Free Chatham Dockyard by Philip MacDougall Page B

material other than timber. As such, it also paved the way for the complete rebuilding of three of the original docks, the No.5 Dock having been deemed better suited to conversion into a slip. The new No.3 Dock had a length of 225ft and a maximum width of 90ft. Internally it was characterised by stepped stone sides, these rising from the base of the floor to the ground level of the dockyard. Essential features of the dock, the steps were used both as a working area and as wedges to hold the shores resting against the sides of the otherwise unsupported vessel that stood within the dock. Divided into an upper and lower set, the top section of four steps was much steeper than a lower group of thirteen. Dividing the two sets was a broad central step that was primarily designed as a walkway. Access to the dock was by means of six sets of stone staircases, these positioned at the head, mid and aft sections of the dock. Finally, for the easy delivery of material to be used in the repair and building of ships, there were a number of stone slides (or ramps) which, at varying intervals, ran over the top of the stepped sides.
    A particularly unusual feature demonstrated by the draft plan of the new dock was the use of a comparatively sophisticated entrance arrangement. As far as the existing docks were concerned, the entrance of a vessel was through a single pair of gates that were set either side of the slightly narrowing aft end of the dock. Held shut by a series of timber shores wedged between the gates and side walls of the dock, these gates were opened and closed through the application of manual power. The new dock dispensed with such a simple, if labour-intensive arrangement, introducing an entry neck that could be sealed at both ends. On the side of the neck closest to the dock was a pair of timber gates that could be opened and closed through the use of a capstan and chain device. The capstan was turned by only a small number of labourers, while the chains adhering to the gate held them firmly closed and overcame the necessity of using numerous timber shores for wedging purposes. According to Rennie’s draft plan, these gates spanned an entrance area of 57ft and opened out on to a special recess that prevented them becoming accidentally entangled with any vessel floated into the dock. 3
    The river side of the short entry neck was sealed by a floating chest known as a caisson. Filled with water, a caisson would rest on the bed of the river, but once pumped free of water, could be floated and repositioned. Fixed into grooves that ran along the sides and bottom of the entry neck, it had the advantage of being completely removable and so allowed for the creation of a much wider entrance which was free of the additional need for a gate recess point. The use of a caisson was, during the second decade of the nineteenth century, a fairly novel innovation but, over time, completelyreplaced the earlier entry gate system. That Rennie chose to use a caisson as the first of two entry seals results from a further drawback in the use of gates, this being that no matter how well they were sealed, water always leaked both through the central join and hinged sections. By introducing a short neck, any water gaining access past the caisson could be drained away before it reached the gates.

Ground plan of Brunel’s saw mill, which dates to 1814.
    By way of his illness and eventual death in October 1821, Rennie was never to see the first stone dry dock at Chatham completed. Yet its continued existence within the historic dockyard serves as a suitable tribute to this great engineer and the frequent visits he made to Chatham to ensure its overall progress. Without his insistence that the dock was to be built entirely of stone and finished to the highest standards, it could not possibly have survived in virtually its original state for over 180 years. Indeed, even at the time of writing, the dock is still in use, having been engaged in the repair of