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STERN GLANDS  

(Stuffing Box , Packing Box)

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The first part of this is about stern glands and some information on them ,  the second part further down the page is how to adjust your stern gland and the third section is bit of general information and how to detrmine the shaft M/S to select the correct packing. 

 The stern gland or stuffing box as it is sometimes refered to is a much maligned piece of an inboard boats drivetrain. Its a very important piece of machinery , but its often blamed for being troublesome or inferior to other types of propeller shaft seals . 

 However a lot of its bad reputation is created purely by lack of attention , maintenance and complete misunderstanding by many boat owners and builders. 

It is often overlooked when it comes to part of a boats maintenance. We see fastidious owners changing engine oils and filters , treating fuel with algacides and scrubbing and antifouling , but when it come to the stern gland it just gets a pump of grease or "nipped" up a bit more to tame the leak a bit.

This is ok if your packing material is fresh, but how long ago was it changed?.

Old packing material is the number one cause of problem leaking and shaft wear and damage.

To understand this we need to understand the dynamics or the properties and the workings of the packing material itself .  Most packing materials are impregnated with a form of lubricant from new. This lubricant "flows" during use to the point of friction ( where the shaft and packing are in contact) to lubricate this point . After time the lubricant becomes depleted or less effective , thus friction builds up further breaking down the packing fibres and burning, scoring and all sorts of problems arise.  Herein is where some of the misunderstandings lay, where many people believe packing lasts forever and just keep tightening and tightening the gland which only adds to compound the problems of increasing the friction between shaft and packing .  Flax packing types also are subject to rot as they are plant fibre or just going hard over time .

Gland packings do not last forever and shouldnt be treated as if it does . It does need replacing as part of routine maintenance to your boat. Routine maintenance means less trouble at sea and a more enjoyable day out or long holiday afloat. 

Replacing the packing isnt a hard job , and its not expensive either .  The average 1.5" shaft stern gland would cost between $15 to $25 depending on the packing type used. This is better than replacing a $1,000 + propeller shaft!!.

I would look at replacing packing on a well used boat around yearly or sooner if any sign of bad leaking despite gland tightening starts , or every 2 years on the average used boat. I dont like to excede the 2 year mark purely because the packing will break down just with age or harden from being under constant tension.

Of course it should also be changed if there has been any sign of problems as well such as excess leakage or a blocked water supply up the stern tube.

One of the most important advances in stern tube packing technology is the introduction of GFO® type packing material.

The GFO fibre packing is quite a revolution for boating . This new style packing can be run virtually dripless and without the need of additional lubrication . It does not need as much tension in the gland either which reduces the friction and drag and extending its useful life far beyond that of flax and PTFE packings .

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GFO fibre packing is receiving rave reviews in forums from happy boat owners world wide and particularly in the USA . Many are reporting that they are able to run the GFO completely leak free and have dry bilges without any sign of heat in the gland itself. It is also great for helping where there is already shaft wear that cant be stopped leaking with traditional packings.

So before you go thinking about spending a fortune on a "dripless" seal, or pulling your hair out trying to get rid of that stern gland dribble ...i suggest giving some GFO packing  a go. You will probably be quite surprised at the result !. 

 And it will cost you a whole lot less than replacing the whole thing for another type.

While we are on the subject of shaft seal types , id like to share my personal experiences and thoughts on "dripless" type seals .

Most of these are either a sealing face rotating against a fixed face or of a lip seal type, not unlike an ordinary oil seal found in your car engine.

The manufacturers will make big claims about perfect seals etc, however they dont tell you about their shortcomings and safety . The big focus is always on the "dry bilge" which seems to be a holy grail amongst boat owners and they use that as the marketing ploy .

Well i hate to say it, but ive had plenty of personal experiences with rotataing face and lip type seals both in our own boats and those ive had to help repair as well as in heavy industry in my 29yrs of trade work that field and i can safely say they are far from what they are cracked up to be.

They have some shortcomings, they wear , they dont handle linear movement very well and they can also be very easily damaged . Sure you will hear someone down the boat club saying they have an XYZ brand dripless seal and its great , and it probably works fine for him.  But you often wont hear the other guy admitting hes had problems with his expensive purchase and still gets water in his bilge.

My biggest concern with most of the dripless types is that when they fail, they can fail disasterously allowing thousands of litres of water an hour in to the bilge and there isnt much you can do to stop it other than wrap it up in rope or a sheet or something, but that usually means you cant motor then either . The only way they can then be repaired is to haul the boat out of the water , whereas in most cases packing can be changed out on the water and rarely suffers a catastrophic failures as other types can do.

