Sheave is the maritime term for a pulley, much the same as galley is the maritime term for kitchen. Ships, boats, and even un-manned scows do not have pulleys on board, they have sheaves.
I have a few maritime dictionaries asquired over the years and the word pulley does not appear in any of them. Sheave is the only word used for the object. I do not purport to know the reason in the way that I know why a floor is not a deck ( a floor being something completely different than a deck on a vessel).
de Kerchove's Maritime Dictionary, which runs close to a thousand pages, describes a sheave thusly:
A grooved wheel in a block, mast, yard, and so on, over which a rope passes. The sheave, which is bushed, rotates upon the pin. Sheaves are made of wood, bronze, or galvinized cast iron or steel. for running riggin where severe or heavy intermittent strains are expected, as in the case of runners and topping lifts, brass sheaves are used.
Wooden sheaves are made of lignum vitaem wood bushed with bronze. Iron or steel sheaves are used with flexible wire rope to prevent galvanic action, which would soon damage the rope. They are of much greater diameter than those for fiber rope (15-20 times the rope's diameter). The groove should fit the rope accurately so as to support it for one third of it's circumference. in practise the diameter of the groove at the bottom should be 1/16th inch greater than the diameter of the rope, with an angle of flare from 50 to 60 degrees. end quote
Elevator pasengers may refer to pulleys, those who wish not to appear lubberly should call a sheave a sheave. Thank you for your time and attention.
...and we need to know this because........??