Dry dock

A dry dock (sometimes drydock or dry-dock) is a narrow basin or vessel that can be flooded to allow a load to be floated in, then drained to allow that load to come to rest on a dry platform. Dry docks are used for the construction, maintenance, and repair of ships, boats, and other watercraft.

The use of dry docks in China goes at least as far back as the 10th century A.D. In 1088, Song dynasty scientist and statesman Shen Kuo (1031–1095) wrote in his Dream Pool Essays:

At the beginning of the dynasty (c. +965) the two Che provinces (now Chekiang and southern Chiangsu) presented (to the throne) two dragon ships each more than 200 ft. in length. The upper works included several decks with palatial cabins and saloons, containing thrones and couches all ready for imperial tours of inspection. After many years, their hulls decayed and needed repairs, but the work was impossible as long as they were afloat. So in the Hsi-Ning reign period (+1068 to +1077) a palace official Huang Huai-Hsin suggested a plan. A large basin was excavated at the north end of the Chin-ming Lake capable of containing the dragon ships, and in it heavy crosswise beams were laid down upon a foundation of pillars. Then (a breach was made) so that the basin quickly filled with water, after which the ships were towed in above the beams. Then (breach now being closed) the water was pumped out by wheels so that the ships rested quite in the air. When the repairs were complete, the water was let in again, so that the ships were afloat once more (and could leave the dock). Finally the beams and pillars were taken away, and the whole basin covered over with a great roof so as to form a hangar in which the ships could be protected from the elements and avoid the damage caused by undue exposure.

The Greek author Athenaeus of Naucratis (V 204c-d) reports something that may have been a dry dock in Ptolemaic Egypt in the reign of Ptolemy IV Philopator (221-204 BC) on the occasion of the launch of the enormous Tessarakonteres rowing ship. However a more recent survey by Goodchild and Forbes does not substantiate its existence.

But after that a Phoenician devised a new method of launching it (the Tessarakonteres), having dug a trench under it, equal to the ship itself in length, which he dug close to the harbour. And in the trench he built props of solid stone five cubits deep, and across them he laid beams crosswise, running the laces whole width of the trench, at four cubits' distance from one another; and then making a channel from the sea he filled all the space which he had excavated with water, out of which he easily brought the ship by the aid of whatever men happened to be at hand; then closing the entrance which had been originally made, he drained the water off again by means of engines (organois); and when this had been done the vessel rested securely on the before-mentioned cross-beams.

It has been calculated that a dock for a vessel of such a size might have had a volume of 750,000 gallons of water.

Before the 15th century, when the hull below the waterline needed attention, careening was practised: at high tide the vessel was floated over a beach of hard sand and allowed to rest on one side when the tide receded. An account of 1434 described how a site near Southampton with a bottom of soft mud was selected for the warship Grace Dieu, so that the hull would bed itself in and remain upright at low tide. A timber, brushwood and clay wall was then built up around the hull. The first early modern purpose-built European and oldest surviving dry dock still in use was commissioned by Henry VII of England at HMNB Portsmouth in 1495. This was a timber-lined excavation, with the seaward end closed off by a temporary revetted bank of rock and clay that had to be dug away by hand (an operation taking typically 29 days, working night and day to accord with the tides) to allow the passage of a ship. Emptying was by a pump, possibly in the form of a bucket-chain powered by horses. This dry dock currently holds First World War monitor HMS M33.

Possibly the earliest description of a floating dock comes from a small Italian book printed in Venice in 1560, called Descrittione dell'artifitiosa machina. In the booklet, an unknown author asks for the privilege of using a new method for the salvaging of a grounded ship and then proceeds to describe and illustrate his approach. The included woodcut shows a ship flanked by two large floating trestles, forming a roof above the vessel. The ship is pulled in an upright position by a number of ropes attached to the superstructure.