Wednesday, 11 November 2015

MAIDEN VOYAGE GM 0.375 FT.

On the 30th November, Messrs. Lund wrote as follows to Messrs. Barclay, Curle, & Co.: 

"We have now marked on a plan the manner in which the cargo was distributed in this vessel, and shall be very pleased to go into the same with one of your representatives at the earliest possible moment for our guidance for the future loading of this vessel." 

It does not appear whether an interview took place or not, but apparently in response to this request Messrs. Barclay, Curle, & Co. made calculations based upon the loading on the first outward voyage, and embodied the result, with certain modifications, in a curve, marked "Condition A," which, with others, they sent to Messrs. Lund on the 17th December, 1908. 

Particulars of the curves are set out below. 

Condition I. Ship light. Fresh water, stores, and crew aboard. G.M.=2 1/2 inches. (It should be noted that this very slightly differs from the figure given by Mr. Barrie in his evidence already referred to.) The righting lever steadily increases to a maximum of 8 inches at an angle of 38º, then diminishes till it vanishes at 64º. 

Even if the Waratah listed to an angle of 45 degrees (Claude Sawyer claim in some sources), in unstable light condition, according to the righting lever figures, she still had a further 19 degrees to play with. In light condition, however, Waratah had a vanishing point of 64 degrees 

Condition II. All coal (including reserve). All water ballast, except forepeak, and 1,200 tons heavy cargo in bottom of hold. G.M.=6 inches. The righting lever reaches 2 feet 6 inches at an angle of 53º, and does not vanish till beyond 90º. 

The builders advised that the Waratah not go to sea with a GM less than 0.83 ft. In this instance, even with a GM of 0.5 ft., it is still encouraging to read that the vanishing point was beyond 90 degrees, but remember, all water ballast, included. 

Except the forepeak - why? - presumably due to the heavy refrigeration machinery up front.

Condition III. Ship loaded to disc with cargo at 40 cubic feet to the ton in holds, and not less than 80 cubic feet to the ton in 'tween decks. All coal, including reserves. Fresh water, stores, &. aboard. G.M.=4 1/2 inches. The maximum righting lever is 10 inches at 48º, and it vanishes at 73º.

Note that cargo at 80 cubic feet to the ton in the 'tween decks + coal reduced the vanishing point to 73 degrees!

Condition IV. Condition III. with all coal burnt out and tanks Nos. 5 and 8 full. G.M.=15 inches. 

The righting levers are moderate for small angles of heel, but grow fast from 20º to 53º, where there is the maximum lever of very nearly 3 feet. At 90º there is still a righting lever of 9 inches. 

It is often said that the Waratah, at the end of a long voyage, with most of the coal burned out, was in relatively light condition. If 401 tons of ballast water was maintained or filled at sea, the resulting stability was extraordinary, with a righting lever of 9 inches at 90 degrees! This was further enhanced by coal burned out in the 'tween decks. But this might not have held true, read on...

Condition V. Loaded to disc with all coal, fresh water, stores, passengers, crew, and 1,500 tons heavy cargo in bottom of holds, remainder measurement cargo. G.M. = 6 inches. There is a small upsetting lever to 27 1/2º, then a small righting lever, never more than 3 inches, vanishing at 57º, after which there is an upsetting lever of rapid growth. 

The full component of coal as per design plans was never going to be a good thing for the Waratah. In fairness, 1500 tons of heavy cargo in the bottom holds, is insufficient, particularly without full ballast tanks - or at least tanks 5 and 8 full. It was becoming more and more obvious that the additional lead concentrate component was essential.

Condition VI. Condition III. but with coal in bridge space burnt out. G.M.=9 1/2 inches. The maximum righting lever of 18 inches is at 50º, and vanishes at 84º. 

Note how the vanishing angle improves from 73 to 84 degrees. Bridge space = spar deck.

Condition A. Loaded as on first outward voyage but with bridge coal put in upper 'tween decks, cargo in remainder of upper 'tween decks, at 80 cubic feet per ton except in No. 5, cargo at 100 cubic feet per ton in bridge. G.M.=about 4 1/2 inches. The maximum righting lever of 15 inches is at 50º and vanishes at 78º. 

This is very important. 

It tells us that there was no spar deck coal on the maiden voyage, outbound, but there was coal in the 'tween decks. The GM was 0.375 ft. which was observed by all, including Prof Bragg, who thought it was as low as 0 ! Cargo and coal in the upper 'tween decks and cargo on the spar deck was not a good recipe, without enough concentrated dead weight lowest down. The cargo on the spar and 'tween decks must have been limited taking into consideration almost 700 emigrants. The vanishing point was still a reasonable 78 degrees. Despite Waratah being in such tender condition the chances of her rolling over were still slim.

A note appears on the stability graph, "If in any condition it be considered that the vessel has insufficient stability, the filling of No. 8 tank increases the G.M. by about 4 inches".

Such a simple solution!! Further enhanced by lead concentrates, final voyage. 

It will be noted that Conditions III. and IIIA. of the stability statement correspond with Conditions III. and VI. of the stability curves, but in the former case the range of stability is stated to be 36º and 50º respectively, whereas the curves of stability show that the ranges were 73º and 84º respectively. The first-named statement only reached the Court at the end of the Inquiry, when there was no opportunity of seeking an explanation of the discrepancies. 

This is alarming! It altered the safety 'net', considerably. The figures of 36 and 50 degrees were borderline dangerous. It proved that the Waratah could not operate safely with the full coal component, including reserve 'tween decks and spar deck bunkers, full. At this stage, the all-important ballast tanks, 5 and 8, were empty and no lead concentrates. 

The Court should have made time to explore this outrageous revelation and discover if the figures presented were accurate or not, considering that so much was at stake when it came to stability. 

It is no surprise, in conclusion, that the Waratah gained a reputation as top heavy and unstable during her maiden voyage. The reputation stuck, despite subsequent and very significant improvements made to the overall GM stability of the steamer. The Waratah looked top heavy, and behaved as such on the maiden voyage. The fact that the GM figure improved to a more than satisfactory 1.5 ft. by the time the Waratah departed Durban, 26 July, 1909, she still looked the same, and people could not comprehend that acceptable stability could be created without lopping off her third superstructure deck.



The metacentric height is an approximation for the vessel stability at a small angle (0-15 degrees) of heel. Beyond that range, the stability of the vessel is dominated by what is known as a righting moment. Depending on the geometry of the hull, Naval Architects must iteratively calculate the center of buoyancy at increasing angles of heel. They then calculate the righting moment at this angle

There are several important factors that must be determined with regards to righting arm/moment. These are known as the maximum righting arm/moment, the point of deck immersion, the downflooding angle, and the point of vanishing stability. The maximum righting moment is the maximum moment that could be applied to the vessel without causing it to capsize. 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. (wikipedia)


G Z = righting arm

No comments: