Would anybody be able to advise me what is going on with my stern tube? From the attached picture it will be seen that some black liquid appears to have been spraying out whilst it is in operation.

Perhaps it is just a water leak with abraded black rubber in it?

My guess is a bit of debris on the carbon face of the seal is leaking a bit of seawater through. Think I have read somewhere cleaning the face up with a bit of sandpaper will sort it out.

I haven't ever done it though, so don't blame me if your boat sinks

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cammd said..
.... cleaning the face up with a bit of sandpaper will sort it out.

Thank you very much for that video and advice.

The seal in the video however looks a bit different as it has some kind of water injection hose going to it.

These seals are pretty much the only part of the boat that I have never yet pulled apart and I don't really fancy doing it whilst the boat is in the water unless I know what I'm going to find.

I presume that if I pull the bellows back I see some sort of stuffing box with an adjustment screw. I think the one in the picture may be of a dripless design and therefore may not be applicable to what I will find in mine. Am I way off base and does someone have a schematic of what I might expect mine to consist of?.

The surface needs cleaning, the one in the video as water inlet, little one do not get hot enough not need it.
The bellows provide the compression to keeps the surfaces of the seal together.

Thank you :)

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julesmoto said..

cammd said..
.... cleaning the face up with a bit of sandpaper will sort it out.

Thank you very much for that video and advice.

The seal in the video however looks a bit different as it has some kind of water injection hose going to it.

These seals are pretty much the only part of the boat that I have never yet pulled apart and I don't really fancy doing it whilst the boat is in the water unless I know what I'm going to find.

I presume that if I pull the bellows back I see some sort of stuffing box with an adjustment screw. I think the one in the picture may be of a dripless design and therefore may not be applicable to what I will find in mine. Am I way off base and does someone have a schematic of what I might expect mine to consist of?.

The PSS seal looks different to yours. The tube from the PSS seal is not a water injection tube, but an air relief tube to let bubbles out of the seal. The top of the tube is above the water line. When re-seating a PSS seal, you need to "burb" it by squeezing the bellows to get the air out. Only then will it seal properly.

Is that the gear box from a Bukh diesel? Your photos are bringing back bad memories of a Bukh diesel and a mucky engine bay in a Cavalier 28. You might be able to clean off the black by using a rag with a small amount of diesel fuel as a cleaning fluid. I know it won't solve the problem, but you might be able to start striking off possible causes and/or seeing the area a little more clearly.

I can't figure out what is going on with your seal, but it looks cactus to me. If the electrics in your boat are below knee-level in the cabin, it might be time to get a new seal. You can guess how I know that.

It looks to me like the stainless steel rotor has walked down the shaft and has excessively compressed the rubber bellows,,, this will preload the seal excessively and cause the carbon face to rapidly wear hence the black deposits.
Check the grub screws in the rotor and look for witness marks the grub screws may have left on the shaft in the rotors original position.
If the seal is left with excessive preload, the bellows will loose its elasticity and remain compressed becoming limited in the range of preload it can provide. A 1" shaft should use about 20 -15 mm of preload from the bellows. If it has been over preloaded for a period of time, you may be lucky to get 5 mm of preload from the bellows.

Just make sure you re-burp the seal every time it comes out of the water as well.
otherwise as Jolene says.

Looks to me like the motor is way out of alignment with the stern tube.
Note how the seal is skewed sideways a bit.

I should have pointed out too in my previous post that if the rotor is walking down the shaft and has excessively compressed the bellows, it can cause the seal face to leak when the propeller is driving the boat, simply because the prop may thrust the engine forward on soft mounts more than what the preload of the bellows can provide,,, in turn opening up a gap in the seal face. Note that spray pattern in the photo,,, although may not leak when sitting idle..