Stern Tube Explained | Location, Parts and Purpose
Among the many different parts of the ship that serve a waterproofing function, the stern tube is the most important.
In this article, we explore what it is, its location and purpose.
What is the stern tube?
The stern tube is a hollow tube that holds the shaft that connects the engine to the propeller. This shaft is known as the tail shaft or the propeller shaft. The tube is made from either cast iron or mild steel.
As the propeller is submerged at all times, the stern tube connects the inside of the ship (engine room) directly to the seawater around the ship.
We, therefore, need very good sealing arrangements to prevent the entry of seawater into the ship’s interior. Nitrile rubber works best as the seal material. Failure to maintain a leak-proof seal will result in rapid flooding of the ship’s engine room, sometimes enough to sink a ship.
In small ships, gland packings are enough to prevent leakage. In big ships, however, we may need header tanks that provide pressure against the external seawater to maintain the engine room’s integrity.
Location of the stern tube
The stern tube is located at the rear or aft end of the ship. The forward part of the tube is supported by the aft peak bulkhead and the rear part is supported by the stern frame.
The tube opens up into the engine room of the ship, usually at the lowermost deck. Several factors such as the arrangement of the engines, power requirement, dimensions of the propeller shaft and sealing arrangements determine the dimensions and other specifications of the stern tube.
Purpose of a stern tube
Withstand the load
The stern tube supports the load of the propeller shaft and the propeller. The overhanging length of the propeller shaft and the propeller apply a downward force on the
These stresses can become very high during rough weather as the tube must bend along with the ship’s hull while also enduring higher forces from the propeller shaft and the propeller.
Prevent water leakage
The stern tube houses the sealing arrangements to prevent water leakage into the sea. The arrangement includes liners that rotate with the propeller shaft, rubber seals, and oil cavities. In addition to water leakage into the ship, it must also prevent oil leakage into the sea.
Provide minimum resistance around the shaft
The sealing arrangements and specifications of the stern tube must provide minimum resistance to the rotation of the propeller shaft. An optimum amount of resistance is one in which minimal to no leakage occurs without imparting an undue frictional force on the shaft to affect its rotation.
What is a stern tube bearing?
Stern tube bearings are journal bearings that are placed between the propeller shaft and the stern tube to transfer the load of the shaft and the propeller to the stern tube. They also facilitate the smooth movement of the shaft in the tube. The bearings are usually made of white metal.
Types of stern tubes
There are several ways to classify stern tubes. But the most popular among them is based on the type of lubrication. They are of two main types: Sea water lubricated stern tube and oil-lubricated stern tube. Let’s get a brief overview of each.
Sea water lubricated stern tube
In this stern tube, we use sea water in conjunction with wood bearings to lubricate the shaft in the stern tube. The hardwood is sourced from a tree known as the lignum vitae (Tree of Life) and is fitted between the stern tube and the shaft. Compared to an oil lubricated tube, the sea water lubricated tube provides better cooling and environmental friendliness.
However, it has a relatively short service life as the sea water is abrasive in nature. Thus, more maintenance and overhauls of the stern tubes are required in the same time frame for a water-lubricated setup over an oil-lubricated setup.
The sea water lubricated stern tube was primarily used on older ships but it is seeing a comeback as more durable materials have been developed recently. These materials include polymers and composites. They work with sea water lubrication but satisfy the requirements for durability and functionality.
Some new systems are also using sea water purification systems to eliminate the abrasive nature of sea water and improve durability.
Oil-lubricated stern tube
As the name suggests, the oil-lubricated stern tube uses oil-based lubricants for stern tube lubrication.
Even though oil is less dense than water, it has a higher viscosity. Where water flows readily, oil does not. This helps it to maintain a thin film under pressure where water would have escaped resulting in metal-to-metal contact.
This property is especially important for our stern tube application. The properties of oil enable hydrodynamic lubrication even at extremely low speeds and high bearing loads. This means that large vessels with low propeller shaft RPM can be lubricated well by oil.
The oil-lubricated stern tube has a much longer life. Even with the most advanced steels, however, we cannot completely avoid oil leakage into the sea.
But there have been workarounds that have reduced the impact of this leakage over the years. Environmentally Acceptable Lubricants (EALs) can replace mineral oils in some systems but their cost can be up to eight times higher than mineral oils.
Types of Stern Tube Seals
Stern tube seals prevent the leakage of oil into the sea or water into the stern tube. These are of three main types:
The stuffingbox uses a gland packing similar to a valve. By using the right size of packing, it is possible to completely stop the leakage. But a slight leakage is allowed to ensure cooling of the packing and the forward area of the stern tube.
Should the stern tube start leaking more, the engineers have the temporary option to tighten the gland packing to a certain limit before replacement becomes due.
The lip seal is a more advanced type of seal that presses against a liner which is in contact with the shaft. It is supported by a garter spring as well as the fluid pressure against the shaft.
There are usually four seals at the propeller end of the stern tube and two at the engine end. The propeller end seals are known as aft seals and those at the engine end are called forward seals.
They are made from a wide range of materials. Some of these are Neoprene, Viton, FKM, PTFE, Nitrile, and EPDM.
Radial face seals
Radial face seals have a flat surface in contact with the shaft to provide sealing. The faces are held in contact against the shaft through special springs.
A unique feature of radial seals is the split construction of all the components. This enables ease of installation, inspection and maintenance.
Stern tubes are a key component of the propulsion system for any ship. In addition to safety, they are also very important to maintain high operational efficiency. They must, therefore, be designed well to ensure their integrity and smooth operation.
By keeping a vigilant eye on the system, it is possible to spot signs of leakage and excessive wear well in advance. With proper care, the stern tube overhaul can be extended from the routine 5 years to well more than that.