3.4.1 Presented in figure 3.10 is the angular
velocity during free-fall as a function of different launch angles.
These data were computed for a lifeboat that is about 11 meters long
and is normally launched from a ramp at an angle of 35°. During
free-fall, the angular velocity is constant; it does not change with
height. As the launch angle increases, the angular velocity imparted
to the lifeboat decreases. Also, as the CG is moved aft (the relative
values of Land D change), the angular velocity decreases. This occurs
because in both cases the time during which the lifeboat rotates at
the end of the launch ramp decreases. The time decreases as the launch
angle increases because the lifeboat is moving off the end of the
ramp at a higher velocity. As the CG moves aft the time decreases
because the time at which rotation begins is closer to the time at
which the lifeboat is no longer in contact' with the ramp. Rotation
begins when the CG passes the end of the launch ramp.
Figure 3.10 Angular Velocity During Free-Fall
3.4.2 Although the free-fall height does not affect
the angular velocity, it does affect the angle at which the lifeboat
enters the water. Presented in figure 3.11 is the water entry angle
that results from free-fall launches from different heights. The initial
launch angle in all cases presented was 35°. These data were computed
with the same lifeboat represented in figure 3.10. As the free-fall
height increases, the difference in the water entry angles increases.
In each of these launches, the lifeboat made positive headway after
water entry (Nelson, 1992).
Figure 3.11 Water Entry Angle Versus Free-Fall Height for Three Conditions of
Load
3.4.3 Most of the change in the angular orientation
of the lifeboat occurs during the free-fall phase. Although there
is a considerable change in angular momentum during the rotation phase,
there is little change in angle. Presented in figure 3.12 is the orientation
of the lifeboat at the end of rotation versus the angle at which it
was launched. The bold line on figure 3.12 represents the angle from
which the lifeboat is launched. As shown on the figure, the angular
change is small, even at shallow launch angles. As the launch angle
increases, the angle at the end of rotation becomes asymptotic to
the launch angle. Further, the difference between the launch angle
and the angle at the end of the rotation phase decreases as the launch
angle increases and as the CG moves aft. The velocity of the lifeboat
at the beginning of rotation accounts for these effects (Nelson, et.
al., 1992).
Figure 3.12 Orientation at the End of Rotation Versus Launch Angle