Thursday, 6 February 2014

Waratah revelation - SHE WAS NOT UNSTABLE.

"Mr. Barrie gave an account of the way he performed the experiment, and the precautions he took to ensure its accuracy. From his account it would appear that all proper measures were adopted to obtain the correct vertical position of the centre of gravity of the vessel in her then condition."

"The vessel was about to go on trial and was practically complete, all refrigerating machinery on board, insulation finished, and boats in place. it was estimated that weights yet to be put into the ship amounted to 20 tons."

"The metacentric height was found to be .556 feet. from the curve of transverse metacentres. The metacentre in this condition was ascertained to be 26.38 feet above the base line, the centre of gravity thus being 25.82 feet above the base."

"Calculations were made to find the effect on the centre of gravity of removing weights, including water ballast in Nos. 3 and 8 tanks, which were to come out of the ship, and of adding the 20 tons above referred to."

"The metacentric height of the ship in a light condition, viz., vessel empty except that 300 tons of fresh water, stores and crew were on board, was then shown to be .26 feet."

"On the 21st October, 1908, a load line certificate was issued by Lloyd's. The centre of the disc was to be 8 feet 1 inch below the spar deck line."

"Mr. F. W. Lund was on board during this trip. He said that so far as he could recollect the water ballast tanks were full, and that about 3,000 tons of coal were on board."

"Mr. Shanks, the superintending engineer, who also made the trip, said she had 2,900 tons of coal on board, some in those permanent bunkers situated below the spar deck and the rest stowed partly in the spar deck bunker and partly in No. 3 hold, with some in the 'tween decks."

Mr Shanks later corrected himself by denying that any coal was in fact stored in the spar deck bunkers.

The Board of Trade Inquiry quoted the following figures:

"Upon these figures the metacentric height of the ship appeared to have been, when leaving Durban, about 1.5 ft. In this condition the vessel had a maximum righting lever of 3.15 ft., at  an angle of 55 degs., and a range of stability of 90 degs. The bases of these calculations were not free from ambiguity and possible sources of error, but the Court thought no serious error had been made."

We need to remind ourselves that the spar deck was the upper most continuous deck, with emphasis on the word continuous, which excluded the bridge, promenade and boat decks (which were not continuous throughout the Waratah) above the spar deck.

This post is very technical and revisits the concept of the Waratah's metacentre. If we are to fully understand the question of Waratah's stability it is important to revisit these terms and what they signified. Take a deep breath, because we are going to delve into this issue in detail.

The metacentric height (GM) is a measurement of the initial static stability of a floating object. This is the vessel's tendency to 'remain' in its intended floating position, and 'return' to this position after listing. It is calculated as the distance between the vessel's centre of gravity and that vessel's metacentre. A greater metacentric height implies greater initial stability against the degree of listing and overturning. This measurement has a direct correlation with the natural period of rolling of a vessel.

A very stable and large metacentric height is associated with shorter, sharper periods of roll (or listing) which is uncomfortable for passengers. Conversely a relatively lesser metacentric height is associated with a longer more comfortable roll, but is less stable. The designers would have to have taken this into consideration when designing the Waratah for the comfort of her first class passenger component. The builders had to weigh up the pros and cons to create a metacentric height sufficiently high but not excessively so.

The Waratah like all floating objects had its own unique pendulum frequency (like a metronome). This frequency also links in with the metacentric height. To calculate the metacentre one needs to determine the ratio between the inertia resistance of the vessel and the volume of the vessel. In other words this is a measurement of how the waterline width of the Waratah would resist overturning.

A narrow and deep hull would have a high metacentre. The Waratah had a narrow hull, but visually a relatively shallow hull (see attached image of Waratah model). This could be compensated by cargo loading and ballast. Slow rolls associated with this configuration would have been considered 'tender' and inclined to overturn, if ballast and cargo were not taken into account. Adequate and appropriate ballast and cargo loading were thus crucial to the stability of a steamer such as the Waratah.

When a ship heels over or lists to one side, the centre of buoyancy moves laterally. When the vessel lists, the metacentre remains directly above the centre of buoyancy if the angle of listing is not excessive. The Board of Trade Inquiry quoted the following figures:

"Upon these figures the metacentric height of the ship appeared to have been, when leaving Durban, about 1.5 ft. In this condition the vessel had a maximum righting lever of 3.15 ft., at  an angle of 55 degs., and a range of stability of 90 degs. The bases of these calculations were not free from ambiguity and possible sources of error, but the Court thought no serious error had been made."

In the following reference  books.google.co.za/books?isbn=3861950936 metacentric heights are quoted for an ocean going steamer (525 ft in length) circa 1908:

- light                                                             0.4 ft

- loaded to lower deck                                   0.7 ft

- deep, fully loaded                                         0.62 ft

The Waratah when 'light' (water, stores and crew) was reported by Mr Barrie to have had a metacentric height of 0.26 ft, which is 35% less stable than the 'standard' steamer (1908) of roughly the same length. But the Waratah would not go to sea in this condition.

When the Waratah was loaded with full water ballast, 3000 tons of coal and the addition of 20 tons making up the operating weight, her metacentric height was 0.556 ft, which is 20.57% less stable than the quoted steamer, loaded to the lower deck. But bear in mind this figure for the Waratah does not include cargo loaded and would not go to sea in this condition.

The Waratah was 46% more stable when she was loaded compared to 'light'. This is an important concept to grasp because steamers such as the Waratah were in effect a number of vessels in one. Without cargo the Waratah was very different from a fully loaded and ballasted version of the same ship. The same applies to whether she carried emigrants or a predominance of cargo. When the Waratah departed Durban for the last time she was in only one condition (version), fully loaded and ballasted with a metacentric height of 1.5 ft.

If this is true, even taking into account a standard deviation of error, the Waratah was considerably more stable than the 'standard steamer' quoted which had a metacentric height of 0.62 ft when deep, fully loaded. It is not a leap of faith to assume that with her full cargo component adding to stability, the Waratah had in all probability a metacentric height of at least 0.62 ft or more, which places her squarely within the acceptable range of metacentric height stability for a steamer of that era and size.

The Waratah, when she departed Durban, was NOT unstable.




to be continued...







There are a number of recent posts which cover this important issue in more detail:

http://waratahrevisited.blogspot.co.za/2015/11/waratah-inquiry-continued-part-36-more.html

http://waratahrevisited.blogspot.co.za/2015/12/thorny-issue-of-cargo-and-weight-crux.html

http://waratahrevisited.blogspot.co.za/2015/12/waratah-inquiry-continued-part-40.html

http://waratahrevisited.blogspot.co.za/2015/12/waratah-inquiry-continued-part-44.html

http://waratahrevisited.blogspot.co.za/2015/12/waratah-inquiry-continued-part-46-spar.html

http://waratahrevisited.blogspot.co.za/2015/12/gm-finale-and-conclusion.html

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