Slipway
From Wikipedia, the free encyclopedia
A slipway, boat slip or just a slip, is a ramp on the shore by which ships or boats can be moved to and from the water. They are used for building and repairing ships and boats. They are also used for launching and retrieving small boats on trailers and flying boats on their undercarriage. The nautical term ways is an alternative name for slipway. A ship undergoing construction in a shipyard is said to be on the ways. If a ship were scrapped there, she is said to be broken up in the ways.
As the word "slip" implies, in theory the ships or boats are moved over the ramp, standing on a sledge, with help of grease. Slipways are used to launch (newly built) large ships, but only can dry-dock or repair smaller ships. Pulling large ships against the greased ramp would require too much force. For dry-docking large ships, one must use carriages supported by wheels or by roller-pallets. These types of dry-docking installations are called "marine railways". Nevertheless the words "slip" and "slipway" are also used for all dry-docking installations that use a ramp.
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[edit] Simple slipways
In its simplest form, a slipway is a plain ramp, typically made of concrete, steel, stone or even wood. The height of the tide can limit the usability of a slip: unless the ramp continues well below the low water level it may not be usable at low tide. Normally there is a flat paved area on the landward end.
When used for building and repairing boats or small ships (i.e. ships of no more than about 300 tons), the vessel is moved on a wheeled carriage, which is run down the ramp until the vessel can float on or off the carriage. Such slipways are used for repair as well as for putting newly built vessels in the water.
When used for launching and retrieving small boats, the trailer is placed in the water. The boat may be either floated on and off the trailer or pulled off. When recovering the boat from the water, it is winched back up the trailer.
[edit] Lifeboat slipways
To achieve a safe launch of some types of land-based lifeboats in bad weather and difficult sea conditions, the lifeboat and slipway are designed so that the lifeboat slides down a relatively steep steel slip under gravity. It is winched back up afterwards.
The Royal National Lifeboat Institution in the United Kingdom currently operates 3 different classes of lifeboat from its slipways. The Tyne, Mersey and most recently the Tamar.
[edit] Ship construction
For large ships, slipways are only used in construction of the vessel. Normally they are arranged perpendicular to the shore line (or as nearly so as the water and maximum length of vessel allows) and the ship is built with its stern facing the water. The slipway then takes the form of a plateway with the two plates nearly horizontal. They are actually arranged in the form of a very flat 'V' to give stability to the vessel being worked upon. The plateways extend to well below the water level taking into account tidal variations. The vessel is built upon a wooden frame that is constructed as the ship extends up from the keel.
The process of transferring the vessel to the water is known as launching and is normally a ceremonial and celebratory occasion. It is the point where the vessel is formally named. At this point the hull is complete and the propellors and associated shafting are in place, but typically the engines have not been fitted and the superstructure has not been built.
On launching, the vessel slides backwards down the slipway on the frame until it floats by itself.
Some slipways are built so that the vessel is side on to the water and is launched sideways. This is done where the limitations of the water channel would not allow lengthwise launching, but occupies a much greater length of shore. The Great Eastern built by Brunel was built this way as were many Landing Craft during World War II. This method requires many more plate runs to support the weight of the ship.
In both cases heavy chains are attached to the ship and the drag effect is used to slow the vessel once afloat until tugboats can move the hull to a jetty for fitting out.
The practice of building on a slipway is dying out with the very large vessels introduced from about 1970. Part of the reason is the space requirement for slowing and manouvering the vessel immediately after it has left the slipway, but the sheer size of the vessel causes design problems, since the hull is basically supported only at its end points during the launch process and this imposes stresses not met during normal operation.
[edit] Different means for dry-docking and launching of ships
- Mobile boat lift, also called a Travelift, a dry-docking and transport on yard, with a maximum capacity of 250 tons weight.
- Slipway, for dry-docking small boats and for launching large new ships.
- Marine railway, for dry-docking and launching ships, with a capacity up to 2,000 tons
- Floating dry dock, for repair of ships, with a maximum capacity up to 5,000 tons weight
- Graving dry dock, for building and for repair, with unlimited size of ships.
- Shiplift, the most modern way of dry-docking and launching, for ships weighing of 800-25,000 tons.
