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application. Each arm is mounted on rollers which run
on an inclined trackway that is mounted on the deck.
The incline on the trackway(s) is sufficient for gravity
to cause the boat and arm(s) to move down the track-
way(s) from the inboard position to the outboard
position so the boat may be lowered into the water.
Depending on the model, the boat davit arms are
connected by either a strongback or a span wire
(fig. 4-13 and 4-15). Trackway boat davits are gravity
boat davits.
BOAT DAVIT SAFETY DEVICES
Boat davit installations have various safety and
protective devices. These safety devices are visual,
electrical, and mechanical in nature. We will describe
some of the safety devices in the following paragraphs.
Safe Hoisting Position Stripes
Safe hoisting position stripes are usually red in
color and 2 inches wide, and they are used as a visual
aid for the boat davit operator. They are painted on the
davit frame and the davit arm(s) at a minimum distance
of 8 inches from either the two-blocked position or the
solidly compressed position of the buffer spring. They
indicate when the electric motor must be de-energized
during hoisting to avoid a two-blocked condition. A
two-blocked condition is where the boat fall(s) are pre-
vented from movement either by design or obstruction.
Continued hoisting against a two-blocked condition
could result in over stressing or failure of davit
components.
Slewing Position Stripes
Slewing position stripes are used for a slewing boat
davit (SLAD) as a visual aid to indicate when to de-
energize the electric motor during slewing. There are
three stripes, usually red in color and 2 inches wide. One
stripe is painted on the arm and two stripes are painted
on the pedestal. One of the two pedestal stripes in-
dicates when the arm is slewed to the STOW position
and the other indicates when the arm is slewed to the
LOWERING position.
Emergency Disconnect Switch
The emergency disconnect switch is located at the
boat davit operation station to allow the operator to
interrupt power to the motor. It is used in an emergency
situation to prevent a two-blocked condition if another
control component fails to function properly.
Double Break Feature
Electrical contacts subjected to momentary jogging
service are prone to sticking or welding. This can cause
uncontrolled operation of the winch. The double break
feature is the arrangement of two independent
contactors in the supply leads to protect against this
danger. When the motor power supply is interrupted by
the master switch the supply leads are opened in two
places by contactors which are not interlocked. In the
Figure 4-16.–Fall tensioning device.
4-16
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