An overview of the shaft line arrangement on a ship
This is the first in a series of articles about the shaft line. When sail began to give way to steam in the early 19th century, one of the first problems facing the new breed of ship engineering designers was how to get the power from inside the hull to where it was needed outside.
Drawing on methods used to move water as opposed to moving objects in water, for most of the fist 50 years of mechanised water transport it was paddle wheels that dominated. Some inventors experimented with ways to employ an Archimedean screw and over time this concept evolved into the disc propeller that has become the accepted means in all but a small handful of propulsion applications.
Paddle wheels, whether side or aft mounted could have their driving arrangements above the waterline but disc propellers and screws relied on power being taken through the hull below the waterline by way of a stern tube. Quite clearly this involved a need to have an opening in the hull through which the shaft line attached to the propeller could transmit the propulsive power but such an opening would also have the undesirable effect of allowing water in. This the problem of developing a means of sealing was posed.
This was not the only problem that needed to be overcome because there also needed to be a way in which the power and motion from the steam – and later diesel – engine could be taken to the propeller. With most ships the weight of the engine and its effect on the trim dictated a location amidships and this necessitated a long propeller shaft line passing through a shaft line tunnel to the propeller at the rear. Through the second half of the last century, the loss of cargo space attributable to the shaft line tunnel has seen the engines gradually moved aft and the effect on trim offset by forward ballast tanks and cargo holds.
Propeller shaft lines connecting the engine to the propeller must be supported at points along its length and the whole arrangement must be designed to absorb and compensate for the various forces that will be acting upon it, the ship’s structure and the engine or gearbox. In a direct drive situation, the propeller shaft line is bolted to the flywheel of the main engine and then passes through the thrust box.
The purpose of the thrust box is to transmit the torque produced by the propeller into the structure of the ship and to reduce axial movement of the shaft line in order to protect the engine from being damaged by the forces. The shaft line may consist of more than one intermediate shaft lines then passes through the stern tube and the propeller is attached. In between the thrust box and the stern tube there will be one or more intermediate bearings supporting the shaft line and absorbing some of the forces. These intermediate bearings also aid in reducing the effect of any minor misalignments.
The stern tube itself has traditionally had two bearings one at the forward end and one at the aft. In recent times it has become common for the forward bearing to be omitted and only a single aft bearing use in the stern tube. Some engineers have questioned the wisdom of the new arrangement, but it is permitted by most classification societies. It is appropriate to state here that the shafting arrangements are governed by classification society rules and the calculations and restrictions involved are extensive. As an indication, the DNV GL rules for shafting arrangements are contained in a 72-page document.
Although uninformed persons consider that a ship and its components are rigid structures, this is not the case. In operation, a ship’s hull flexes through various axes and even components as large and heavy as the main engine are not completely rigid. The propeller shaft line and its bearings must be flexible enough to absorb vibrations and accommodate and allow the movements caused by hogging and sagging, different degrees of propeller immersion caused by pitching and rolling in different sea states and the strain exerted by the propeller when manoeuvring. In addition, the inevitable expansion and contraction of the propeller shaft line due to heat both caused by friction and ambient temperature is an issue to be taken into account.
Ensuring shaft line alignment
It is important for the original shipbuilder and subsequent repair contractors to ensure that the shaft is correctly aligned between the propeller and the engine to avoid premature failure of the shaft line or excessive bearing wear. A misaligned shaft line can be caused by even quite small errors in location of bearings, the thrust box or even the main engine. Checking the alignment is a skilled process that is best perform by specialist. If the shaft line is found to be mis-aligned remedial action should be taken. This may be done by work on the bearings or in extreme cases by machining of the shaft line.
Propeller shaft lines and the couplings are generally constructed from forged steel but hot rolled steel can be used in some applications. The diameter of the shaft line will depend upon the installed power and propeller size and weight. Until quite recently a diameter of 500mm would have been considered high but there are now ships with shaft lines up to 800mm diameter.
A range of different materials are used for the bearings the shaft line runs in. A long-time material that is still in use in some ships is the hard wood lignum-vitae. Lignum-vitae is incredibly hard wearing and has the added advantage of containing natural lubricants. Today the original lignum-vitae species have become rare and substitute species are used instead. As a consequence, most ships now make use of other materials such as white metal, synthetic rubber, resin or composites.
All bearings need to be lubricated and for the thrust block and intermediate bearings where there is no risk of leakage to the sea, this is not a problem. The question of stern tube lubrication is more controversial. Except for Lignum-vitae, most stern tube bearings have traditionally been lubricated using mineral oil.
Despite the stern tube having a seal at the outer face, there is an inevitable amount of leakage as the seals deteriorate or are damaged. This has been identified as a pollution risk and in most areas of the world, a sheen of oil at the stern of the ship spotted by PSC inspectors will see the ship detained until the leaking seal is replaced.
In the US, the advent of the VGP regulation now means the use of mineral oils is now only permitted in special circumstances. To ensure compliance with the regulations, lubricant manufacturers have developed environmentally acceptable lubricants (EALs) as a replacement for mineral oil.
An alternative to EALS are the water lubricated bearings that are made by a small number of companies such as Wartsila Seals & Bearing and Thordon Bearings. Water lubricated bearings are not a pollution risk under any circumstances.
EALs have been well received but an increasing number of propeller shaft line failures on ships making use of them has raised questions which are currently being investigated by class societies and by insurers who are obliged to cover the costs that arise if a shaft line fails.