Modern ball bearing pulleys offer a much lower coefficient of friction than those without balls and are recommended for maneuvers that require constant adjustment (mainsail sheet, downhaul, cunningham, spinnaker essentially).

For the mainsheet which is not always fully tightened, or even never on some boats which require constant adjustment (29er, 49er, Class A,...) as for the spinnaker sheet, favor automatic or non-automatic winch pulleys, which considerably save muscles and gloves thanks to a retention that can go up to 70% of the workload.

Choose the diameter of the pulley according to the diameter of the rope it will guide.

For example, depending on the support and workload, a 40 or 57 sheave is very suitable for a 6 to 10mm mainsail sheet.

For Cunningham or downhaul type maneuvers, 16 or 22 sheaves are suitable.

Stainless steel, carbon, composite, it's essentially a matter of weight, with an impact on the price: it all depends on the importance you attach to it.

If the pulley is to guide a cable, use aluminum sheaves which will be much more resistant than composite materials.