"THE LOSS OF THE WINSTON CHURCHHILL"
We
now have enough information that we can apply the same analyses
to the Winston Churchill.
We can estimate
the speed of a breaking wave (but not a non-breaking wave) if
we know the height of the wave. The breaking wave that destroyed
the Churchill was estimated by several observers to have a height
of at least 45 ft. Such a wave would be moving at about 30 mph.
Therefore, when the boat had been picked up by the wave it would
be moving at that speed..
As the Churchill
slid down the face of the wave on its side, there would be very
little friction or drag, because the water supporting the boat
would be moving at the same speed and would accelerate with the
boat. With no friction the boat could reach a speed of 67 mph
by the time it reached the trough. If we assume only half of this
speed increase, the boat would strike the green water in the trough
at over 50 mph.
This velocity
is equivalent to a free fall from over 70 ft. This clearly explains
the sequence of events which destroyed the Churchill. A boat striking
green water at this speed can incur a force of over 200,000 lbs.
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Winston
Churchill With Drogue
It
is not feasible to design a drogue which will prevent a boat from
being picked up by the wave and carried up to wave speed. The
loads would be prohibitive. Therefore it is necessary to design
a drogue which is capable of decelerating the boat to a low speed
before it plunges into the trough.
Fig . 4
shows the Churchill in the trough of a 45 ft breaking wave. A
series drogue has been deployed and the boat is dead in the water.
The wave face is moving toward the boat at over 30 mph. The drogue
device consists of 164 5in. diameter cones concentric with the towline
and attached to 348 ft.of double braided nylon line tapered from
7/8 to ¾ to ½ in.diam. A 30 lb. weight, usually
a length of chain is attached to the end.
Fig. 5
shows the boat as it reaches the wave face. A heavy boat such
as the Churchill is not thrown ahead of the wave but is caught
up by the wave and brought up to wave speed. The loads on the
boat when struck by the crest are not high enough to cause damage.
The boat rides up the face and is near wave speed when struck
by the moving water at the crest. In the more than 15 years that
the drogue has been at sea, no boat has ever been damaged. In
particular the rudder, transom, cockpit and companionway doors
have all been unscathed.
At the position
shown in Fig. 5, the drogue has picked up a load of approximate
5000 lbs.
This is sufficient
to avoid yawing and broaching but not sufficient to prevent the
boat from being driven up to wave speed.
Fig. 6 shows the Churchill surfing down the face of the
wave. The crest has broken and the surface water is moving with
the wave. Without a drogue the boat would accelerate rapidly.
However, at this point the drogue has straightened out and is
reaching the peak load, approximate 25000 lbs or half the displacement
of the boat. The boat now decelerates and reaches the trough at
a moderate velocity and with little roll or yaw.. No high loads
are imposed on the hull or rigging.
Fig. 7
shows the Churchill without a drogue impacting the trough at a
speed of over 50 mph. This is why the crew reported that "It
felt like we had struck another boat".
The wave characteristics
discussed here and shown on these figures are taken from a computational
fluid dynamics simulation. Although actual storm waves will have
local surface variations, the energy level and dynamic behavior
of large waves such as those that struck the Churchill are now
well understood and predictable for engineering purposes.
