Dripless Shaft Seals - Basic Question

RWS

RWS

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I've been running PSS Dripless Shaft Seals since the refit with no significant problems other than a cleaning of the surfaces one time.

Two questions:

1) TWIN INBOARDS - there has been some discussion about installing a line between the raw water source of the two systems to provide sealing and lubrication in the event of running on one engine and the disabled engines propeller freewheeling. (transmission lubrication is NOT an issue with my particular model Hurth)

While this makes sense, what is the possibility of the running engine's raw water pressure creating engine damage in the stopped engine via raw water going into a stopped raw-water system? Could the water build up and then backup into the exhaust/risers/manifolds and ultimately into the engine's internals?

2) SEALING WHEN AT REST: If the raw water pressure in the bellows is maintaining the dripless seal while underway with the engines running what, other than the mild "spring" tension of the bellows against the seal is maintaining the seal when the boat is in the water with the engines NOT running?

Questions to contemplate on a cold winter Sunday.

THANKS !

RWS
 
Although I just had these installed and only put a few hours on them...

#1 - good question. I dont' think linking the two together for a freewheeling prop is necessary (but I'm not an expert). Since it's likely that your speed will be around 6kts on one engine (limping home), the freewheeling prop probably isn't spinning so fast that friction will create a great deal of heat/wear.. Note that sea water is present in the bellows via the shaft log at all times. We actually adjusted the bellows on one side while underway at 6kts to fine tune the installation. The shaft was barely spinning. We would have stopped for the adjustment, but we were in 20kt gusts and getting blown out of a narrow channel.

It would be great to hear from someone with more hours on this specific issue.

#2 - My understanding is that the mating surface is so smooth between the graphite disc and stainless disk is so precise, the tension from the bellows is more than sufficient to maintain the seal. My observation is that this near perfect mating is what caused the graphite dust to appear during the sea trial. I suppose it could be compared to a drop of water between two sheets of glass - All but impossible to pull apart, but easy to slide apart.

Anyhow, that feedback is worth what you paid for it :-)
 
The water line is there to prevent a vacuum inside the bellows at high speed. I would guess that you would be running at low speeds on one engine so the auxiliary water wouldn't be necessary. The outside water pressure and bellows compression seal the surface at rest (and at low speeds).
 
When I had mine installed I checked the PSS website and they had a max speed listed for a non water-injected installation. I don't remember the exact speed and can't find it on a quick glance at their website, but I decided it wasn't necessary for my boat to have the crossovers.
 
I thought pressurized raw water is used for lubrication, not sealing.
Enlighten me please...
 
1.) When I originally put mine in about 8 years ago, PSS noted that the water line was for lubrication. They also had a guideline for the model of unit you needed and it was based on shaft rpm, with the lower rpm version being the same as the high rpm version just without the water tube. Net of all this is that you don't need a water line for just freewheeling.

2.) The water pressure is not what is creating the tension to seal. That's just not accurate. The tension is from the bellows itself. You slide the bellows back and compress it when installing and then lock the metal ring in place to keep the tension. Thus, if the locking screws in the ring slip and you lose tension, that is a potential failure point. You will sometimes see the practice of putting a regular hose clamp on the shaft just behind the ring as a stopgap measure to keep the ring from sliding, just in case. In any case, the water pressure has little to do with any of this.
 
At the advice of my mechanic, I installed the crossover tube last fall. I can see the water in the crossover line but it is not a large volume or under great pressure. I would describe it as a strong trickle...just enough to lubricate. I seriously doubt that it could cause a problem with the non-operating side.

I like the idea of installing a hose clamp as backup measure of holding the ring in place. That failure point has been a concern of mine as well.
 
Low rpm shaft seals don't come with water line points. I wouldn't think freewheeling would be a problem.
 
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