Hm – much food for thought. 15cwt is about 3/4 of a ton, taking a safety factor of at least 10 and the lesser typer of steel in Machinery would indicate 1/2″ wire rope (the groove in the pulley would take a larger rope, but the fixing loop on the drum suggests smaller.
A calculation for pressure on drums or sheaves in Machinery is
P=2*Tension/(sheave dia x rope dia)
With a 12″ sheave and a 1/2″ rope and 15cwt tension I make the pressure 550lbs. The safe limits given for cast iron with different types of wire rope vary from 300 to 800lbs.
With the winding drum itself being 8″ diameter, this suggests a wire rope at least 3/4″ diameter for safe running, and there’s no way you could get away with a small roller at the top if it was cast iron or ordinary steel.
For manilla rope you would need 1″ dia for a 3/4 ton SWL, perhaps that’s the answer, but a tight fit in the loop on the winding drum.
I’m leaning towards adding a second 12″ pulley on some sort of added frame.
Neil