Friday, 6 May 2016

FELL RATHER THAN ROLLED.

The Argus (Melbourne) Monday, 19 December, 1910.

Mr. Crossley, of Melbourne, detailed a
conversation he had had with the chief 
officer of the Waratah. The latter was dis-
satisfied owing to the peculiar habit of the
vessel in getting on one side, without righting 
immediately. The Waratah fell rather
than rolled.

We need to revisit GM and acknowledge that it is a complex issue prone to over-simplification by the layman - myself.

The metacentric height (GM) influences the natural period rolling of a hull. The GM is the distance between the centre of gravity and metacentre. The greater this value, the greater the initial stability against overturning. A large GM is associated with shorter periods of roll - not comfortable for passengers, whereas a reduced GM creates a longer, more gentle rolling pattern. This can be equated to a weight on a spring, where the GM increases, the spring gets stiffer and hence the increased frequency of heeling and more powerful righting force. A reduced GM would reduce the frequency and 'stiffness' of righting force - ie 'falling into a list'. We know that waratah had a relatively low GM during her first three voyages, hence the comment. 

Top heaviness, raising the centre of gravity, has a significant effect on GM, which is correspondingly reduced. It has to be compensated by shifting the centre of gravity down by means of dead weight lowest down in the hull. Wind force on the superstructure decks also needs to be taken into account and its effect on heeling. If the wind force equals the righting force, the vessel will remain in a list. Top heaviness and wind force are not the only factors influencing the heeling pattern. The hull design and shape have an effect on the GM - wide, shallow hulls or narrow deep hulls should have relatively high GM values. Let us return to the Inquiry to examine a very interesting sentence taken from a letter written by the owners to the builders regarding Waratah's stability: 

'and we will record again our protest to your objection to supply us with a copy of the lines of this vessel's hull'.

Previously I have wondered if the Waratah's hull was too narrow. But the opposite might be true in terms of GM. We know that Waratah's hull was largely based on that of Geelong, just roughly 12% larger. Geelong's hull was appropriately designed to support two superstructure decks, whereas Waratah had three. Waratah's beam was 9% broader than Geelong's. Perhaps if Waratah's had been 54.5 ft. there would have been less of an inherent GM problem? A narrow deep hull is more GM stable. The owners' use of the words 'lines of this vessel's hull' suggests that there were concerns which went beyond beam measurement and delved into the complexity of establishing a suitable GM. Clearly there was not enough sheer on the hull to establish adequate buoyancy with three superstructure decks above. 

Why did the builders object to supplying the owners with Waratah's hull lines ? We may never know the truth, but one thing is clear, the builders and owners were not on the same page.

see:

http://waratahrevisited.blogspot.co.za/2016/05/hang-at-end-of-roll.html



Ship stability diagram showing centre of gravity (G), centre of buoyancy (B), and metacentre (M) with ship upright and heeled over to one side. Note that for small angles, G and M are fixed, while B moves as the ship heels, while for big angles both B and M are moving.

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