To prevent a bush from being worn excessively by solving such a problem that, though a clearance between a sleeve and a bush is 0.75 mm, because exposed to sea water, grease dissolves into sea water contaminating sea, though the dissolved amount is small, due to frictional heat when the sleeve and bush slide each other, and lubricating oil between the sleeve and bush becomes zero, and a clearance between the sleeve and bush forms an air layer because the entry of sea water is sealed by an O-ring.

A specified air sump is provided in a boss 6 leading to a clearance between a sleeve 9 and a bush 7. The air in the air sump is compressed by a hydraulic pressure at a depth of water, and sea water enters the clearance between the sleeve 9 and the bush 7. Thus, by the lubrication of sea water, the amount of wear of the bush 7 can be reduced.