What is a Ship's Stern Tube?

The propeller is the only part of ship’s machinery, apart from the rudder, that is located outside the ship’s engine room. Have you ever seen the propeller of a ship and wondered how it is connected to the inside of the ship? Or ever thought where does the shaft holding the propeller go when it vanishes into the ship’s hull? If so, then you have come to the right place.

The propeller, a part of the propulsion system of the ship, needs a power source to rotate it. A marine diesel engine located inside the ship supplies the power to the propeller. Both the diesel engine and propeller are very heavy in weight and locating them both at the same place inside the ship will disturb the ship’s stability. It is for this reason a stern tube along with a propeller shaft is used.

The stern tube, as the name suggests, 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.

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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.

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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.

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