Patent slip

The new Big Chute Marine Railway
The old Big Chute Marine Railway, showing the cradle and rail system.
Patent slip at Arbroath Harbour
"Slipping" a vessel. Cartoon of a vessel in profile view through the course of being slipped.

The patent slip or marine railway is an inclined plane extending from shoreline into water, featuring a "cradle" onto which a ship is first floated, and a mechanism to haul the ship, attached to the cradle, out of the water onto a slip. The marine railway was invented by Scot Thomas Morton in the early 19th century, as a cheaper alternative to dry docks for marine vessel repairs, in particular below waterline. Larger modern marine railways can handle vessels of thousands of tons.

History

Invented by shipwright Thomas Morton in 1818,[1][2] the marine railway offered an alternative to the expensive and time-consuming process of dry docking a ship to perform maintenance or repairs to its hull below waterline. The means and mechanisms over time became various, but always include a "cradle" onto which the ship is floated, and a mechanical mechanism for transferring the ship from water to land up an incline. The destination where work was performed was termed the slip.

Thomas Morton petitioned to extend the duration of his patent in 1832, before a Select Committee of the House of Commons. The committee, chaired by the Rt. Hon. Sir George Cockburn, was convened and heard the claim that the slow construction and implementation of such slips required the extension, that the cost of using such slip was one tenth that of using a dry dock, and that by hauling completely onto a clear area it was easier to carry out maintenance. The committee was sympathetic, in a particular regarding the small return that Morton had seen during the initial patent period, but ultimately did not support the requested extension.[3]

Operation

The process of slipping a vessel is an inexpensive and straightforward way to take a large vessel out of water for inspection or repair. In tidal harbors and ports, it is normally necessary to wait for high tide. In many cases, it is possible to take the vessel out of the water on one tide, and to make repairs and return it to the water on the next tide.

The first step in use of a patent slip or marine railway system involves a "cradle" being lowered to the bottom of the inclined plane (the slip/slipway), at which point the vessel is moved into position directly above the cradle. The vessel is then be moored to the cradle with a number of ropes fore and aft to prevent the vessel from moving in any direction. Large marine railways can handle vessels of 6,000 tons.[4]

Once the vessel is secured to the cradle, the process of hoisting the cradle out of the water and up the slipway begins. Care is taken when the cradle starts to bear the weight of the vessel; if the vessel is not sitting correctly in the cradle, it may damage the cradle or fall from it when fully out of the water. The vessel will usually sit on large wooden wedges when the lift out of the water begins; in the case of larger vessels, they may be temporarily welded to the cradle by divers. When slipway engineers and staff are satisfied that the vessel is correctly cradled, it is hoisted to the top of the slipway, normally beyond the high tide mark.

Originally, men or horses and block and tackle were used to provide mechanical advantage to drag the cradle and vessel up the slipway. With the advent of the steam engine, most marine railways were converted to steam powered operation. For remaining railways in existence, electric or electro-hydraulic winches are the norm.

Reasons for use

A typical reason for slipping a vessel is so as to clean and paint it, and in particular, to apply anti-fouling protection to the hull. Other uses include repairs below the waterline, the replacement of propellers, inspections for insurance purposes, or the fitting of cathodic protection. Using a marine railway/patent slip avoids the need to pump out a dry dock, saving time and money.[4]

Examples

One of the earliest surviving marine railways is the Creque Marine Railway on Hassel Island in the Caribbean.

References

  1. ↑ Prosser, R. B. "Morton, Thomas (1781–1832), shipbuilder and inventor of a ship-building slip". Oxford Dictionary of National Biography. Oxford University Press.
  2. ↑ Morton, Thomas & John Barclay (12 March 1824). "Infringement of a patent: notes of a trial before the Jury Court at Edinburgh". W. Reid & Son.
  3. ↑ Stuart Cameron (2005) "The Patent Slip", ClydeSite Magazine, Issue 5, Article 4 (23 March 2003), see , accessed 30 June 2014.
  4. 1 2 Shipbuilding Technology International (UK) journal article, see
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