Shaft alignment optical

Optical shaft alignment uses a telescope or laser line of sight bored through the stern tube and shaft bearings to establish and check the straightness of the propulsion shafting before and during installation. The optical reference defines the design bore line; bearings, the stern tube, and the main engine are set to it within tight offset and slope tolerances so the shaft runs without overloading any bearing. Modern yards verify the result with strain-gauge bearing-reaction measurement and jack-up tests, but the optical (or laser) sighting establishes the geometric line first. Misalignment outside tolerance overloads an aft bearing, runs it hot, and can wipe the white metal in service.