Stern Gear - Mechanical Seal / shaft life

[RAB]

Stern Gear - Mechanical Seal / shaft life

 

Was discussing with a friend how water is prevented from entering along the prop shaft a few weeks ago where I explained the two basic principals; Older and dare I say traditional packing gland, with a greaser, and the mechanical seal that’s water cooled. I explained the basic pros / cons but didn’t really know how much of an issue shaft wear is with a mechanical seal and assuming correct maintenance how long the seals really last for before replacement. Would be interested in peoples experiences of years / running hrs?

[IanD]

15 minutes ago, RAB said:

Stern Gear - Mechanical Seal / shaft life

 

Was discussing with a friend how water is prevented from entering along the prop shaft a few weeks ago where I explained the two basic principals; Older and dare I say traditional packing gland, with a greaser, and the mechanical seal that’s water cooled. I explained the basic pros / cons but didn’t really know how much of an issue shaft wear is with a mechanical seal and assuming correct maintenance how long the seals really last for before replacement. Would be interested in peoples experiences of years / running hrs?

There are several types of modern water-lubricated stern seals. The cheaper ones (Vetus/Volvo and similar -- though not exactly cheap...) normally fitted on narrowboats use synthetic lip seals which do wear, need regular lubrication (a bit of silicone grease every few hundred hours), and have been known to fail catastrophically on occasion. The more expensive ones (PSS and similar) use face seals, typically stainless steel on carbon, don't need any maintenance, and seem less prone to failure, but are not often used on narrowboats, maybe because they cost more...

 

https://www.pbo.co.uk/gear/dripless-shaft-seals-pbo-buyers-guide-17357

[cuthound]

My Vetus seal failed spectacularly after 12 years after a hard burst of reverse power.

 

However when I bought the boat if came with a tube of silicon grease and a glass syringe with the tip missing...

 

I strongly suspect that the missing syringe tip had been lurking in the seal for at least the 5 years I had owned the boat and was responsible for the failure.

 

It was replaced by a Radiche seal, which is similar but cheaper.

[ditchcrawler]

Engine alignment and drive shaft arrangements have a lot to do with it.

[Bee]

If the budget allows its worth doing it properly, plummer block to keep the shaft unstressed and exactly aligned, Aquadrive or Python including thrust bearing for many reasons and if you have a long shaft even more plummer blocks and a flexi bearing to protect the rubber engine mounts and gearbox bearing.  Add that lot up and its much the same cost as a good gearbox. If the budget is tight then a cheap Chinese plummer block exactly aligned so the tail shaft spins true, some sort of shortened car drive shaft and a load of Nyloc nuts bolts and washers for shims will do well enough.

[bizzard]

Generallly the more modern boats sacrifice engine and transmission space for cabin space,  so more often than not there is no room for a proper marine engine and drive line set up.

[J R ALSOP]

In my personal experience I built my my boat in 1979, I have a Bukh 2G105 29 HP Danish built engine, I have only put one ring of 1/2" packing in my stuffing box, the total amount of water dripping through my stern gear is about 1 gallon, I have have used plenty of grease as I think grease is cheap, stern gear isn't.

The engine is mounted solidly the shaft moves backwards and forwards through the stuffing box about 1" a time every time I change from forward to reverse.

My boat is available for anyone to inspect at any time, at the moment I am moored on the Northern Coventry Canal

[Slim]

52 minutes ago, J R ALSOP said:

In my personal experience I built my my boat in 1979, I have a Bukh 2G105 29 HP Danish built engine, I have only put one ring of 1/2" packing in my stuffing box, the total amount of water dripping through my stern gear is about 1 gallon, I have have used plenty of grease as I think grease is cheap, stern gear isn't.

The engine is mounted solidly the shaft moves backwards and forwards through the stuffing box about 1" a time every time I change from forward to reverse.

My boat is available for anyone to inspect at any time, at the moment I am moored on the Northern Coventry Canal

I'm intrigued, where does the 1" or so come from? Also, do you mean that 1 gallon has leaked into the boat via the gland in 30 years?

[blackrose]

I wonder how much grease has leaked into the waterways over those 30 years?

