As for AIRGUARD 3AS, the oil in the oil chamber between #2 and #3 seal rings flows through under #3 seal ring. Then the oil pressure in the oil chamber is constantly higher than the pressure in the stern tube by 15 kPa in like manner of the aftermost seal ring which blows the air through under the lip.
Furthermore, a suitable air pressure, which is constantly higher than the air pressure in the air chamber, is added to the oil in L.O. tank with the pressure bias (signal) output of the air relay to control the oil pressure in response to the change of draft. By this method, the air in the air chamber can not enter into the oil chamber through #2 seal ring in ordinary case, and #2 seal ring is always maintained under the stable condition. Though the differential pressure on #2 seal ring is set up 40 kPa in case of Fig.6, if necessary, it can be easily
