The patent slip or Marine Railway was invented by Scot Thomas Morton in 1818 as a cheaper alternative to a dry dock for ship repair. It consisted of an inclined plane, which extended well into the water, and a wooden cradle onto which a ship was floated. The ship was then attached to the cradle and hauled out of the water up the slip.
In 1832, a House of Commons Select Committee, under the chairmanship of the, Rt. Hon. Sir George Cockburn, was convened to adjudicate on a petition by Thomas Morton to extend the duration of his patent. The committee eventually did not support the patent extension. It was claimed that not only was the cost of a slip one tenth of a dry dock but also by hauling completely onto a clear dry area that it was easier to carry out the maintenance.
Early designs would have used manpower and block and tackle to provide mechanical advantage, while later solutions used steam engines. One of the earliest surviving marine railways is the Creque Marine Railway on Hassel Island in the Caribbean.
The process of slipping a vessel is an easy, cheap and straightforward way to take a large vessel out of water for inspection or repair. In many cases, it is possible to take a boat out of the water on one tide, make any repairs and return the boat to the water on the next tide. In tidal harbours and ports, it is normally necessary to wait for high tide. The first step involves the cradle being lowered to the bottom of the inclined plane (or slipway) at which point the vessel will move into position directly above the cradle. The vessel will then be moored to the cradle with a number of ropes fore and aft to prevent the vessel from moving in any direction.
Once the vessel is secured to the cradle, the process of hoisting the cradle out of the water and up the slipway will begin. 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 off it when totally out of the water. Normally the boat will sit on large wooden wedges when she starts to lift out of the water, although in some cases, larger boats may be temporarily welded to the cradle by divers. When the engineers and staff who operate the slipway are satisfied that the vessel is sitting correctly in the cradle, it is then hoisted up to the top of the slipway, normally beyond the high tide mark.
Originally, men or horses would have been used to drag the cradle and vessel up the slipway, but with the advent of the steam engine, most patent slips would be converted to steam powered operation. Today electric or electro-hydraulic winches are the norm.
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 patent slip avoids the need to pump out a dry dock, saving time and money. Large marine railways can handle vessels of 6,000 tons. 
- Morton, Thomas, and John Barclay (12 March 1824). Infringement of a patent: notes of a trial before the Jury Court at Edinburgh. W. Reid & Son.
- Prosser, R. B. "Morton, Thomas (1781–1832), shipbuilder and inventor of a ship-building slip". Oxford Dictionary of National Biography. Oxford University Press.
- (See http://trid.trb.org/view.aspx?id=401682, Shipbuilding Technology International (UK) journal article)
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