The Probert Encyclopaedia of Ships

DOCKS

Docks are usually artificial inclosures for the reception of vessels, and provided with gates to keep in or shut out the tide.

They are called wet-docks when they are intended to receive vessels for loading and unloading, the gates being in this case constructed so as to keep in the tide, and thus preserve the water within the docks as nearly as possible at the uniform level of high water.

They are called dry-docks, or graving-docks, when they are intended to admit vessels to be examined and repaired, the gates in this case being such as to keep out the tide while the shipwrights are engaged on the vessel.

There is another kind of dry-docks called floating-docks, which float on the surface of the water, and may be sunk sufficiently to allow of a vessel being floated into them, and then raised again, by pumping the water out of the tanks round the sides.

One of the chief uses of a wet-dock is to keep a uniform level of water, so that the business of loading and unloading ships can be carried on without any interruption and without danger of damage to the vessel from straining, low tides, storms, etc. The first wet-docks constructed in England were those later called the Commercial Docks, in London, which existed in a much less extensive form so early as 1660. In 1800 the West India Docks were constructed, and were followed by the East India Docks, Millwall Docks, London Docks, the St Katharine Docks, and the Victoria Docks, affording, together with those at Tilbury, more than 600 acres of water accommodation, besides wharf and warehouse grounds, where all kinds of appliances and machinery for the speedy and convenient transfer of goods and cargoes were formerly in use. Some of the warehouses were extremely capacious, the tobacco warehouse of the London Docks was itself nearly five acres in extent.

Next after the London docks came those of Liverpool, which extended more than six miles along the north bank of the Mersey, and covered, together with the Birkenhead docks, nearly as large a total acreage as those of London. The other important British docks were those at Southampton, Bristol, Cardiff, Hull, Great Grimsby, Newcastle, Shields, Barrow, Leith, Glasgow, Dundee, etc.

Graving-docks are built of strong masonry, and their entrance is closed either by swinging gates opening in the middle, and when shut presenting a salient angle to the water in the river or harbour from which the dock is entered, or by a framework called a caisson, built like the hull of a ship, with a keel and a stem at both ends. When the caisson is empty it floats, and may be removed to admit of a vessel being floated into the dock. The caisson being then placed at the entrance and filled with water, again sinks into the grooves intended for it and closes the graving-dock. The water is then pumped out, leaving the ship dry and supported by wooden blocks and props. With regard to floating-docks, a common type of construction was the iron floating-dock built in water-tight compartments, and not closed in at either end. It was sunk to the required depth by the admission of water into so many of the compartments, until the vessel to be docked could float easily above its bottom, and it was then raised by pumping out the water until the ship can be propped up as in a dry-dock.

A kind of dry-dock, called the hydraulic lift dock, was popular in the Victorian era and consisted of a double row of iron columns, each of which contained a hydraulic press. All these hydraulic presses could be worked simultaneously by a powerful steam-engine, and their combined action had the effect of raising a series of transverse iron girders stretching from the columns on one side to those of the other. An iron pontoon was first floated above these girders, and then sunk so as to rest on them, and the ship to be docked was floated above the pontoon and supported by blocks resting only upon the pontoon, so that the ship was in no way connected with the columns on each side. The hydraulic presses were then set to work, the. girders with the pontoon and ship were raised high enough for the water to be run out of the pontoon, which was then sufficiently buoyant to float the ship. The pontoon was then floated away clear of the dock, and another took its place. By this plan a number of vessels could be floated for overhauling and repairs in very shallow water and at comparatively slight expense.
Research Docks