Found on an older sailboat ceiling as you enter the cabin.

My nephew bought this boat and we’ve been puzzling over some of its parts. As you on the ceiling as you climb down the cabin is this device which clearly has an intended purpose. The maximum diameter is about 3 inches and it’s firmly screwed into the underside of the deck / cabin top ceiling. You can unscrew the nut with your fingers and retighten. There’s a conical washer on top of the nut.

Any ideas?

BTW there’s only one of these on the boat.