What Are a Shaft Coupling and Alignment?
The connection that joins the gearbox flange to the propeller shaft — and the adjustment of that joint down to tenths of a millimetre.
The coupling is the connection between the gearbox output flange and the propeller shaft. Alignment is the setting of the two flanges so their centre lines match; it is checked with feeler gauges, adjusted with the engine mounting nuts, and the acceptable tolerance is at the level of tenths of a millimetre.
A misaligned shaft turns power into vibration and wear: the shaft seal wears out early, bearings run hot, and gearbox seals start to leak. In complaints of 'vibration at a certain engine speed', alignment is always one of the first two suspects, along with propeller balance.
Alignment schedule
When engine mounts are changed.
When seals or bearings are replaced.
After haul-out and relaunch, if there is any doubt.
Whenever there is a clear vibration complaint.
A critical point: a 'perfect' alignment done ashore can be misleading, because the hull shape changes once the boat is back in the water. Final alignment should be carried out afloat, after the boat has settled for a few days. The measurement itself takes half an hour; if it is skipped, the cost is spread over years through seals, bearings and the gearbox.