It is actually a tripple pulley. Two wood pulleys side by side with a much smaller metal pulley bolted on at the base.
Not certain how this piece functions. I just was intrigued by the size and weight of this rustic well worn item. The item "WOOD IRON Block & Tackle VTG ANTIQUE Wheel Ship Pulley Barn Rope Nautical Huge" is in sale since Thursday, January 17, 2019. This item is in the category "Collectibles\Tools, Hardware & Locks\Tools\Pulleys, Block & Tackle".The seller is "rustiquerooster" and is located in New River, Arizona. This item can be shipped to United States.