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  • STERNTUBESTERNTUBE

    Dr Abdus Samad

  • Contents Stern tube

    Bearing, seal

    Pollution

    2IIT Madras

  • IIT Madras 3

  • 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 ships engine room. A long shaft known as the propeller shaft is used for connecting

    the ships 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

  • IIT Madras 5

  • 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 ships 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 ships 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 vessels hull at aft peak.

    This allows the tail shaft to rotate smoothly in the bearing area for uninterrupted propulsion.

    IIT Madras 8

  • 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.

    IIT Madras 9

    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.

    IIT Madras 10

  • Oil lubricated bearings

    IIT Madras 11

  • 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?

    IIT Madras 14

    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

    IIT Madras 15

  • Sterntube seals

    IIT Madras 16

    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

    IIT Madras 17

  • Lubrication

    IIT Madras 18

  • Oil lubricated stern tube bearings

    IIT Madras 19

    Currently, this sealed oil system is the standard for

    stern tube bearings

  • Oil pollution

    IIT Madras 20

  • Stop accidental discharge

    IIT Madras 21

  • Proven available technology

    IIT Madras 22

  • IIT Madras 23

  • Q?

    IIT Madras 24

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