Understanding Stern Tube Arrangement on Ships

Everyone knows that a ship is propelled with the help of a propeller located at the aft of the ship. However, many have a doubt as to how the propeller is connected to the engine located inside the ship. A stern tube, situated at the aft of the ship, helps in this purpose.

Introduction

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.

What is a Stern Tube?

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.

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

References:
Marine Aux machinery by H.D.McGeorge

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