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Single-Arm Trackway Gravity Davit
The typical boat davit system is made up of five major subsystem. These subsystems are the electrical system, the winch system, the boat davit arms and sheaves system, the boat falls system, and the stowage system. The primary function of the davit arm, (or arms, as applicable) is to swing out the boat from the inboard position to a point outboard of the ship's side from which the boat may be lowered. The reverse of this process occurs when the boat is hoisted. Hoisting operations are controlled by the winch and boat falls from which a hoisting hook (or hooks) is (are) suspended. The number of hoisting hooks is dependant on whether the boat davit is  the  single  arm  or  double  arm  configuration.  The  falls lead from the boat davit winch (the source of power), through the boat davit arms and sheaves to the hoisting hooks. A drum type winch is used with all boat davits having a wire rope fall or falls. A gypsy type winch is used with some boat davits when the falls are synthetic fiber  rope.  Boat  davits  are  either  of  gravity  or mechanical  type. A gravity boat davit requires only the force of gravity to move a boat suspended from the hooks at the inboard position to the outboard position and down to the  water.  The  lowering  evolution  from  the  inboard position to the water is controlled at the winch through the boat falls. The winch's manual brake controls the boat's descent speed and prevents the davit arms from slamming into the outboard stops. The manual brake is also used to stop the boat before it reaches the water to allow the coxswain to start the boat's engines. The power to hoist the boat from the water is provided by the winch. Handcranks can be attached to the winch for hoisting the boat by hand if a loss of electrical power should make it necessary. Gravity boat davits are either of the overhead suspended, trackway, pivoted, or pivoted link type, and may be of the single or double arm  configurations.  Figure  4-13  depicts  the  typical operation  of  a  trackway  gravity  boat  davit. The mechanical boat davit requires the application of an external force to move the boat and davit arms from the inboard to the outboard position in preparation for lowering the boat to the water. Movement of the boat outboard with mechanical boat davits is not under control of the boat falls. Mechanical boat davits of the pivot  sheath  screw  (occasionally  called  crescent)  and radial designs are no longer being built for Navy ships. However, these older design mechanical boat davits are still used on some Navy and merchant vessels. The newest type of a mechanical boat davit being used by the Navy is the slewing arm boat davit. This boat davit is often called the SLAD and it handles the rescue boat called  a  rigid  inflatable  boat  (RIB). The  double-link  davit  generally  handles  two  boats. The double-link configuration can handle both the larger and heavier types of boats (officer/personnel, utility cargo) as well as the lighter weight rescue boat. A double-link pivoted gravity davit is shown in fig- ure 4-14. Figure  4-14.–Double-link  pivoted  gravity  davit. 4-13

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