3.5.1 The geometry of the lifeboat as it impacts
the water and the forces acting at that time are shown in figure 3.13.
The desired occurrence during water entry is that the lifeboat remain
upright, return to even keel, and continues to move away from danger
without using its engine. SOLAS (IMO, 1990) requires that the lifeboat
make positive headway immediately after water entry.
Figure 3.13 Geometry of the Lifeboat During Water Entry
3.5.2 As a result of the launch and free-fall,
the lifeboat has kinetic energy at the time it contacts the water.
During water entry, the fluid forces do work on the boat which causes
the kinetic energy to change. The changes that occur can be represented
conveniently in terms of impulse and momentum, namely (Nelson, 1992):
The left side of Equations 3.9 through 3.11 is the momentum
of the lifeboat after it has been acted upon by the fluid. The first
term to the right of the equal sign is the momentum when the lifeboat
first contacts the water and the integral is the impulse caused by
the fluid acting on the boat. During the time the lifeboat is entering
the water, the vertical and rotational momentum (computed with Equations
3.10 and 3.11) become zero and the horizontal momentum (computed with
Equation 3.9) should remain positive. The lifeboat stops rotating
and moving vertically but continues moving in a positive horizontal
direction.
3.5.3 When the lifeboat impacts the water, a righting
moment is caused by the fluid forces. This righting moment is the
integrand in Equation 3.11. Its magnitude is dependent upon several
factors including the longitudinal location of the CG, the magnitude
and direction of the fluid forces, and the orientation of the lifeboat
at the time of water contact. As seen on figure 3.14, the magnitude
of the righting moment decreases as the water entry angle increases
(as the CG moves forward). This has the effect of causing the lifeboat
to return to even keel slower. If the entry angle is steep enough,
or if the CG is too far forward, the line of action. of the fluid
force can pass beneath the CG which causes the fluid force to produce
an overturning moment instead of a righting moment. 'In an extreme
situation, the lifeboat can over-rotate and become inverted when it
impacts the water.
Figure 3.14 Righting Moment on Lifeboat During Water Entry
3.5.4 When conducting tests with full-scale and
model lifeboats it has been observed that the headway made by the
lifeboat after water impact decreases as the water entry angle increases.
In some cases the lifeboat has been observed to move backward after
entering the water. It has also been observed that the lifeboat dives
deeper into the water as the water entry angle increases. During previous
studies (Nelson, et. al., 1991) it was seen that when the water entry
angle increases, the orientation of the lifeboat becomes more parallel
to its direction of flight.
3.5.5 These effects can be seen in the data presented
in figure 3.15 for a free-fall lifeboat launched from a height of
25 meters at an angle of 35°. The water entry angle and the trajectory
angle both increase as the rate of rotation during free fall increases.
Recall that decreasing ramp length causes the rate of rotation to
increase. Under extreme conditions (an unusually short ramp) the water
entry angle actually exceeds the trajectory angle. During these launches
the lifeboat made positive headway when 'the trajectory angle exceeded
the water entry angle by more than five degrees. The headway increased
as the difference in the water entry angle and trajectory angle increased.
Similar trends have been noted during analytical studies with the
lifeboat being launched from different heights.
Figure 3.15 Water Entry Angle, Trajectory Angle and Righting Moment Versus Ramp
Length
3.5.6 The "knee" of the righting moment curve
on figure 3.15 appears to separate the regions of positive and negative
headway. Similar results were found during launches from other heights.
When the ramp length was equal to that at the knee of the curve, the
lifeboat became dead in the water after water entry. As the length
of the launch ramp was increased from that at the knee of the righting
moment curve, the lifeboat made increasingly positive headway after
water entry. When the ramp length was decreased from that at the knee
of the curve, it made increasingly negative headway.
3.5.7 As the relative values of L and D changed,
differences were noted in the acceleration forces experienced by occupants
within the lifeboat. Consider the acceleration force data presented
in figure 3.16 that occurred in an 11 meter lifeboat. These data are
the maximum Z axis accelerations that occurred at the CG during water
entry. The acceleration data are presented in the axes of the lifeboat.
The Z axis is the vertical axis of the lifeboat and is perpendicular
to the keel.
3.5.8 There was a significant increase in the
acceleration force as the CG moved aft and an even more significant
decrease as it moved forward. The overall behavior of the lifeboat
was acceptable (the boat entered the water upright made positive headway
after launch) in each of the launches reported in figure 3.16.
Figure 3.16 Maximum Z Axis Acceleration During Water Entry in the Axis of the
Lifeboat
3.5.9 It is interesting to note that the data
presented in figure 3.16 indicate the accelerations increase with
free-fall height up to a certain point and then begin to decrease.
For the lifeboat evaluated, this change occurs at about 28 meters
when the CG is in the normal location. The change occurs at about
23 meters when the CG is forward. This phenomenon occurs because of
two factors. First, as the free-fall height increases, the velocity
at which the lifeboat impact the water increases and, as such, the
magnitude of the fluid force increases. Secondly, with increased free-fall
height there is an increase in the water entry angle. It has been
shown that increasing the water entry angle causes the maximum fluid
force to decrease. The interaction of these two factors affects the
final performance of the lifeboat.