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The Masthead
March 2012 Page 13
June 2012 Page 13
Post-Capsize Safety Lessons
(continued)
The righting-arm curve and wind heel-
ing arm curves for various steady
wind speeds are shown in Diagam 1
for
Concordia
on the day of the occur-
rence.
The steady-heel angle is where they
intersect. Up to about 27 knots of
wind, the response is as one would
expect: more wind induces more heel
which in turn induces more righting
force, resulting in a reasonable and
steady angle of heel. But, somewhere
between 27 and 37 knots, the two
curves substantially coincide over a
significant range of heel, starting at
about 38 degrees. In strengthening
near-gale conditions, the angle of
heel rapidly increases to a partial
knockdown.
As the heel approaches about 70 degrees, the righting arm in-
creases as the deck houses start to submerge and provide addi-
tional buoyancy (provided that the hull and deck houses are wa-
tertight). Indeed, the model suggests that
Concordia
would have
retained positive buoyancy after being knocked down onto its
beam ends and would, therefore, likely have recovered once the
wind abated.
Limited buoyancy
However, the hull and deckhouses were not secured. Not only
were the leeward doors open, but so were the engine room sky-
light and numerous vents. To reflect this, the model was modi-
fied to remove the buoyancy of each of the deck houses once
water reached the first openings. The deck houses still provide
some residual buoyancy but over a more limited range, as
shown in Diagram 2.
This would not last for long, however, because of downflooding
into the hull through other unsecured openings.
But that still may not account fully for the rapid increase in heel
between 70 degrees and 90 degrees. So was another factor
potentially at play? Although a microburst did not occur, squalls
associated with thunderstorms can contain some downdraft
element that reaches the ocean surface (see Diagram 4).
So, what happens if the winds are inclined? Although not imme-
diately intuitive, this can be shown by shifting the heeling arm
curve for a horizontal wind to the right on the x-axis. For example,
to look at the effect of winds inclined to 30 degrees from the
horizontal, the horizontal wind heeling arm curve can be shifted
30 degrees to the right. As can be seen in Diagram 3, when this
is done, any residual buoyancy from the deck houses is totally
overcome, and the vessel goes over onto its beam-ends.
So what we have from the
Concordia
model is:
- A vulnerability to a rapid increase in heel angle at a rela-
tively modest wind speed;
- A loss of buoyancy due to downflooding through open
doors, windows, and vents; and
- A possible inclination of the wind that overcomes any
remaining residual buoyancy provided by the deckhouses.
All well and good—as explanations go—but
Concordia
was no
spring chicken; rather, it was a sea-
soned veteran of many ocean voy-
ages. While we do not know if the
vessel had been knocked down be-
fore and recovered, we do know that,
by reputation, the vessel had always
been sailed conservatively, with
minimized heel angles so as not to
adversely affect classes.
Here's a final element to consider:
Concordia
was built in Poland and
was originally flagged in the Baha-
mas. The Bahamas followed the UK
rules and required that the vessel
have “squall curves” included in its
stability book (maximum steady heel
angle to prevent downflooding in
gusts and squalls). These rules were
introduced in the UK following an
inquiry into the loss of the
Marques
Diagram 1
Diagram 2