doug748
Well-known member
20yrs, give or take.
Recommend me a stern gland
- Thread starter purplerobbie
- Start date
20yrs, give or take.
ghostlymoron
Well-known member
Not all stuffing boxes have a rubber boots. Both boats I've had with them the gland is bolted to the boat. Having a rubber boot seems un necessarily risky unless there's another way of absorbing the torsion.
sailorman
Well-known member
I only have the engine flexi mounts + R & D coupling + cutlass bearing, so the stuffing box needs to "float" via the very thick walled rubber hose
tidclacy
Active member
Leakage and wobble
ghostlymoron
Well-known member
I thought the general rule was to have 2 out of 3 flexible.
Tranona
Well-known member
This is an aside and should just be taken as such, but Manecraft are the leisure version of a seal produced by Deep Sea seals, who are in turn evolved from Crane Packing, and ultimately Wartsila - perhaps the finest marine diesel engineers on earth at present. The original deep sea seals are the best ever produced and were used for nuclear subs including ballistic boats and had the best performance of any seal on earth - the limit was reached at a shaft seal diameter of +/- 1 meter at full test deep diving depth and the 'emergency - backup' was designed to be used after the main seal springs (they don't just use the compression of rubber at those depths /diameters) had shattered following a depth charge attack - that inspires confidence in me if not Tranona!Don't take the post too seriously in the context of a leisure shaft seal - I use a PSS and it has now been fitted for a couple of decades.
But for ultimate use there is no shaft seal company on earth that can compete with Crane / DSS. Use includes Cruise ships and Ice-Breakers.
http://www.wartsila.com/products/marine-oil-gas/seals-bearings/sealing-solutions/wärtsilä-stern-tube-seals-water-lubricated
While the principle of face seals is good, using it in small marine applications is problematic and the original Deep Sea seal did not do it well. The two problems to overcome are maintaining the face contact at the right pressure and preventing the faces from sticking through a build up of calcium type deposits when the shaft is not being turned. Face seals are best used where they just seal a rotating shaft running in water and used either continually or regularly so the faces are not stationary against each other for extended periods nor required to move in any other direction.
In a small boat application the shaft not only rotates but also in many cases is subject to angular movement as well as fore and aft movement. So to keep the faces in contact bellows are used to mount the faces to both apply the correct pressure and cater for the movement. Many leisure boats spend long periods of time in the water not being used, leading to build up of calcium type deposits on the faces - remember they are normally kept apart by a film of water.
Failures of this type of seal are usually of three types. First is the faces parting because of inadequate pressure, either because the bellows are not compressed correctly (a point stressed in the installation instructions) or the shaft moving forward and extending the bellows too much. Second is the bellows failing because of cuts splits or material failure. Third is damage caused by the faces sticking and not being freed off before engaging gear.
To illustrate the first type. Consider many modern boats with semi floating propshafts attached to the engine at one end and supported by a P bracket with an open stern tube - that is no bearing in the tube. Normally there is enough waterflow with a feed from the engine to ensure that the faces are kept lubricated, but the flow is aft out of the tube. However if there is a sudden reversal of flow the pressure can be enough to force water back up the tube and open the faces. Two situations I have seen where this happened were first on a big sailboat that had been surfing off the top of a wave and dropped into a trough and second a twin screw planing powerboat in an emergency stop where water pressure was enough to squirt water out from between the faces.
The second type of failure is also common in flexibly mounted stuffing boxes where the water is kept out by a hose. This is a vulnerable component and failure here is more common than failing of seals around the shaft.
I was reminded only last week of the dangers of build up of deposits on bearing faces when they are not used for some time. I took the rope cutter off my saildrive which had little use over the last 18 months. There are two stainless faces with a delrin bearing between and are water lubricated and cooled. The build up of deposits was such that the unit could not be turned by hand and had to be forced apart in order to clean the faces and replace the bearings. There are reports of faces sticking together on seals and when the engine is started and put into gear, staying stuck leading to the bellows tearing. This is a particular problem on the original small boat Deep Sea where the bellows were plain rubber.
The PSS design goes a long way toward solving these problems with well engineered bellows to cope with the movement. However as the instructions say they are not fit and forget. The bellows must be compressed accurately, regularly inspected for damage and the faces opened up after periods of inactivity. They are at their best in installations like Vyv's where the shaft is not connected directly to the engine and fixed fore and aft so the seal only has to cope with rotation. This means the bellows are only required to maintain pressure. Extra care is needed with more powerful, flexibly mounted engines in view of the greater range of movements, and here the Tides lip seal type is arguably more suitable.
Apologies for the length of the explanation but there is a lot of confusion about the merits of different kinds of seals and I hope this is helpful.
GHA
Well-known member
Ta.
Just fitted a nice shiny new one so was wondering if I should schedule in replacing the hose after a few years, might be OK for a bit yet then
JimC
Well-known member
I have the Volvo seal but when I replace it I'm thinking of going to the Radice one as I dry my boat out often and it would be nice not to have to burp the seal. One question: if a leak develops in the breather pipe between its connection to the seal and the water level, could this sink the boat?
Sandy Bottom
Active member
As I said, deep sea seals are the best engineered shaft seals on earth. The small (tiny) leisure versions (Mane seals) are not really proper deep sea seals (either literally or as in DSS seals the company).
They (DSS) are the only seals that can operate at a combination of of three of 1. I meter shaft diameter 2. Deep diving Test pressure (I'm prevented from giving at precise figure but 0.47psi on the area of 1 meter diameter shaft at say 1,000 feet deep) - and then 3. Take a 'shock load' of an exploding depth charge at various depths.
You cannot use rubber bellows in those circumstances because the pressure on the seal varies enormously with depth - at say 3 feet from the surface you will have less than 2psi on the seal - dive and you will have considerably in excess of 500psi on the seal - then add an exploding depth charge pressure wave to that. There is a simple but ingenious answer to cope with this - can you think what it might be.
(Sorry about the fred drift folks - just leave me and Tranona to natter about it).
I'm happy about the principles of the leisure one's - we fitted thousands when I owned the boatyard. :encouragement:
Steve
Tranona
Well-known member
Are you sure about the "thousands" bit? Wonder why they are no longer active in the market.
Sandy Bottom
Active member
Bad wording on my part, apologies - thousands of shaft seals of various makes not just DSS one's (we lifted about 1,000 boats a year). DSS in their larger company (commercial) versions are still the biggest on the world market :encouragement: (did you follow the link that I provided?) They are very active but haven't bothered with the tiny leisure market (though they are just about to come back into it).
http://www.wartsila.com/products/marine-oil-gas/seals-bearings/sealing-solutions/wärtsilä-stern-tube-seals-water-lubricated
Worked out how to do it at 0 - 500 psi and more differential pressure yet? That depth change gives about 90 ton differential loading between the seal faces.
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Gwylan
Well-known member
Fitted a Radice to replace drippy, short lived Volvo seal. Hope I have made the right decision.
So far, so good!
ghostlymoron
Well-known member
The Volvo shouldn't have dripped unless the shaft is worn. Hopefully the new one will be on an unworn area and not have the same fault.
doug748
Well-known member
Yes, it could, tho it might take a while.
Another thing:
A forum member felt he missed the support of a conventional seal when he fitted a PSS and found his prop shaft hitting the sides of the tube - it was a particularly snug fit and a single cylinder engine. So if you have this arrangement, another point to bear in mind.
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