[blackrose]

6 hours ago, IanD said:

There are several types of modern water-lubricated stern seals. The cheaper ones (Vetus/Volvo and similar -- though not exactly cheap...) normally fitted on narrowboats use synthetic lip seals which do wear, need regular lubrication (a bit of silicone grease every few hundred hours), and have been known to fail catastrophically on occasion. 

 

 

And of course we shouldn't forget that there have been more than a few cases where poorly maintained traditional packing stern glands have also failed catastrophically...

 

[Keeping Up]

We have the traditional stern gear, just a brass fitting with greaser (none of your modern water-lubricated stuff) and a plummer block but no thrust-bearing. After about 20 years and 20,000 miles of travelling it was getting difficult to stop the drips so it was inspected carefully; the prop-shaft had worn down to about half its original diameter so the shaft and bearing were replaced. Once again it doesn't leak more than a few drops per hour and I don't expect to be around when it needs replacing again.

[IanD]

14 minutes ago, blackrose said:

 

And of course we shouldn't forget that there have been more than a few cases where poorly maintained traditional packing stern glands have also failed catastrophically...

 

Indeed, but these are usually checked daily so there's often warning of an increasing leak. With the "maintenance-free" ones the temptation is not to bother with this "because they don't leak"...

 

I was really pointing out the difference betweeen the lip seal and face seal types, the second is reportedly more robust and less susceptible to damage/leaks, and also doesn't wear the prop shaft.

Edited March 21, 2021 by IanD

[blackrose]

5 hours ago, IanD said:

Indeed, but these are usually checked daily so there's often warning of an increasing leak. With the "maintenance-free" ones the temptation is not to bother with this "because they don't leak"...

 

But they do leak if they are not maintained, and as we all know anything that's not maintained is liable to fail. That includes traditional packed glands which are also often neglected. 

 

Some people might check them daily, but lots of people who don't move very much might go months or years without checking theirs.

Edited March 22, 2021 by blackrose

[Tony Brooks]

10 hours ago, J R ALSOP said:

In my personal experience I built my my boat in 1979, I have a Bukh 2G105 29 HP Danish built engine, I have only put one ring of 1/2" packing in my stuffing box, the total amount of water dripping through my stern gear is about 1 gallon, I have have used plenty of grease as I think grease is cheap, stern gear isn't.

The engine is mounted solidly the shaft moves backwards and forwards through the stuffing box about 1" a time every time I change from forward to reverse.

My boat is available for anyone to inspect at any time, at the moment I am moored on the Northern Coventry Canal

 

Then something is probably wrong. Even gearboxes that use prop thrust to engage the clutches like the old Volvo Pentas MDs don't normally allow that degree of movement. If its really is rigidly mounted then most gearboxes allow no axil shaft movement as you go in and out of gear.

 

The fact its rigidly mounted means that the shaft can't wave about radially so that goes a long way to minimising stern gear wear. In that resect one of the packless glands plus a single Cutless bearing will help minimise gland wear and vibrations transmitted to the boat hull but I don't like that system for cabals.

[pearley]

Our boat was built in 2007 with a Volvo Shaft seal and Centafle coupling. The seal was replaced at 4230 hours in June 2014 due to excess water dripping in.

In June 2018 we hit something under water at Brownhills and badly damaged the prop, just 2 weeks after coming out of the drydock.

As there was quite a bit of sideways movement on the shaft we decided to change shaft, cutless bearing and prop plus a new seal. As the prop had Vetus stamped on it I assumed that the shaft and bearing were too.  However it was a mix of Volvo and Vetus, the OD of the cutless bearing being 1mm different. Fortunately the right way to allow it to be turned down. The seal is mow not genuine Volvo, having a much stiffer rubber.

 

Edited March 22, 2021 by pearley

[J R ALSOP]

10 hours ago, Slim said:

I'm intrigued, where does the 1" or so come from? Also, do you mean that 1 gallon has leaked into the boat via the gland in 30 years?

The 1" travel comes from the thrust between forward and reverse clutches when they are selected, the 1 gallon in 42 years through the stern gland packing to the interior of the boat is true, I only have to dust the area.

