vibration. That being said it was reliable over the 3 year ownership period.

Parts still available and quite a bulletproof motor. I ran mine dry by not checking the lube oil, and seized it twice in one night. Filled with oil in the morning it was none the worse.

Simple motor, but a little noisy and vibration because of the single cylinder but otherwise tough motor. Cheap to maintain, and injectors are only about $280 to re-do.

Anybody with experience of the YSM8 engine willing to comment?

That's MEEEEEE!
Well actually I have a YSE 12. But almost the same.

The Yanmar YSM8 is the little old single cylinder diesel dating back to circa 1978. This is the horizontal cylinder engine.

That is pretty well correct. But the YS 8 is only 8hp. Which is among the smallest diesel engine available (Volvo, Bukh and a few other were on the market). So If you want a really small diesel engine it is great. Other wise it is gutless.

The YS Yanmars were built as multi-purpose industrial engines. They were (and are) used on, pumps, whacker (road ballast compactors), welders. generators. air compressors, refrigeration condensing units, various agricultural equipment and marine engines.
The horizontal single cylinder Yanmars are still made and used in all applications (with modernized design) except as marine engines.

The YS models were designed to be cheap, reliable, fuel efficient and easily repairable. For all those reasons they are a great engine.
They are not designed to be smooth running or quiet, so they aren't.
They knock, rattle and roll.

To your question, "Are they any good?" I would say absolutely. But I am biased ( by experience).

My engine is 40 years old. Before I owned her, when Jono was refloated after the tsunami that hit Japan, Mori san took pity on her and fixed her up.
When he opened the engine is was full of sand he reconditioned her and she now runs like a dream. What I am saying is that YS engines are infinitely repairable and can be brought back to as new condition.
I can do any repair with no special tools.
I can (and do) crank start it).
I have been offered a free GM 10 Yanmar but I wouldn't swap the YS for anything.

Now to a comparison to the alternatives.
Volvo and Bukh parts are unbelievable expensive and many are simply not available,
Both are prone to corrosion in the heads and barrels. In the case of barrel / sleeve corrosion the engines are a write off.
They also noisy and vibrate just like the Yanmars. .
So all in all I think the small old Yanmars are the best of the lot.

Sorry for the long rave.

PS if anyone needs Yanmar parts from Japan and are prepared to pay the freight I will source them

gary

Thanks Gary. Much appreciated. Might actually buy the boat.

May I ask how long/ heavy she is?

gary

Its a Marieholm 26, the version of the Folkboat with raised topsides and a bigger cockpit. 26ft, but narrow beam.

I second Gary, I too have a YSE12 in my 1973 Compass 28. The thing is bullet proof, easy to service and reliable as the sun coming up. Noisy and full of vibration but they don't stop.

I've got a YSM8 in my 42-year old Yamaha 25 (second of these yachts I've owned). When I got it 7 years ago the Yanmar was low on compression, so my brother-in-law and I stripped it down in the garage over the winter, Got parts from Minards Diesel in Newcastle (they have nearly everything), and now it runs like new, i.e., noisy, rattily, and only 8 HP. That said, it starts everytime, even hand crank, uses a tank of fuel a season, and as long as you're not trying to cross an ocean, will push the Yamaha at about 5 knots.