A Klabautermann is a water sprite (or nix) who assists sailors and fishermen on the Baltic Sea in their duties. He is a merry and diligent creature, with an expert understanding of most watercraft, and an unsupressable musical talent. He also rescues sailors washed overboard.
His image is of a small sailor in yellow with a tobacco pipe and woolen sailor's cap. This likeness is carved and attached to the mast as a symbol of good luck.
The Klabautermann nowadays 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 center of rather ghastly stories.
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.