Originally Posted by Nos4r2
The collar into the deck looks a little like part of one too.
Might sound like a daft question but is there anything on the underside of the hull that corresponds with it? My yacht had a similar looking tube under the front berths. It actually housed the log, but the top of the tube was allegedly above the waterline so you could clear a foul whilst afloat. I never had the balls to test that particular theory.