Procedure for lining up an engine bedplate, main bearings, gearbox, thrust block, propeller shafting and tail-end shaft on a new ship:

1. Preparation: The ship is in the building dock/berth with the hull settled. The engine room floor/seatings are prepared, the foundation is checked for level and cleanliness, and the datum lines (centreline of the shafting and the engine) are established from the ship's baseline and the propeller shaft centreline. The shafting centreline is set from the stern tube to the engine.
2. Lining up the propeller shafting and tail-end shaft: The tail-end shaft (propeller shaft) is fitted through the stern tube and the propeller is mounted. The intermediate shafting is then aligned to the tail-end shaft using the "sag and gap" method or by optical/laser alignment: the shafts are coupled and the alignment is checked by measuring the sag (deflection) and the gap (angular misalignment) at each coupling flange, and by the use of a dial gauge/straightedge. The intermediate shaft bearings are adjusted (by chocks/shims) so that the shafting is in a straight line (or with the designed sag) from the stern tube to the engine.
3. Lining up the engine bedplate: The engine bedplate is lowered onto its seatings and aligned to the shafting centreline. The bedplate is levelled (fore-and-aft and athwartships) using precision levels, and its height is set so that the crankshaft centreline coincides with the shafting centreline. The bedplate is then chocked (with steel/epoxy chocks) and the holding-down bolts are tightened.
4. Main bearings: The main bearings are fitted in the bedplate and the crankshaft is installed. The crankshaft is checked for alignment by measuring the crank web deflections (the change in the distance between the crank webs at different crank angles) - the deflections must be within the maker's limits, indicating the main bearings are correctly aligned. The main bearing clearances are checked and adjusted.
5. Gearbox (if fitted): The reduction gearbox is aligned to the engine output and the propeller shafting. The gearbox is levelled and aligned so that the input shaft aligns with the engine crankshaft (or via a flexible coupling) and the output shaft aligns with the propeller shafting. The gearbox is chocked and bolted.
6. Thrust block: The thrust block (thrust bearing) is aligned so that the thrust collar is correctly positioned relative to the shafting and the engine; it is levelled and chocked so the thrust is transmitted correctly to the ship's structure.
7. Final alignment and coupling: The shafts are coupled (engine to gearbox to thrust block to intermediate shaft to tail-end shaft) and the final alignment is checked by measuring the sag/gap at each coupling and the crank web deflections. The alignment is adjusted by the chocks/shims until all are within tolerance.
8. Final checks: The holding-down bolts are torqued, the chocks are set, the alignment is re-verified after tightening, and the system is tested (turning gear, then running). The alignment is recorded for future reference.