Good info. - thanks! Do you think the flex coupling helped reduce vibration - and noise transmission? That would be my only reason for installing one. The alignment has always been good, but the old shaft - 40 plus years old - was worn down at the cutlass bearing as well as the key sheared off at the transmission end last year damaging the shaft and coupling so we are replacing it all. The original 2QM15 is a noisy and unbalanced beast anyway so a flex coupling may just be a waste of money in my case. There was a note in the Yanmar service manual that claims a flex coupling must be installed if the engine is mounted with flexible mounts - which started me thinking that our installation should have had one. It has a steady throb at cruising speed (2200 rpm) that rattles the whole boat. Prop was serviced and balanced by Martek recently.
