Screw Lifting Frame
The ss Great Britain had originally been fitted with a six-bladed propeller, and with this machinery she successfully completed her maiden crossing of the Atlantic. However, the design of screw propellers for ships continued to evolve as ship-builders developed their practical understanding of the new technology. By 1852, when the ss Great Britain set sail on the first of her many voyages to Australia, she had already been fitted with five models of propeller in succession. Each of these represented a practical improvement on an earlier version.
In 1857, the ship underwent a major refit after her return from trooping voyages to the Russian Crimea. At this stage, the ship’s owners had an opportunity to introduce improvements, and to address one major problem associated with the use of a propeller. This problem arose from the fact that the propeller was not always in use. Whenever the wind came from the right quarter she could be sailed, and this had the advantage that she did not need to carry enough fuel to steam for the whole journey. However, when the engines were not in use, the propeller caused drag, slowing the ship’s progress under sail. There was considerable competition between shipping companies to achieve short journey times to Australia, and so the drag caused by the propeller was a serious economic disadvantage.
In order to get round this problem the ship’s owners replaced the existing propeller with a new, two-bladed model, which was supported in a special frame. The one critical innovation associated with this propeller was that when it was not in use it could be lifted up into a cavity within the ship’s stern. This was achieved by first disconnecting the propeller from the shaft inside the ship which linked it to the driving wheel, and then winching it up from above. This was a labour-intensive process, which occupied up to 30 men for half an hour each time it was performed. However, it was also very efficient, and the improvement to the ship’s performance was such that she continued to use this mechanism for another 18 years.
After the ship herself, the screw lifting frame is one of the largest artefacts in the ss Great Britain Trust’s collection. Until 1970 it was part of the ship, although it had been covered with planking after 1875, when the propeller was no longer in use. However, when the ss Great Britain was salvaged and returned to Bristol, those responsible for her conservation decided to remove the lifting frame together with the rudder attached to it, in order to replace these with replica versions of the original 1843 propeller and rudder designed by Brunel. After their removal from the ship, the frame and the rudder were propped against the wall of the dry dock, and there they remained for the next 30 years.
In late 2002 it was decided that the lifting frame would form the ideal centrepiece for a new exhibition focusing on the conservation of the ss Great Britain. The logistics of moving this object, which is five metres high and weighs sixteen tonnes, presented a major challenge, but it was in any case necessary to move it before commencing the construction of the glass plate which will cover the dry dock. An eighty-ton crane was hired to raise the lifting frame out of the dry dock and onto a specially built cradle, weighing 2 tonnes, which can support it in a semi-upright position. The lifting frame in its cradle was then lifted onto a low-loader and hauled out of the dockyard, round some very tight corners. The most difficult part of the operation, however, was lifting and winching it into the exhibition space, through a doorway slightly too small to admit the artefact with ease. This was all good practice for the artefact’s next move (in 2004) into a new dockyard museum – when it will probably have to go in through the roof!
Interpreting the Screw Lifting Frame
In 1857, the addition of the lifting frame to the ss Great Britain was a very important technological innovation, which increased her efficiency and her economic viability as an emigrant ship. The fact that the innovation was superseded in 1882 by the ship’s conversion to sail is also an important part of the s Great Britain’s story, and the decision taken in the 1970s to cut the lifting frame off the ship tells us much about recent changes in our ideas about conservation. It is important to help visitors to the exhibition understand the function and historical significance of the screw lifting frame.
In order to illustrate the function of the lifting frame, a replica propeller has been commissioned for the artefact. The design for the replica propeller, and our understanding of how the lifting frame originally worked, are based on a number of nineteenth-century sources. Although there is no original plan for the design of the ss Great Britain’s screw lifting frame, there are a number of important contemporary analogues which, together with the evidence provided by the object itself, can help us to reconstruct the propeller. We are also fortunate to have journalistic and diary accounts of the arrangement and use of the screw lifting apparatus. By drawing on such sources, we can help modern visitors see the lifting frame through the eyes of nineteenth-century travellers.
The installation of this propeller will reintroduce an important dynamic element to the lifting frame, and in time we plan that it should be made to turn slowly on the shaft. It may even be possible eventually to show the screw being lifted up and down in the frame, by replicating some other elements of the apparatus which are missing at present. More research is needed, however, in order to develop the designs for fully illustrative replica elements.