STERNTUBE
Dr Abdus Samad
Contents • Stern tube • Bearing, seal • Pollution
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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
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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. 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. IIT Madras
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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
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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
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Shaft Bearings
CAN STERN TUBE LUBRICATING OIL DISCHARGES BE ELIMINATED? •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. IIT Madras
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Ship stern tubes today
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Sterntube seals Stern tube seals have 2 purposes:
There are many different types of sealing systems, but the majority of ships use lip seals.
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Stern tube seals
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Lubrication
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Oil lubricated stern tube bearings
Currently, this sealed oil system is the standard for stern tube bearings
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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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