First submarine was a stone Boat. LOL !
Actually, as others have said, it's a sled used for hauling stones out of a field or anywhere that you wanted to move them.
I'm a city kid, and I'm not a farmer now, but I certainly know about a Stone Boat. Used to help my Dad move rocks with one as a youngster.
Dad had an old Packard Touring Car & he'd hitch an old surplus military trailer to it and we'd drive to the L.A. river-bed. Dad would block the wheels & jack up the back end of the car. Then he removed a rear wheel & replaced it with two rims welded together that had cables wrapped around them. Then he got a 25 pound Danforth boat anchor that had a snatch-block on it, and the Stone Boat out of the trailer. He'd haul the Stone Boat, anchor, & cable out into the river-bed; set the anchor, put the cable thru the snatch-block and hook it to one end of the Stone Boat. The cable from the other rim went straight to the front of the Stone Boat. Dad had said this type of set-up was called a "Drag Line". We'd load large flat stones onto the Stone Boat & Dad would start the car, put it in gear and reel in the loaded Stone Boat, and we'd put the stones in the trailer. Then Dad would put the car in reverse & run the Stone Boat back out again.
Slickest set-up I've ever seen, never forgotten it.
We used the stones to build stone walls and a large stone patio on the back of the house.