I have noticed that air tend to always be present in the prop shaft tubes whenever I check them. After drying out , scrubbing off etc, I pinch the seals and the air is heard to escape, when water flows I stop. If I then check them may be a few days or a week later there is air there. I believe water is supposed to be round the seal at all times to ensure cooling and lubrication. I did notice last summer when sailing in clear water at reasonable speed 6 knotts, that when looking overboard I could see long white trails appearing every 10/ 20 seconds. After a lot of thought I believe it to be a trail of air being sucked out of the stern tubes, possibly venturi effect. Has any body else had this problem?? How did you cure it? Did it reduce the seal life? I guess I could fit a permanent vent pipe to the stern tube but this is rather messy.