A davit is any of various cranelike devices used on a ship for supporting,
raising, and lowering boats, anchors, etc.
Davit systems are most often used to lower an emergency lifeboat to the
embarkation level to be boarded. Davits can also be used as man-overboard safety
devices to retrieve personnel from the water.
The lifeboat davit has falls that are used to lower the lifeboat into the
water. Davits can also refer to single mechanical arms with a winch for
lowering and raising spare parts onto a vessel and for lowering any other
equipment from the deck of a vessel or a pontoon to the water. The maintaining
and operation of davits is all under jurisdiction of International Maritime
Organization. The regulations are enforced by the country's own Coast
Guard. Development
The development of the davit from its original "goose neck form" to the
current devices advanced greatly when A.P. Schat patented a number of systems
in 1926 that allowed the lifeboat to glide over obstructions on a ship's
hull, known as the "Schat Skate". This was followed by a self-braking winch
system that allowed the lifeboat to be lowered evenly. The standard became so
common that shipyard specifications call for Schat-type davits which are
available from various sources. Similar systems developed by Schat companies are
used on offshore oil or gas rigs, being placed around the structure.
Development of the davit has also been in terms of material. Traditionally
davits have been made in aluminium or steel but recent advances in composite
material have led to the manufacture of davits in carbon fibre which has an
excellent power to weight ratio. This means davits can be stowed away when not
in use and the same davit used in multiple deck sockets fitted permanently
on deck. Lifeboat Davit types
Davits are designed to fit into deck spaces that the naval architects deemed
necessary: • Radial — Hand powered davit. This type
was used on the Lifeboats of the RMS Titanic. Each arm must be rotated out
manually; uses manila rope falls. Goose-neck shape to the arm that is
swung out. • Mechanical — This type is like the
radial davit, but both arms are moved out at the same time using a screw
system; uses manila rope falls. • Gravity — There are multiple forms;
one man can operate; uses wire falls. • Roller — Davit slides down a track,
bringing the davit to the embarkation deck.
• Single pivot — One pivot point where the lifeboat is moved over the side of
the craft. • Multi pivot — Common on Promenade
decks of cruise ships. Useful where space is limited.
• Free Fall — Lifeboat slides right off vessel. Lifeboat must be an enclosed
type. Main type of Davit on merchant ships now. This type does not use falls.
• Fixed — Common on Oil Rigs. Lifeboat is hung above the water and lowered into
the water. Basic Parts
Liferaft: These can be Enclosed, partially enclosed, or open.
Frapping Lines: These lines are used on all davits except the fixed and freefall
davits. The frapping line are used to pull the lifeboat over to the
embarkation deck along with the tricing pendent to be loaded.
Gripes: Ropes used to hold the Lifeboat in the stored position while underway.
Tricing pendants: Lines used to initially pull the lifeboat over to the
embarkation deck so that the Frapping lines can be connected.
Releasing mechanisms There are 3 basic systems used to
release the lifeboat from the davit. On-load: For this style of release
mechanism, the lifeboat can be released at any point from the davit. This type
of system allows a lifeboat to be released when it is not in the water,
whether this is because of the emergency or an accident. Because of this, during
an evacuation the release mechanism must be watched to make sure there is not an
accidental activation. Offload: This release mechanism requires
the weight of the lifeboat to not be on the hook when it is released. This
includes the titanic era Monomony hook design that requires someone to remove
the hook from the lifeboat by hand. But this type also includes the hydrostatic
system many lifeboats use now. For this, a float is raised up and engages the
release once the craft is in the water to the right depth.
Free Fall This type of release mechanism is very
basic. The lifeboat is on a ramp and slides down and off of the ship when
engaged. This is done by pumping a lever that is inside the lifeboat by the
pilot. If there is not enough hydraulic pressure to release the stop fall. A
pump on the inside must be rotated to build up the hydraulic pressure to
release the lifeboats stopfall hook. Once the stopfall hook is released the
lifeboat will slide off the ramp and into the water. This type of lifeboat is
more common due to its quick deployment and ease of operation. This is the type
of davit used for lowering the enclosed lifeboat in the movie Captain Phillips.
Procedure For all lifeboats using a roller gravity
davit and Rottmer releasing gear, this is the procedure:
1. Make sure the Davit tracks are clear of debris
2. Remove the lifeboat cover if applicable
3. Put in the lifeboat plugs 4. At this time the Rottmer releasing
gear is checked to be secure. 5. Attach the sea painter to the bow of
the ship. 6. Remove the gripes from the lifeboat.
7. Make sure the gripes preventing bar is free from the track.
8. Have the assigned brake man life the braking bar and lower the lifeboat to
the embarkation deck. 9. Now that the Tricing pendants have
pulled the lifeboat in close, attach the Frapping lines to the wire falls by
passing them to a worker who is on the lifeboat.
10. Load the lifeboat. Load one person at a time and seat them so their weight
is distributed between the two sides. 11. Have the brake man raise the
lifeboat slightly so the workers can release the Tricing pendants.
12. Brake man lifts bar and lowers the vessel into the water.
13. Operate the Rottmer releasing gear and release the davit on the crest of a
wave. 14. If ship is sinking, pull toggle pin
to release the sea painter from the ship.
During this procedure, if the lifeboat has air-cooled engines start it at the
beginning. If it has a water-cooled engine start the engine after the
lifeboat is in the water. Gallery
References