In-depth | SAFETY
day to ensure reasonable accuracy. Te results were intriguing. Te righting arm curve and wind heeling
arm curves for various steady wind speeds are shown in diagram 1 for Concordia on the day of the occurrence. Te steady heel angle is where they intersect.
Up to about 27knots 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 37knots, the two curves substantially coincide over a significant range of heel, starting at about 38degs. In strengthening near-gale conditions, the angle of heel rapidly increases to a partial knockdown. As the heel approaches about 70degs, the
righting arm increases as the deck houses start to submerge and provide additional buoyancy (provided that the hull and deck houses are watertight). Indeed, the model suggests that Concordia would have retained positive buoyancy aſter 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 skylight and numerous vents. To reflect this, the model was modified to remove the buoyancy of each of the deck houses once water reached the first openings. Te 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 70degs and 90degs. So was another factor potentially at play?Although a microburst did not occur, squalls associated with thunderstorms can contain some downdraſt element that reaches the ocean surface (see image, page 19). So, what happens if the winds are inclined?
Although not immediately intuitive, this can be shown by shiſting 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 30degs from the horizontal, the horizontal
Diagram 3 18 Ship & Boat International March/April 2012
Diagram 1
Diagram 2
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