Understanding Stern Tube Arrangement on Ships

The propeller of the ship is fitted at the aft and attached to a crankshaft coming from the main engine. This is done so that the rotating motion of the Main Engine can be converted into thrust to propel the ship. The propeller shaft or tail shaft is supported by a bearing arrangement which acts as an intermediate phase between the sea and the ship.

The stern tube is a hollow tube passing at the lower stern part of the ship carrying tail shaft and connecting it to the propeller out at sea, bearing for the tail shaft, lubrication arrangement and most importantly the sealing arrangements.

The stern tube bearing arrangement and sealing plays a vital part in ship’s operation and pollution prevention.

Stern Tube

The two main purpose of the stern tube bearing are:

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.

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.

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.

You may also like to read-What is a Prop Guard?

 

 

22 Comments

  1. what type of lubrication system do stern tube applies? does it have the gravity fed type?

  2. Yes. A gravity tank is also fitted in the system along with a stern tube oil circulation pump.

  3. Anish please explain the need of time delay and relief valve in fixed co2 fire fighting system….pleas explain….waiting for ur reply
    Thanks vimal
    3 rd engg
    Wallem ship management

  4. – CO2 system is designed with a pre-alarm system and a time delay function to allow the evacuation of the hazard area and avoid danger to the crew.
    – To avoid the risk of liquid CO2 entrapment in the pipe work, a relief valve is provided which is set at around 150 bar.

  5. Hi Anish,
    Can you advise, if there is any rule requirement regarding the steel temperature during installation of the stern tube bush? If the steel temperature is 0 degree celcius, is it acceptable? Does this only depend on Maker’s instruction(if they have)?
    Thanks in advance
    Inspector

  6. Please can you explaine these terms for me,staoilifer,beans and beans knees,stearing gear,bilges and tanks

  7. Dear sir,
    How to remove shrunk fitted chrome liner out of stern tube. Or could you explain the procedure for chrome liner renewal of stern tube please.
    Thanks & best regards.

  8. @Vaibhav: Cast steel, cast iron and cast copper alloys are generally to be selected as the material for sterntube bearing housing of composite material.

  9. Dear sir can you tell me what action we want to take if stern tube bearing high temperature alarm triggered.

    thank you very much.

  10. @dissa: This can be due to –

    1. Cooling system not functioning properly
    2. Friction in stern tube and its bearing

    Check the cooling pump working, all valves in the line. Check the current drawn by the pump motor, it will INCREASE as the pump becomes worn and the water recirculates from the discharge side back to the suction side inside the pump. Check the oil temperature record for any indication. udden or gradual increase in oil temperature can also give a clue.
    If cooling system is all ok, it might be due to the friction in shaft, due to bearings failure or issue in shaft or propeller out of balance. Even damaged or stuck propeller will lead to this issue.

  11. In your article,
    “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.”

    Could you cite this statement?

    The forward, header and aft tanks are located at various heights. To the best of my knowlede, the heights of the oil tanks are approximately as follows:
    AFT TANK: 0.3 – 05 m
    HEADER TANK: 15 – 27 m
    FORWARD TANK: 1 – 1.5 m

  12. Hi sir,
    We re having big waves currently and propeller goes to air and go back in to water again, again and again 🙁
    And we started to get s/t l.o seal pot level high alarm, i drain it and water comes. In abt 14 hours 8liters water gets into the pot.
    I can’t figure out why(maybe becoz of its my first contact) i tried to search but couldn’t find any info. Do u have any idea sir? Thanks.

  13. Please xan you explain the use of stern tube cooling water tank? And how to refill and check the level. Thank you in advance

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