Sunday, 6 December 2015

FINALLY, A GM OF 1.5 ft

The documents already mentioned (manifests, plans, &.) have been used by the experts consulted by the owners, as the basis of calculations to arrive at the disposition of the cargo, and the centre of gravity of the ship when leaving Durban on the last homeward voyage; and their results are in substantial agreement with the independent figures of Mr. Larcombe, the Board of Trade Surveyor, based on the same documents. Upon these figures the metacentric height of the ship appears to have been, when leaving Durban, about 1.5 feet. In this condition the vessel had a maximum righting lever of 2.15 feet at an angle of 53º, and a range of stability of 90º.

Finally we arrive at a GM of 1.5 ft. and a range of stability of 90 degrees. In terms of GM, Captain Ilbery had achieved his goal of creating a steamer without the alarming associations of top heaviness. The centre of gravity had been shifted down due to absence of 1819 tons of coal (42 cubic feet to the ton) in 'tween decks reserve bunkers, substituted with additional cargo graded from heaviest lowest down to roughly 100 cubic feet to the ton, highest up. In my opinion, to achieve a GM of 1.5 ft. required an additional, essential component. The Waratah needed to be loaded to the recorded draught of 28 ft. 3 in. which included the essential dead weight component 1300 tons of lead lowest down at 11 cubic feet to the ton !

The bases of these calculations are not free from ambiguity and possible sources of error, such as the following:

1. The draught of the ship on leaving Durban. This was assumed in the calculations as being correctly taken, and necessitated taking the total displacement as 170 tons in excess of the known weights in the ship. This difference was regarded as being made up of 129 tons of water ballast, 20 tons of bilge water, and 21 tons of additional stores and baggage. If, however, the draught was in error to this extent the metacentric height (without the 170 tons) would be reduced to about 1.32 feet. Considering, however, that the draught was taken by a government official in calm weather, the draught given seems likely to have been accurate.

This is a very illuminating paragraph. 'Necessitated taking the total displacement as 170 tons in excess of the known weights in the ship', suggests that the draught calculation was simply not adding up with the given cargo and coal component. Even if the GM was reduced to 1.32 ft. the figure is still substantially higher than the average (for this size steamer) of about 0.7 ft. Which ever way one chooses to view the figures, taking into account expert statements, the Waratah departed Durban with a very adequate GM stability. 


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