"The caisson is a hollow gate which closed off the entrance to the dock. Shaped like a ship's hull, it is made of wrought iron and hollow inside.
When filled with water it sank down onto the grooves at the entrance to the dock, so sealing it. When the dock was closed of, it was possible to cross the dock by walking along the wooden deck on top of the caisson. When it was pumped empty by a steam pump, it became buoyant and floated up to allow the ship to be drawn in and tugged out of the dock. Constructed in 1867, it is one of the oldest surviving vessels made by Harland and Wolff. It was launched by the firm in the same way as a ship and was designated 'Hull No. 50' on their shipping list."
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"Caisson -
The primary function of a caisson is a device used for repairing outside damage to the hull at, or below, the waterline while a vessel is afloat. In the case of a graving dock a caisson is an iron or steel structure used for closing the entrance. It is constructed with buoyancy chambers and ballast tanks so that by means of valves and a pump or ejector the weight of the contained water ballast can be varied at will for floating or sinking the caisson into position.
A total of four of these types of caissons were constructed between the 1860s and 1910s for the three graving docks on Queen’s Island.
Hamilton Graving Dock
The oldest surviving example on Queen’s Island is only a short distance from the Titanic Belfast building. Called the Hamilton, it was named after the Chairman of the Belfast Harbour Commissioners, Sir James Hamilton, JP (1815-1882).
The contract for the works was awarded to Thomas Monk & Company in December 1863 with excavation work beginning the following year. The graving dock cost a total of £40,000 to construct and created employment for about 250 men many of whom were Catholics excluded from employment in the shipyards in Belfast on the grounds of their religion. The work was hard, difficult and often dangerous and although the dock claimed only one life during construction there were a large number of accidents resulting in serious injury.
The pumping machinery for emptying the graving dock of water was designed and manufactured by Benjamin Hicks & Sons of Bolton at a cost of £1,200 and housed in a small stone built building with a tall chimney near the entrance. The first steaming of the engine and testing of the pumps took place at the end of May 1867.
The iron caisson for the Hamilton was constructed by Harland & Wolff and assigned the yard number 50. The completion of the caisson, in July 1867, had been delayed several months while the builders awaited delivery of brass control valves from England.
The caisson survives to this day and is believed to be the oldest surviving example of Harland & Wolff’s marine engineering. The structure is practically a vessel in its own right. The wooden ‘keel’ of the caisson, designed to fit perfectly in the granite groove at the entrance of the graving dock was constructed using heavy logs of African Green Heart timber². The frames, plating and rivets connecting each piece of the structure are made of iron and the same method of construction employed as used to build ships. A roadway on the weather or top deck of the caisson was laid originally in heavy oak beams. The pumping machinery, valves and water ballast tanks were also all of the same design and type as fitted by the yard in the vessels under construction at the time. A builder’s plate attached to the shell plating survives to this day but the dock’s name, in large white lettering and indicating its outward face, has long since disappeared under countless coats of protective paint."
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