Many new industrial machines are still fitted with packing gland type seals because they are reliable and easily serviced and can handle a wider range of extremes and materials/liquids than a lot of other types of seals.

Basically dripless types work perfect in perfect conditions , but vary the conditions and it can all be downhill from there.

I read a recent article where a largish boat was in heavy seas and a tool box or an object like that moved and fell against the belows of a dripless type seal and held the sealing faces open and the boat sank in literally minutes . That would not have happened with a stuffing box.

Well anyway, thats my a little of my experiences annd thoughts on the topic of dripless vs stuffing box types.

For my money i think you cant go past a well maintained and serviced packing type stern gland packed with the new GFO fibre . They would rival if not be better and safer than most commonly available "dripless" seals . In fact packing type seals are still specified in heavy industry machinery to this day for reliability of service.

You may get a drip of water, but its a known quantity . If you know what your dealing with , its easily managed.

We have fitted our new twin engine boat with packing box type seals and packed them with GFO fibre packing .

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 # The Stern Gland in our own vessel . It is a flexible self aligning type installation and packed with GFO® Type gland packing.

Not shown is a catch tray system that sits under the gland and collects any water that may leak ( just about zero with the GFO) and it then runs to a larger tray with its own float switch and pump.  No water from the gland sits or collects in the bilge at all as its all contained in the small containment and discharge system. There is also the double security of if the first water discharge system fails it simply overflows in to the bilge where the boats main bilge alarm and automatic bilge pump can take over. A system with a backup system.

I will do an article on this system and add it to the knowledge pages as time allows. Keep an eye out for it.

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Repacking your Stern Gland

## NOTE:- The following information is a guide only and covers general stern gland installations .

Its content may need to be varied to suit your particular installation and use .

If you have any problems or questions contact your closest marine service provider or contact us .

 

The first thing to do is to get rid of ALL the old packing from inside the packing gland .

 This can be pulled out with the help of special tools or you can make your own "flax pick" from a piece of metal rod around 2.5 to 3.5 mm thick (a welding rod with the flux removed makes a good rod ) and hammering one end a bit flat then bending the very end of the flat over at 90 deg forming a hook at the end. Use this end to pull at the old packing and remove it . The proper tools are still better, but that method works ok when the tools arent available.

Flexible corkscrew type removal tools are the best for pulling larger packing and good for confined space on packing around 1/4" and up .

 

 

Be carefull not to score the shaft or inside of the gland when using flax pics etc. If damage is done then this will need to be sanded smooth before repacking or it will simply tear the packing to pieces when repacked. Particularly if the the scratching is on the shaft.

Next cut the rings of new packing material .  Theres a few ways to do this.  The basic way is to cut it as a "butt" fit with square ends that butt against each other in the gland .

The best way is a 45 deg angle cut or scarfe cut as it is often called, so that the ends of the packing meet at the 45 deg forming a smooth flowing join. The best way to make and acurate length cut is to use a piece of exposed shaft where nothing runs on it or a piece of metal round bar the same diameter as your shaft and wrap the packing around it one turn and some more till it overlaps a bit, then cut across the 2 widths of packing at a 45 deg angle with a sharp knife or stanly knife , or in the case of thicker packing make a mark across the packing then unwrap it and cut it along the marked lines. Repeat this till the right number of rings are cut.

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I recomend using the 45 deg ( scarfe) join method, particularly on GFO fibre type packing . This better controls leaking at a looser pressure.

Now install the rings of packing . Push each ring in one at a time using something that wont damage it till it is in place . Space the joins out on each ring of packing  at 180 deg to each other . Dont put all the joins together.

## Do not just wrap packing around the shaft in one continuous piece as doing this can turn your gland in to a pump and not a seal if the rotation of the shaft and the spiral of the packing are in the right orientation to each other !!!.

Some people use this method , but i feel its just out of a bit of laziness for taking the time to properly cut seperate packing rings and i do not recomend it at all . 

At the very least it may seal very poorly and at worst overheat or fill your boat with water very rapidly .

Once the packing is installed , fit the nut or gland....  and tighten it a little to bed the packing in . Then back it off fully again. Now adjust it back up so it is loosely pushing on the packing .

The rest of the adjusting will have to be done while running the boat .

