It's made of steel, a little over a foot diameter, weighs at least 20 lbs. It was in my dad's shed, in an area with lots of farming and also on the Chesapeake Bay, so I'm thinking maybe farm or nautical related. But, he also liked to hoard all sorts of random "cool-looking old stuff", so really it could be anything.
Image search gives lots of things that are similar but a completely different size, and thus wouldn''t have the same function as whatever this is.