ENGINEERING - Royal Australian Navy

N A VY E N G I N E E R I N G B U L L E TI N FE B R U A RY 2 0 02
4 3
TABLE 1 F R E Q U E NCY OF WAVE HEIGHTS IN AU ST RALIAN WAT E R S
H1/3 [m] Average Cumulative Sea State
Occurrence [%] Probability [%]
6.0 3 100 7 and above
H OW THE FREMANTLE CLA S S
P E R F O R M S
Both the numerical and survey
results indicated that the
Fremantle Class exhibits the
typical performance of a small,
relatively fast monohull however
certain areas of the design could
be improved. The ship is no
longer able to safely complete a
full range of tasks in a significant
wave height of 2.5metres (Top of
Sea State 4). Moving into Sea
State 5, tasks such as launch
and recovery of sea boats
becomes dangerous and even in
the higher range of Sea State 4
some areas of the ship become
particularly uncomfortable. Both
the survey and numerical
analysis indicated junior sailors
spaces and ships of fice and
communications centre are are a s
w h e re many ta s ks become
d i fficult in modera te seas.
These areas suffer the most
f rom ship motions and the
result is increased difficulty when
wo rking in these areas. It is not
s u rp rising to note that th e s e
a reas are fo rwa rd in the ship
w h e re higher acceleration leve l s
a re encounte re d .
From a seakeeping pers p e c t i ve
t ravelling in fo l l owing seas re s u l t s
in the lowe st motions, howeve r,
dynamic stability considera t i o n s
s u ch as avoidance of bro a ch and
s u rf riding must then be
c o n s i d e red. The survey
responses from the commanding
o ffi c e rs indicated that opera t i n g
in modera te beam seas wa s
u n d e s i rable due to the MII
caused by considerable roll.
For this reason bow or ste rn
qu a rte ring is often a bette r
heading. Despite the imp l i c a t i o n s
for slamming and deck we t n e s s
the commanding offi c e rs of th e
FCPB have indicated head seas
is often the most desira b l e
heading as far as crew safety
is concern e d .
The commanding officers have
indicated that when selecting any
heading, crew safety is the
highest priority. Following this,
in head seas, equipment damage
and slamming were of concern,
in beam seas roll and equipment
damage and in following seas,
broaching and surf riding were
in the top three concerns.
The dilemma facing the FCPB is
that roll is the gre a te st concern
when considering the ta s ks to be
c o mp l e ted on board but pitch
and heave motions become
s i g n i ficant for crew during ‘re st ’
p e riods. The result of inadequ a te
re st is increased fatigue and
motion sickness. It was re p o rte d
that a combination of roll, pitch
and heave in bow and ste rn
qu a rte ring seas (which can cre a te
a cork screw effect) will ofte n
reduce the magnitude of any one
c o mponent but usually will gre a t ly
i n c rease sickness incidence.
EFFECT OF SHIP MOT I O N S
The survey, completed by 138
crewmembers from eight of the
RAN FCPB fleet, asked the
usual degree of seasickness
experienced. The categories
included:
• Never;
• Mild (headache, more than usual
tiredness);
• Moderate (Sleeplessness, more
than usual irritability, nausea);
and
• Extreme (actual vomiting and
general sea sickness enough to
prevent you from performing your
normal duties).
In response 30% claimed never
to suffer seasickness, 26% mild
doses, 28% moderate and 16%
extreme sea sickness when
seasick on the FCPB.
Frequency of seasickness was
questioned with responses
sought for calm (0-1.25m seas),
moderate (1.25-2.5m seas),
rough (2.5-4.0m seas) and ver y
rough seas (4.0-6.0m seas).
The response could be; not at all,
occasionally, frequently, nearly
always or always.
In calm conditions 6% indicated
they have some degree of
seasickness either occasionally or
frequently whilst the remaining
94% never get seasick. This
decreases to 67% never getting
seasick in moderate seas, 44%
in rough seas and 35% in very
rough seas. In rough conditions
31% frequently to always suffer
seasickness and this increases
to 39% in very rough seas.
Apart from these statistics two
other clear trends came from
the survey. They were that in
moderate seas (Sea State 4)
73% of respondents believed
they took longer to complete
tasks and 76% used more
caution to complete tasks.
These results clearly indicate
that adverse seakeeping
characteristics have a significant
influence over operating
efficiency. In the operating
environment that can be
expected 36% of the time
(ie. Sea State 4), one third of
the crew could be expected to
be suffering some degree of
seasickness and most of the crew
are taking additional time and
caution to complete tasks.