Engine vibrations at low rpm

[Philip_Oceanis 361]

I get some engine vibrations at low rpm, from idle to around 1400rpm. Above that everything is quiet. The engine is a 3 cylinder Volvo Penta MD2030D, engine mounts are relatively new and engine-shaft alignment is fine, though these vibrations have nothing to do with the alignment because are there no matter if you engage or not the gear (it’s the engine that vibrates).
Any idea what to check and how to get rid of them?

 

[RichH]

Not much you can do as its a dynamic/harmonic imbalance that occurs below a 'critical' rpm of the engine. 3 cylinder and other 'un-naturally' balanced engines commonly do this, some more so than other designs.
Once the engine rotates faster than this 'critical' rpm, the vibrations will mathematically cancel out one another. Your crit. rpm is apparently near 1400 rpm.
Its a 'vector mathematics' phenomenon.