Under 46 CFR §61.20-23, a vessel has propelling machinery located aft and a water-lubricated (non-rubber) stern tube bearing on a shaft of 200 mm (approximately 7.9 in) diameter. At what clearance must the after stern tube bearing be rebushed?

Why C is correct

For machinery located amidship, a shaft of 229 mm or less requires rebushing at 6.4 mm clearance per 46 CFR §61.20-23(a)(1). When propelling machinery is located aft, §61.20-23(a)(2) reduces that limit by 1.6 mm (0.0625 in), yielding 6.4 mm minus 1.6 mm = 4.8 mm (0.1875 in). The 200 mm shaft falls in the 229 mm-or-less category.