Wikipedia
A Klabautermann is a water kobold (or nix) that assists sailors and fishermen on the Baltic and North Sea in their duties. It is a merry and diligent creature, with an expert understanding of most watercraft, and an unsupressable musical talent. It is believed to rescue sailors washed overboard. The name comes from the Low German verb klabastern meaning "rumble" or "make a noise". An etymology deriving the name from the verb kalfatern ("to caulk") has also been suggested.
His image is of a small sailor in yellow with a tobacco pipe and woollen sailor's cap, often wearing a caulking hammer. This likeness is carved and attached to the mast as a symbol of good luck.
Despite the positive attributes, there is one omen associated with his presence: no member of a ship blessed by his presence shall ever set eyes on him. He only ever becomes visible to the crew of a doomed ship.
More recently, the Klabautermann is sometimes described as having more sinister attributes, and blamed for things that go wrong on the ship. This incarnation of the Klabautermann is more demon- or goblin-like, prone to play pranks and, eventually, doom the ship and her crew. This deterioration of image probably stems from sailors, upon returning home, telling stories of their adventures at sea. Since life at sea can be rather dull, all creatures - real, mythical, and in between - eventually became the centre of rather ghastly stories.