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 can only 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.
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.
Lifeboat slipways
To achieve a safe launch of some types of land-based
lifeboat 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 three different classes of lifeboat from
its slipways: the Tyne, Mersey and, most recently, the
Tamar.
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. Modern slipways take the form of a reiforced concrete
mat of sufficient strength to support the vessel, with two
"barricades" that extend to well below the water level taking into
account
tidal variations. The barricades
support the two launch ways. The vessel is built upon temporary
cribbing that is arranged to give access to the hull's outer
bottom, and to allow the launchways to be erected under the
complete hull. When it is time to prepare for launching a pair of
standing ways are erected under the hull and out onto the
barricades. The surface of these ways are greased. A pair of
sliding ways is placed on top, under the hull, and a launch cradle
with bow and stern poppets is erected on these sliding ways. The
weight of the hull is then transferred from the build cribbing onto
the launch cradle. Provision is made to hold the vessel in place
and then release it at the appropriate moment in the launching
ceremony, these are either a weak link designed to be cut at a
signal or a mechanical trigger controlled by a switch from the
ceremonial platform
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 dependant on the depth of water, stability and weight the
engines might have not been fitted or the
superstructure may not be
completed.
On launching, the vessel slides backwards down the slipway on the
ways 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 sets of ways to support the weight of the
ship.
In both cases heavy
chain 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 maneuvering 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.
Different means for dry-docking and launching of ships
- Mobile boat lift, (also called a Travelift), for vessels up to
1000 metric tons)[86138]
- Marine railway, with a capacity
up to 2,000 tons
- Floating dry dock, with a maximum
capacity up to 5,000 tons weight
- Graving dry dock, with unlimited size
of ships.
- Shiplift, the most modern way of
dry-docking and launching, for ships weighing 800-25,000 tons.
See also
External links
Wikis
Quiz
Map
Search