[Bee]

10 hours ago, Keeping Up said:

We have the traditional stern gear, just a brass fitting with greaser (none of your modern water-lubricated stuff) and a plummer block but no thrust-bearing. After about 20 years and 20,000 miles of travelling it was getting difficult to stop the drips so it was inspected carefully; the prop-shaft had worn down to about half its original diameter so the shaft and bearing were replaced. Once again it doesn't leak more than a few drops per hour and I don't expect to be around when it needs replacing again.

Cor! Thats a lot of wear. It is surprising how much wear a bit of greasy packing can inflict. If you are lucky then you can shift the shaft a bit so that the worn bit is in /out of the packing point. Could be overtightening the thing? 

[J R ALSOP]

1 hour ago, Tony Brooks said:

 

Then something is probably wrong. Even gearboxes that use prop thrust to engage the clutches like the old Volvo Pentas MDs don't normally allow that degree of movement. If its really is rigidly mounted then most gearboxes allow no axil shaft movement as you go in and out of gear.

 

The fact its rigidly mounted means that the shaft can't wave about radially so that goes a long way to minimising stern gear wear. In that resect one of the packless glands plus a single Cutless bearing will help minimise gland wear and vibrations transmitted to the boat hull but I don't like that system for cabals.

The prop thrust does not help engage the the clutch, that is done with lever, the gearbox is/was a ZF fitted to engine by Bukh on their G range 1-6 cylinder.

[Bee]

10 minutes ago, J R ALSOP said:

The 1" travel comes from the thrust between forward and reverse clutches when they are selected, the 1 gallon in 42 years through the stern gland packing to the interior of the boat is true, I only have to dust the area.

If anyone has that sort of gearbox it should still be OK with a Python /Aquadrive as they have a splined joint that allows for that movement . I watched a Scottish fishing boat in a harbour with gin clear water years ago and was intrigued to see the prop and shaft moving backwards and forwards. Took years before I realised that it was actually supposed to do that. Glad I didn't tell the steerer the prop was about to fall off.

Edited March 22, 2021 by Bee
added a bit

[Keeping Up]

1 hour ago, Bee said:

Cor! Thats a lot of wear. It is surprising how much wear a bit of greasy packing can inflict. If you are lucky then you can shift the shaft a bit so that the worn bit is in /out of the packing point. Could be overtightening the thing? 

Yes it's a lot of wear - but a lot of mileage too. 20,000 miles, that's about 10,000 hours of use; in that time a car would probably have travelled 300,000 miles and would probably have worn out a few bearings too.

[Tony Brooks]

2 hours ago, J R ALSOP said:

The 1" travel comes from the thrust between forward and reverse clutches when they are selected, the 1 gallon in 42 years through the stern gland packing to the interior of the boat is true, I only have to dust the area.

 

Thanks, I have never seen that much shaft movement on that type of box, but then I have not seen many of them.  The ones I have seen its more like 1/2" or less.

[J R ALSOP]

6 minutes ago, Tony Brooks said:

 

Thanks, I have never seen that much shaft movement on that type of box, but then I have not seen many of them.  The ones I have seen its more like 1/2" or less.

I will measure exactly the next time I go to the boat.

[nb Innisfree]

I fitted a Python Drive to Innisfree, the results were dramatic, much quieter and smoother, not a single drip of water, I even readjusted to make sure it wasn't too tight. When we sold Innisfree it had done several thousand hours running with no water drips or issues of any kind, brilliant bit of kit. 

 

Just to add - I made sure it had been fitted correctly, properly aligned with recommended offset. 

[Tony Brooks]

30 minutes ago, nb Innisfree said:

I fitted a Python Drive to Innisfree, the results were dramatic, much quieter and smoother, not a single drip of water, I even readjusted to make sure it wasn't too tight. When we sold Innisfree it had done several thousand hours running with no water drips or issues of any kind, brilliant bit of kit. 

 

Just to add - I made sure it had been fitted correctly, properly aligned with recommended offset. 

 

Exactly. It's the radial movement caused by flexibly mounted engines that cause the noise and gland wear. The Aquadrive in effect makes the equivalent of a rigidly mounted engine while still allowing the engine to jump about.

[nb Innisfree]

2 hours ago, Tony Brooks said:

 

Exactly. It's the radial movement caused by flexibly mounted engines that cause the noise and gland wear. The Aquadrive in effect makes the equivalent of a rigidly mounted engine while still allowing the engine to jump about.

Though I used to grease generously.