Abstract: A stern tube bearing for vessels is sealed off in a leakproof aft seal asbly which comprises at least one annular chamber between axially spaced sealing elements. The annular chamber is filled with a liquid for lubricating the sealing elements. The liquid is circulated in a circulation system through a separating tank. The head of the circulated liquid is lower than the head of a lubricant in the stern tube bearing which again is lower than the lowest possible head of the sea-water surrounding the aft seal assembly. Any sea-water or dirt entering into the annular chamber will be removed out of it by the circulated liquid and separated from it in the separating tank which can be provided with devices for supervising the circulation system. A forward seal assembly of the stern tube bearing may be included in the circulation system.