STORM
WAVES
John Rousmaniere
writes, in his excellent book Fastnet Force Ten, "The
calamity in the Western Approaches (The Fastnet Tragedy) seems
to be yet another indication that our positive faith in technology
may be groundless. We appear to be led by transitory successes
into the heresy that we can completely manipulate our environment".
This view of the unworldly fury of a storm at sea has existed
thru the ages. Fortunately it is no longer is pertinent.
When I started
to work on the capsize problem I had no doubt that it would be
possible, in the modern world, to provide equipment to bring a
well found yacht thru a storm like the Fastnet or the Sydney Hobart.
Water is a familiar fluid. Wave speeds and heights are well within
engineering experience. Any competent Aerospace company has handled
problems much more challenging. However, I was not at all confident
that the required equipment would satisfy the constraints of size,
weight, cost, complexity and ease of operation to an extent that
it could be expected to gain acceptance by ocean yachting skippers.
The series drogue has now earned that acceptance, but it took
15 years or so to get there.
In addition
to a feeling of helplessness, there is another irrational attitude
that countered our obtaining a solution to the capsize problem.
The shape and motion of storm waves when viewed from the deck
of a yacht are such that it can lead to optical illusions which
confuse the skipper. A large storm wave approaching the boat appears
to be a dangerous wall of water and the skippers instinctively
tend to head up or run off to avoid being pooped. Actually the
water in the wave is not moving towards the boat and will lift
the boat harmlessly.
A second optical illusion is that a dangerous breaking wave comes
from a direction different from the prevailing wind and sea. The
report of the Investigating Comm. for the Hobart Sydney disaster
states "Exceptional waves were responsible for inflicting
the damage or causing severe knockdown to yachts. These waves
were 20 to 100% larger than the prevailing seas and came from
a direction other than the prevailing wave pattern".
From physical considerations it is virtually impossible for a
breaking storm wave to approach from a significantly different
direction. Breaking waves are formed by the wind and by the addition
of the energy of the smaller waves that they overtake. If a wave
moved across a series of smaller waves it would lose all its energy
in turbulence. We have many aerial views of the sea surface in
the Sydney Hobart storm. If a large wave had moved across the
smaller waves we would see a white streak running across all the
other streaks. There is no such a streak. What actually happens
is that if the boat is lying at some angle to the prevailing sea
as the breaking wave approaches, the action of this wave yaws
the boat until it is abeam. This yawing motion is not observed
by the skipper and he thinks the wave direction has changed, whereas
it is the boat that has moved. It is true, however, that the waves
that caused the damage were "exceptional"
In gale force
winds most of the waves can have breaking crests, but the speed
and height of the waves are such that they do not constitute much
of a threat.. However, in hurricane force winds the sea is generally
blown "flat", but from time to time very large and dangerous
waves appear, often moving in pairs or groups. A boat can ride
for hours without encountering such a wave and then be destroyed
in seconds.
Another optical
illusion is that it is possible in a survival storm to reduce
the hazard by running off before the waves and, by skillful seamanship,
to out maneuver a dangerous wave. This is a particularly unfortunate
choice. The waves are moving faster than the boat can go. A 40
ft .breaking wave will be moving at a speed of approximate 23 knots.
The breaking wave is completely random. Furthermore, by far the
most important concern is that, if the boat is moving through
the water, the chance of being caught by the wave and surfing
to a dangerously high speed is greatly augmented.
In the modern
world we have an understanding of storm waves. There is no more
mystery. Such terms as "rogue waves" serve to confuse
the issue. For over a hundred years we have had an engineering
grasp on non-breaking waves but it is only in the last 30 years
or so that we have been able to determine the position, velocity
and acceleration of every water particle in a dangerous breaking
wave.
A final misconception is the belief that a breaking wave "strikes"
the boat and that the moving water in the crest does the damage.
Actually, the boat is lifted by the forward face of the wave with
no impact. When it reaches the breaking crest the boat velocity
is close to the wave velocity. The crest water is aerated and
has little damage potential. Damage to the boat is incurred when
the boat is thrown ahead of the wave and impacts the green water
in the trough. The leeward side and the deck are struck. A careful
reading of "Fastnet Force Ten" and "Fatal Storm"
will confirm this conclusion.
