Our amazing Lip seal and stern tube lubrication | An InGenious arrangement

What's that red color thing? 
Why there are fins in the Bottom most platform?

Why seawater isn't getting into our engine room through the gaps of the "propeller shaft" and hull? 

Or to make it a heart-pumping question, (usually heart pumps at 0.14 bar)
WHAT IF your stern tube starts leaking when you are on your night-watch 1000 Kms far away from Home?

What's inside these? or which part of the machinery should I be looking out for?

Instead of saying curse words at midnight, What should be my action?

Have a look at this excellent Lip seal and stern tube lubrication

Our simple Stern Tube and lip seal arrangement

If you have tried to push an empty bucket into the water, then you know the upthrust
exerted by the bucket.

Imagine that if you drilled a hole in the side of this bucket,
trying to push it down, 

the water rushes into this bucket.
and remember, the depth of the bucket is just 0.25 meters.

And now imagine our big ship, resting on the water with her cargo. Having a draft of 20 meters. 

Lip seal and stern tube lubrication
Pic showing the aft sealing arrangement -  so that the water doesn't get in

Here the Green color thing called lip seal is pressed against the shaft (or liner)

The whole black color thing that holds the green color seal is called seal housing.
In our case, its called aft seal housing. as it holds aft seal.

Yes. Super easy.!

The orange line drawn here is the chrome liner.

This liner is something similar to the shaft sleeve. Called chrome liner. 
And an important part of our sealing arrangement.

Why chrome liner? 

we don't want any grooving on the shaft right?  and its easier to replace this chrome liner.

The less known information on this is that,
The aft chrome liner is fitted on to the propeller boss.

Where is this Hollow stern tube located?

As you know, this stern tube is a simple hollow like structure
and is located at the aft end of the ship's hull.

forward part of the stern tube is supported by aft peak bulkhead and
aft end of the stern tube is supported by stern frame of the ship

Picture of aft side of the ship

Lip seal and stern tube lubrication system

Stern tube arrangement is inside a cooling water tank.
Which is always filled with seawater.

There is a filling connection as well as an overflow connection.
and we should always ensure there is enough cooling water in this water tank. 
Using the sounding pipe.

2 purpose of a stern tube

Stern tube supports the stern tube bearing and holds the seal-housing arrangement.
The stern tube has to do 2 jobs

A) Withstand the load

The bearing inside the hollow stern tube (a.k.a stern tube bearing ) takes the weight of the propeller
just like all the bearings in the world.

to make it more clear, have a look at this.

Our Enormous Propeller hangs in the 
Stern tube bearing  
Stern tube bearing rests in the hollow stern tube 
Our hollow stern tube is supported by a stern frame arrangement in the aft.

  • The shaft is supported by the two bearings inside the stern tube.
stern tube bearing lubrication system
Note the size of the aft bearing in the above diagram 

As you can see here, 
the aft bearing bush is the border one

As it has to take the majority of propellers and his shaft's weight.

B) Holds the aft sealing housing

There is a sealing arrangement in both the ends of the stern tube. 
For sealing purposes and to keep things apart.

If you have not heard about lip seal, it's just a sealing arrangement to avoid the moment of liquid into the bottom platform through the rotating shaft. 

stern tube lubrication and sealing system
Lip seal is pressing against the shaft to prevent the entry of seawater

The shape of the seal is to be noted. It might be needed for answering questions. Or at the time of seal assembly.

And in order to avoid grooving in the shaft. we have inserted a liner on to the shaft.

stern tube lubrication in ship
This seal is held around the shaft by a spring called garter spring.

As you can see in the below diagram, the whole aft seal housing is underwater.
and the forward seal housing is in Our engine room.

Diagram shown here is the simplest sealing arrangement on a typical ship.

Just a few seals, 2 chrome liner, and header tanks.
stern tube lubrication and sealing system
Diagram showing of both the seals and oil flow arrangement

The only point to note here is
The aft seal housing arrangement is bolted on to the stern tube.
And the forward chrome liner is bolted to clamp ring.
(Clamp ring is just a ring which is clamped on to the shaft in the forward part)

If you ever happened to open up the line diagram in the ECR,
and turned to the Stern tube line diagram

You could only see 6 Black dots representing these seals.
Thankfully, now you have this smart information on the stern tube and seals. 

No one could takeout your dignity by asking these silly questions on 
"stern tube and sealing arrangements" 
Not even your CEO.

Here, only the Shaft, Propeller, and the chrome-liner are rotating.
And the lip seal and lip seal housing are stationary.

As you could see in this video down below,
The lip seal is sandwiched in between these golden colour rings. (exactly at 1:25 )

Have this video to have a complete understanding of lip seal and stern tube arrangement.

Exactly at 1:38, you can see they are fitting the whole lip seal arrangement on to the chrome liner.

Now, For your information,

The seals are numbered in order from the propeller as 1, 2, 3, 3S 4, and 5.
the "S" in the 3S stands for a supplementary seal. 
( Markings in the diagram below )

If the seal 3 is leaking, oil to the seals between 2 and 3 should be shutoff. 

stern tube lubrication and lip seal
Stern tube lip seal lubrication oil system

And one should open the Lub oil line between 3 and 3s. 
So that we can keep continuing to till the next dry dock.

Or in the worst situation, 

fill the gravity tank with a more viscous fluid.
There is a separate grade of oil which can maintain the lubricity even when the oil is contaminated with 40% of the seawater.

We could replace the entire stern tube oil with this high viscous oil.
So we could avoid any ingress of seawater into the engine room.

A great  piece of information for you: the propeller drop is measured using a poker gauge, 

It's easy- 

Insert the stylus (stick) of the pocket gauge in between the lip seals 1 and 2. 
by opening the plug just above it. And measure the depth.

Yes. You visualized it correctly. As the stern tube bearing gets thinner, Shaft goes down.
The readings are written on the poker gauge box as well as in the logbook.

Hope you got a Solid foundation about our stern tube and some valuable information
on this compact article.

Please do a small share if you liked the diagram and explanations
that helps me a lot in producing more awesome content.

Keep smiling. and
Have a nice day. 

Comment down your thoughts.
Ciao for now!

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