Thursday, 6 February 2014

Waratah - specifications revisited (part 2).

If a vessel such as the Waratah listed beyond the scope of her metacentre, the 'righting moment' comes into play which is a factor taken into consideration in the design of the hull; the maximum righting moment is the maximum moment that could be applied to the vessel without causing it to capsize, and the angle at which this occurs is the most stable heel for the vessel.

The point of deck immersion is the angle at which the main deck will first encounter the sea. Similarly, the downflooding angle is the angle at which water will be able to flood deeper into the vessel.

Finally, the point of vanishing stability is a point of unstable equilibrium. Any heel lesser than this angle will allow the vessel to right itself, while any heel greater than this angle will cause a negative righting moment (or heeling moment) and force the vessel to continue to roll over. When a vessel reaches a heel equal to its point of vanishing stability, any external force will cause the vessel to capsize.

A vessel such as the Waratah was at risk for large angles of heel if cargo or ballast shifted, thus contributing to a shift of weight and centre of gravity to the listing side. This reminds us of Captain Pidgeon's claim that should carcasses in hold one have shifted, they had significant potential to destabilise the Waratah, perhaps to a point of vanishing stability or beyond.

The method of loading cargo had a very significant impact on the forces contributing to the degree of listing and Captain Ilbery did in fact comment after the maiden voyage that he had learned how to stow the Waratah properly (perhaps he should have been present at the heeling experiment). This enhanced the Waratah's stability by the second voyage.

A further factor the designers of the vessel needed to take into account were the crew and passengers themselves. This is mobile 'cargo' and as such needed to be factored into the stability curves. For example in a gale passengers might seek shelter on the lee deck of the vessel to which side the vessel would be listing (blown by the gale) and further contributing to the listing angle. Let's not forget about up to 700 emigrants taking the place of cargo on the 'tween, spar and upper decks.

to be continued....

German steamship Roda

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