5.1.1 Appropriate accelerometers need to be selected,
properly placed in the lifeboat, and securely mounted to obtain reliable
and replicable acceleration force data over the complete range of
acceleration forces that occur during the launch of a free-fall lifeboat.
Different types of accelerometers are commercially available. The
type selected must have adequate response for the test being conducted.
In addition, the acceleration force data must be recorded in a manner
so as to accurately represent the measured acceleration field.
5.1.2 During the conduct of free-fall lifeboat
prototype tests, the accelerometers used to measure the acceleration
forces that occur during the launch should have a working range of
20-25 G's. Typically the maximum acceleration forces measured on the
hull of the lifeboat do not exceed about 12 to 15 g's. The basis of
these values is experience with the many freefall lifeboat prototype
tests that have been conducted to date. However, as the mass of the
lifeboat decreases, or as the height of free-fall increases, the acceleration
forces tend to increase if all other factors are held constant. Therefore
the actual launch conditions should be evaluated and appropriate accelerometers
selected.
5.1.3 The accelerometers should be placed in the
lifeboat in groups of three and oriented so as to measure all three
components of acceleration at each location. The accelerometers in
each group are normally oriented parallel with the principal axes
of the boat. Typically three such groups are used. As shown in Figure
5.1, one group of accelerometers is placed by the side-wall of the
boat at the location of the most forward seat. A second group is placed
on the opposite side of the lifeboat near midships. The third group
is usually located near the most aft occupant seat and on the same
side of the boat as the forward group of accelerometers. Sometimes
an additional group of accelerometers is placed at the helm.
Figure 5.1 Typical Placement of Accelerometers in a Free-Fall Lifeboat
5.1.4 Because prototype tests with free-fall lifeboats
are conducted with the ship on even keel and generally in good weather
conditions, the lifeboat predominantly moves in a vertical plane during
the launch. As such, the lateral force imparted to the lifeboat is
generally negligible; the accelerometers oriented in the lateral direction
will indicate negligible acceleration. However, lateral acceleration
force should be measured, if possible, so that unexpected or unusual
behavior can be observed and quantified. Such unexpected behavior
could result from an improperly constructed lifeboat or launch ramp
or from improperly distributed mass within the lifeboat.
5.1.5 When the accelerometers are placed in the
lifeboat, care should be taken to ensure that they are properly oriented
with the lifeboat and that they are securely mounted on a firm support.
A good place to mount the accelerometers is directly over bulkheads
or deck stiffeners. By placing the accelerometers at these locations
unwanted vibrations are minimized to the greatest extent possible.