Home' Afloat : AFLOAT October 2014 Contents 54 AFLOAT.com.au October 2014
by Malcolm Riley*
air winds and following seas’ is usually meant as a
wish of good fortune to the mariner. This was certainly
true in the days of square-rigged vessels that only
travel with the wind. But a following sea in motor vessels
can be hazardous.
On a dark February night in 1945 an Australian warship was
passing Cape Leeuwin (WA) en route from Melbourne to Perth.
Steaming at around 21 knots and turning onto a northerly course
for the final run up the coast to Perth, disaster struck.
Ten sailors were washed overboard and never found and
significant damage was sustained to the port side of the vessel.
The ship was (more or less) turned onto its port side and the
engines were either stopped or reduced to dead slow. Large
amounts of water entered the vessel and the electrical and radio
system failed due to water ingest. Basically the ship was briefly
dead in the water and the Captain thought they had experienced
a rogue wave.
The ship was the HMAS Nizam a destroyer with a displacement
of around 2,000 tons. It was over 100 metres long, 10 metres
wide and had a draught of six metres. In other words, not an
However, what possibly occurred could be a bit more
complicated and those who want an in-depth understanding of
this incident should obtain a book called Man Overboard published
by Red Rose Books. It is a fascinating read.
The weather situation at the time of the incident was an
easterly wind of Force 7. Wind speeds in the Force 7 range are
from 28-33 knots, just below Gale Force. A fully developed sea
with this wind range would be four metres with a maximum height
of 5.5 metres. The ship reported the sea to be near two metres,
suggesting that the sea was not fully developed.
When a wind starts to blow the waves do not form to their
full potential height or speed immediately. It takes in some cases
many hours for the wind to transfer the energy across to the waves.
To fully develop a sea the wind needs to blow from one
direction over a specific distance for a specific time. The graph
below shows that a two metre wave would need the Force 7 wind
to be blowing from the same direction for 50nm for around three
hours. So the Nizam was in a developing sea coming from the east.
As the sea was building from the east it was moving against an
already established moderate swell coming from the southwest.
A moderate swell is between 2-4 metres. There are some rules
of thumb with swell.
The further they travel from their area of generation, their wave
period and wave length increase but the height stays more or less
the same. This means swells of the same height that are closer
to their generation area are more likely to have a shorter period
and steeper wave faces than ones that have travelled further.
The longer the wave period the faster the wave is travelling.
Multiply the wave period in seconds by three to get an
approximation of the wave speed in knots.
Swell often travel in sets of waves. This may be 10 to 15 waves
with the smaller waves (in height) at the beginning and end of
the set growing to the largest waves in the middle of the set.
The sea and swell interact with each other, if the crests come
together the overall wave height will be higher. If the troughs
coincide the troughs can be deeper. If the crest of one wave
meets the trough of another there will be a certain amount of
cancelling each other out.
The dangers of
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