Stern Tube

STERNTUBESTERNTUBE

Dr Abdus Samad

Contents• Stern tube

• Bearing, seal

• Pollution

2IIT Madras

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What is a Stern Tube• The stern tube is a hollow tube-like structure at the stern or rear

part of the ship. A ship needs a propeller to drive it forward

against the waves. The propeller, located outside the ship needs

to be connected to the engine inside the ship’s engine room. A

long shaft known as the propeller shaft is used for connecting

the ship’s engine and the propeller. This narrow hole in the hull

structure at the rear end (aft peak) of the ship, through which the

propeller shaft passes and connects the engine and propeller is

known as the stern tube.

• The propeller shaft (or tailshaft) is supported in a stern tube

bearing of one of a number of designs. The bearing, being at the

end of the shaft, is affected by the overhanging weight of the

propeller. The propeller mass pulls the outer end of the shaft

down, so that there is a tendency for edge loading of the stem

tube bearing to occur. IIT Madras 4

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Inside the Stern Tube

• The stern tube of a ship has a peculiar shape and arrangement

for carrying the propeller shaft. As the shape of the ship is

streamlined at the aft end, there is very little space to

accommodate the propeller shaft arrangement.

• The stern tube is situated at the aft peak of the ship’s hull and

the propeller shaft arrangement is also situated at the same

level. The forward end of the stern tube is supported by the aft

peak bulk head and the aft end of the stern tube gets its

support from the stern frame of the ship. Throughout the

length, the shaft is supported by specially designed brackets

and narrow floors inside the stern tube. 6

Stern tube Bearings and Glands• The total weight of the propeller shaft inside the stern tube is

carried by these bearings which are known as the stern tube. The

weight of the overall stern tube arrangement, along with the

bearings is carried by the stern frame and the internal framing of

the ship’s hull structure at the aft peak area. Moreover, there are

generally two bearings inside the stern tube for supporting the

shaft, located at both the ends. The main function of stern tube

bearings is to allow an unrestricted and smooth rotation of the

propeller shaft.

• Apart from stern bearings, the stern tube also houses water and oil

sealing glands known as the stern glands. It is also to note that

stern tube is that sensitive part of the ship from where the sea

water can easily seep inside the ship. The stern glands thus seal

the area between the stern tube and the propeller shaft. The glands

are attached at the forward end of the stern tube and prevent the

passing of sea water into the ship. The glands also prevent the

leaking of oil from the stern bearings into the sea.IIT Madras 7

Withstand load

• The propeller which hangs at the aft end exerts load on

the shaft, which is supported and withstand by the stern

bearing. The bearing is a cast iron bush lined with a

white metal having excellent load handling and

lubricating property.

• The stern tube is fitted at the stern frame and internal

framing of vessel’s hull at aft peak.

• This allows the tail shaft to rotate smoothly in the bearing

area for uninterrupted propulsion.

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Sealing• The stern tube bearing consists of sealing arrangement

to prevent ingress of water and to avoid the lubricating

oil to escape into the sea.

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Lip seals retain the

oil inside the air

end, whilst at the

same time keeping

air, dust and dirt

away from any

rotating shaft, when

a machine is off

load for instance.

Sealing arrangement• The lubrication system for ships with variable draught (due to

loading and unloading of cargo) consists of header tanks

located at around 2 to 3 meters above the water line so that the

differential pressure ensures no water ingress.

• Different sealing arrangements are used to prevent water

ingress and oil leakage. They are as follows:

• Stuffing boxes consisting of packing material.

• Lip seals in contact with shaft to prevent passage of oil or

water along the shaft.

• Radial face seals supported with springs fitted radially around

the shaft, aft bulkheads and after end of the stern tube.

• Out of these, the lip seal arrangement is most popularly used.

Read more about Lip seals here.

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Oil lubricated bearings

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Oil lubricated bearings• When large propellers are fitted the heavy overhanging weight

greatly increases the load at the after end of the stern tube

breaking down the hydrostatic lubrication causing metal to metal

contact and seizure towards the aft end of bearing. To obviate

this it is usual to angle the shaft downwards for about 8mm over

100m length thus attempting to ensure than the weight of the

bearing is taken on the full length of the bearing. It is good

practice to leave the oil tank open to the stern tube when in port

with machinery stopped, this prevents sea water leaking into the

system. However, water has been known to contaminate

lubricating oil systems causing rusting of tail shaft particularly

when shaft is stopped for periods long enough for water to settle

in bottom of bearing.

• In ships with large changes in draught it is usual to fit two gravity

tanks. The upper tank is used when fully loaded or there is water

leaking in. IIT Madras 12

Shaft Bearings

CAN STERN TUBE LUBRICATING OIL DISCHARGES BE ELIMINATED?

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•Two options claim to reduce stern tube oil pollution by using

environmentally friendly variations on the standard oil

lubricated white metal bearing system.

1) Seal manufacturers have developed more sophisticated

multi lip seals which reduce the amount of oil that escapes,

but shaft seals can still be damaged, oil can still escape into

the sea and owners still have to deal with any oil captured

onboard.

2) Biodegradable oils are also available, but they are still

considered a pollutant and leave a sheen which must be

reported when leaked.

Ship stern tubes today

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Sterntube seals

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Stern tube seals have 2 purposes:

There are many different types of sealing systems, but

the majority of ships use lip seals.

Stern tube seals

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Lubrication

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Oil lubricated stern tube bearings

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Currently, this sealed oil system is the standard for

stern tube bearings

Oil pollution

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Stop accidental discharge

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Proven available technology

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Q?

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