The best method is to run the boat with the loose packing for around 1/2 hr at least to "break in, or run in" the packing, allowing water to drip out of the gland at a good rate of around a drop every 5 seconds or so. Then slowly tightening the gland a little at a time until a drip of water comes out around every 30 to 50 seconds  while running is obtained  for the flax or PTFE style packings . There is some debate about the number of drips per second or minute, but again this can vary in different installations and packing materials , so the best way is to adjust to the least leak while being able to obtain a cool gland . 

The temperature of the gland should always be monitored to ensure it isnt overheating.  A gland is ok to be warm after a period of time running, but it should never be hot. If it is starting to heat up , back of the packing adjustment and run it in some more before starting to retighten again . In some cases the balance will need to be found between the minimum leakage amount and the temperature of the gland itself . Trial and error will be the only way to find this. Once you get it right first time, every subsequent time will be far easier to adjust at a setting that best suits your boat. 

With GFO type packing it will be possible to obtain a lesser drip rate and quite often it may be adjusted to a point where it just stops dripping altogether.  As long as the gland remains cool while running it may be kept adjusted in the no drip state, which is a good result.

In general GFO type packing requires much less adjusting pressure applied to work as well due to its softer more pliabale nature..

In all though , stern gland adjustment is a trial and error thing, and once youve got it learnt it will be a much easier job next time and the time after.

## Any future adjusting as the packing wears in and settles should be done the same way by tightening it a little at a time untill the desired setting is acheived.

Gland Packing Information 

General

Gland packing for stern glands is available in many different types and materials and also contain different built in lubrication .

It is generally available in 1/16" increments of sizing from 1/16" to over and above 1/2" in size.  For example 1/8", 3/16" , 1/4" , 5/16" and so on.

The metric sizing equivalents is available for the packing on the listings themselves on the Stern Gland Packing page

The different material and lubrications  makes them each better suited to each other for a specific use. However with the coming of the newer GFO fibre type packing material , things have changed a little and now the one type can cover many different applications.

We stock the 3 main types common to boat use . These are the traditional style Flax packing which is impregnated with a Tallow lubrication , a PTFE type which is impregnated and coated in a PTFE lubricant , and thirdly the GFO  type. The GFO type is a far superior product to the previous 2 for life of service and sealing capability when used in marine craft.

Full product spefications can be found on the Palmetto Packings Page

# Palmetto GFO®  Gland Packing

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We recommend using the GFO type packing, but also cater for those still wanting the traditional Flax or standard PTFE types for their boat or pump as for some applications these still function exceptionally well.

Of course these packings are also very suitable for rudder post glands, water pump glands and other areas where gland seals are used.

Shaft Speed Explanation

When you look at a spec sheet for Packing materials, you will se a maximum shaft speed rating stated for the particular packing shown for example as  "  12 m/s  ". The  m/s rating refers to the surface speed of the rotating shaft.  It is short form for meters per second .

To dermine the  m/s of your boats propeller shaft follow this formula :-

First find the following ;  1. The shaft diameter in millimeters, 2. The shaft RPM ( divide engine RPM by the gearbox ratio to find this, for example , a engine doing maximum RPM of 2,500 RPM with a 2.5-1  gear ration has a shaft speed of 1,000 RPM ,   2,500 ÷ 2.5  = 1,000 )

Next work out the shaft circumference . this is done using the formula  Π x D , or simply   3.14 x shaft diameter ( mm).

Once the cicumference has been worked out , and the shaft speed in RPM  is also known , use the folowing :-

( shaft circumference ÷1,000 ) x ( shaft RPM ÷ 60) =  shaft M/S .

I will give the following example to better explain this .

For this purpose we will use and engine that has a max RPM of,   2, 500 rpm and a gear reduction ratio of 2.5 - 1 , driving a 2" (50.8 mm ) propeller shaft.

First the shaft speed....  2,500 ÷ 2.5 = 1,000.

Next the shaft circumference .....    3.14 x 50.8 =  159.5 mm .

Next the formula.....   ( 159.5 ÷ 1,000 ) x ( 1,000  ÷ 60 ), which becomes     0.1595 x 16.6 = 2.647

So the shaft  M/S is -  2.647 m/s

So as long as the rating on the packing is higher than the speed of your shaft it is suitable.  In all ,the higher the speed the packing is capable of the better its service life will be.

The same formula can be applied to determine the shaft speed in M/S of a water pump shaft or other rotating shaft, however in the case of a pulley driven or gear driven pump shaft , you will need to work out the shaft speed by dividing or multiplying the ratio of the pulley sizes or gear reduction or step up.