The clearance is measured at the stern tube bearing (white-metal lined or the oil/water lubricated stern bearing). The method: the propeller shaft is dropped to its lowest position in the bearing (i.e. resting on the bearing), usually done by wedging/raising or by hydraulic jacks at a defined temperature or after the shaft has been centred; the vertical clearance is measured by feeler gauges inserted between the shaft and the bottom of the bearing housing, or by a dial gauge with the shaft lifted. The clearance is the difference between the bearing bore and the shaft diameter at that position. On withdrawal, the shaft diameter and the bearing bore are measured directly with a micrometer/an inside micrometer at a number of points, and the clearance is the difference of the actual measurements. The bearing wear-down is also established by referencing the shaft centre relative to a datum on the stern tube (e.g. using a string/plumb or the shaft sag), and the clearances are checked along the length.

1. Shaft diameter and bearing length - large shafts need larger clearances to allow the oil film and to accommodate thermal expansion.
2. Operating oil film thickness - the minimum clearance must keep the shaft and bearing surfaces separated by the hydrodynamic film (depends on load, rpm, viscosity).
3. Misalignment/alignment limits - clearance allowed to accommodate hull deflection and shaft sag.
4. Allowance for temperature: the bearing expands more than the shaft in service, so thermal expansion is allowed.
5. Tolerance for the type of propulsion (fixed/variable pitch), propeller thrust, and the vibrations (shaft whirling) of the stern bearing.

Modern practice gives clearances of about 0.001 to 0.002 times the shaft diameter (e.g. for a 500 mm shaft roughly 1-2 mm wearing allowance) gauged both from maker's recommendation and classification society limits.

1. The tail-end shaft/stern tube bearing lined parts, the shaft at the bearing journals - inspect for cracks, wear, corrosion, and fretting.
2. The flange coupling bores and bolt holes, and the coupling faces - for frettage, cracks and fatigue.
3. The keyway (if any) and the propeller boss/cone area - for cracks, stress concentration and fretting between shaft and cone.
4. The shaft liner and any renewable sleeve - for wear, cracks and corrosion, particularly where it passes the stuffing box/gland.
5. The fillets at shoulders, the change of diameter, and the area near the coupling, where fatigue cracks initiate - closely follow with MPI (magnetic particle inspection)/ultrasonic testing.

Shafts with a smaller boat-deflection factor, lower stress (shafts that are less highly loaded), those equipped with a reliable water/oil-tight stern sealing arrangement, shafts of high-grade materials that resist corrosion/fatigue, and shafts in boats with a protected (enclosed) propulsion line where the tail shaft is not exposed to sea water and given a continuous oil seal, require less frequent withdrawal and survey. In contrast, open (unprotected) shafts exposed to sea water, highly loaded shafts, and shafts with known material issues require more frequent inspection. The interval depends on the survey requirements, condition, material, protective arrangements and the duty.