Shaft alignment (cold/hot)

Shaft alignment is the setting of the propulsion line so the crankshaft, intermediate shaft, and tail shaft run on a controlled curve through their bearings, with each bearing carrying its design share of load. Cold alignment is measured with the engine and hull cold, dockside, by sag-and-gap, strain-gauge, or optical and laser methods; hot alignment accounts for the rise as the engine, stern tube, and hull reach working temperature and the ship floats at draft. Modern large engines use calculated alignment because thermal growth and hull deflection move the bearing reactions enough to overload or unload a bearing. Class rules require alignment calculations and verified bearing loads on long, stiff shaftlines.