This is a 4" stainless steel sheave for a Magnum boat lift. The dimensions for the sheave are:
Diameter: - 4"
Hole Diameter - 1/2"
Thickness - 3/4"
See the Specifications Tab for Boatlift models.
When replacing sheaves, it is a good idea to inspect the bushings as well to determine whether or not they need to be replaced. Your goal is to keep the pulleys turning with as little friction as possible.