Slipway
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.
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 drydocking large ships, one must use carriages supported by wheels or by roller-pallets. These types of drydocking installations are officially called "
Marine railways". Nevertheless the words "slip" and "slipway" are also used for all drydocking installations that use a ramp.
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 usablity 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.
|
Swanage lifeboat being winched back up its slipway after a launch. |
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
RNLI currently operates 3 different
classes of lifeboat from its slipways. The
Tyne,
Mersey and most recently the
Tamar. More details on the
RNLI can be found on their website -
www.rnli.org.ukMore details and photos can be found on Swanage Lifeboat station's website -
www.swanagelifeboat.org.ukFor 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
WWII. 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.
* Mobile boat lift, also called Travelift. Drydocking and transport on yard, with a maximum capacity of 250
tons weight.
*
Slipway, for drydocking small boats and for launching large newbuildings.
*
Marine Railway, For drydocking and launching ships, with a capacity up to 2000 tons
*
Floating drydock, for repair of ships, with amaximum capacity up to 5000 tons weight
*
Graving dock, for newbuilding and for repair, with unlimited size of ships.
*
Shiplift, the most modern way of drydocking and launching, for ships weighing of 800-25 000 tons.
*
Ferry slip*
Port*
Harbour*
boatlaunch - slipway listings
