Volvo shaft seals- air!!!!!!!

[Transcur]

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.

 

[jerryat]

Hi Transcur,

Your bio doesn't show what boat you have, but you mention 'shafts'. If you have two shafts, it could be that in tight turns or when well heeled, one shaft is lifted clear of the water long enough to allow the water that in the stern tube to escape. Obviously once back on an even keel, air could be trapped. Clearly the same would happen on the other tack. The 'long white trails' you mention could well be the props skimming across, or just below, the water.

Don't know if this is the answer, but will be interested in what the final reason is.

Cheers Jerry

 

[boatmike]

Interesting one this! Is the inboard end of the shaft tube below the waterline? I assume the white trails are when the engine is not running. My guess is its more likely turbulence, but it's only a guess. You dont say what happens if you lock the shaft by putting the engine in gear or allow the prop to rotate (assuming you have a fixed prop of course) Does it make a difference?
Sorry I don't have any answers, only more questions!

 

[boatmike]

Hadn't thought of that! Yes we need to know more info on the boat itself....

 

[Transcur]

The shafts are below the water line about 10"at the inboard end. The "trails" are definatley not the props surfacing causing turbulence. They are folding. The shaft are also stationary. When I first noticed the trails I thought it was strips of white material in the water. After a time it was evident that this was not so and the most likley thing was a release of air from the shaft tube. The tubes are 2" glass with a 30 mm shafts. The shafts protrudeabout 2" from the hull. Shafts are inclined at % degrees and parallel with the centre line.

 

[boatmike]

That's wierd...... I still think the white trails are turbulence from the props even of they are folding, but I can't explain where the air comes from. If the shaft seals were gone you would get water in the boat not air in the tube..... I give up!

 

[malcp]

I too observe air buildup behind the Volvo shaft seal on my 35' sailing sloop with folding prop. This occurs over a period of weeks. My local Volvo agent reckons this is not uncommon and says air does get drawn down between the shaft and seal. Perhaps as the shaft vibrates with the engine (assuming solid coupling) some kind of pumping effect takes place between the surface of the shaft and the seal's angled lips whereby small amounts of air get pumped in.

I found this effect makes little difference to the seal's life, especially if it has it's correct grease inserted. Mind you, I replace at 5 years or 500 hours (Volvo recommendation), and there seems plenty of life left. So, maybe just purge the air from time to time and carry on.