How would a compromised bellows sink the boat?

[kd4pbs]

Could some of you smart people help me out here?
Something dawned on me while reassembling this DP-SM I've been working on all summer in my spare time... Of course, it's not so much particular to VP, but I guess could apply to Mercruiser's and Yamaha's I/Os as well. I've done the same thing on 3 other Mercruiser drives before, but never really thought about this...
The transom shield has the cylinder / sleeve cast into it which extends through the inner plate into the inside of the vessel. The bellows is of course attached to the rear (outer) lip of this sleeve, and is sealed against the input to the drive's upper unit. This of course creates a (usually) waterproof seal between the input to the drive and the transom shield.
Inside of this sleeve lives our gimbal bearing, further forward, on the inboard side. In front of this, sealed against the sleeve lives a seal (part # 21) which seals the space between the sleeve and driveshaft. As long as the seal is working, there is no way for any liquid to pass from behind it to in front of it. I'm pretty sure I remember the two Alpha's and the one Bravo I have totally R&Rd had this seal as well, but I might be mistaken.
All of us who have an I/O have been told that a cracked bellows will sink our boat. I don't understand how it would, unless the seal which seals the driveshaft to the sleeve fails. Yes, the bearing would get saturated, as well as the nose of the drive's input shaft/yoke, and the u-joints... everything aft of this shaft seal, but I don't see where water could get into the boat with a good seal. Perhaps older drives didn't have this seal and the fact that they could sink with a compromised bellows just got carried on to the drives with driveshaft seals without considering that important change?
I'm sure my thick skull is missing something here, so what is it?

 

[Scott Danforth]

the exhaust bellows will not sink your boat, however the u-joint bellows can given time, or a bad y-pipe to transom shield seal.

water will make it past the shaft to bearing surface, and end up in the bilge. add any corrosion or poor machining and it flows that much faster. Volvo is better at sealing than Mercruiser, however there are still leak paths.

add a bit of moisture to the housing/gimbal area and you get galvanic corrosion happening, which then corrodes its own leak paths

the leak can be slow, as in a little trickle of about a gallon per hour up to a gusher at 1 gallon per minute. now leave the boat unintended for a few days and you can see where a boat would sink.

 

[kd4pbs]

Thanks, Scott.
I'm not referring to exhaust bellows and exhaust pipe seals here, nor corrosion related issues. Those are easy for even me to understand how they could sink a vessel.
I'm strictly limiting my attempt to understand how water which gets into a compromised u-joint / driveshaft bellows would get past the seal and the driveshaft which would effectively "plug" the sleeve, eliminating water from being able to enter the boat. Indeed, what water gets past the bearing then has to make it past this seal and the driveshaft. If both are not compromised, I can't understand how it would. If one doesn't realize the seal is bad, I do of course see that happening, in which case I'd blame the seal just as much as the bellows.
My old long gone boat with the OMC 800 stringer seems to beat modern I/Os for don't-sink-the-boat-ability I guess. Seems that pretty much the only thing that they could suffer from would be a cracked transom seal or blown water pickup hose - both of which I'd assert are much easier to see and notice than bellows inspections.

 

[Bondo]

Inside of this sleeve lives our gimbal bearing, further forward, on the inboard side. In front of this, sealed against the sleeve lives a seal (part # 21) which seals the space between the sleeve and driveshaft. As long as the seal is working, there is no way for any liquid to pass from behind it to in front of it. I'm pretty sure I remember the two Alpha's and the one Bravo I have totally R&Rd had this seal as well, but I might be mistaken.

Ayuh,.... That seal is just to keep purged grease from the gimbel bearing from splattering all over the bilge,.....
It a rather loose fit, it's not a water tight fit,....
You could run without it, 'n not hurt a thing,.....

 

[tpenfield]

Yea, I did not always put those o-rings on the drive shaft, figuring that they won't really do much if the bellows gets compromised.

An equal concern is the 'Y' pipe gasket, which can cause an even bigger leak.

I don't worry much about that stuff, other than the routine inspections. If the boat sinks, then it will be time to get a new one.

 

[kd4pbs]

Ayuh,.... That seal is just to keep purged grease from the gimbel bearing from splattering all over the bilge,.....
It a rather loose fit, it's not a water tight fit,....
You could run without it, 'n not hurt a thing,.....

Thanks, Bondo... The one on this DP-SM is just as tight as the front crank seal or any other kind of lip seal I've ever used. That drawing doesn't do it justice and makes it look like just a square cross section O-ring or something like that. I checked the fit against the seal surface on the driveshaft and found it to be just like any other lipped seal, and it was a firm press into the transom shield. I'd think it could easily hold back water that is a few inches deep. I don't think I ever really checked it much on a Merc and I never had to tear down the Yamaha I/O I had this far.

Ted, this isn't the O-rings that ride near the bearing. This is a lip seal which lives in the transom shield which seals the driveshaft, and is in front of (towards the bow) the bearing and those O-rings.

It's strange, and while I never plan on testing the theory, I'd wager beers that this seal could prevent sinking. I'd also wager beers that sinkings blamed on bellows failure usually involved more than just the bellows failing.

Thanks, y'all!

 

[kenny nunez]

The 2 “O “ rings on the driveshaft are there to grip the gimbal bearing. There is usually a 1/4” breather hole at the 12 o clock position above the driveshaft. With a torn bellows water can really get into the bilge.

 

[kd4pbs]

The 2 “O “ rings on the driveshaft are there to grip the gimbal bearing. There is usually a 1/4” breather hole at the 12 o clock position above the driveshaft. With a torn bellows water can really get into the bilge.

No breather hole on this DP-SM transom shield. I was quite intimate and up-close with it for a few weeks sandblasting, Alodizing, priming, and painting it. The only hole along the whole sleeve is the one the grease tube lives in to bring grease to the gimbal bearing from the zerk fitting on the right side of the shield.
I don't remember the Mercs that well - that was quite a few years back.