Under 46 CFR §61.20-23, a vessel has propelling machinery located aft and a tailshaft diameter of 200 mm (approximately 7.87 in). At what after stern tube bearing weardown clearance must the non-rubber water-lubricated bearing be rebushed?

Why A is correct

46 CFR §61.20-23(a)(1) sets the amidship-machinery rebushing clearance at 6.4 mm (0.25 in) for shafts 229 mm or less in diameter. Under §61.20-23(a)(2), when propelling machinery is located aft, the bearing must be rebushed when weardown is 1.6 mm (0.0625 in) less than the applicable amidship clearance, giving 6.4 mm minus 1.6 mm = 4.8 mm (0.1875 in).