Friday, 6 March 2015

Waratah - could she have broken her 'back'?

It is well recorded that some heavily loaded steamers from the Waratah era sustained complete fractures of the hull (snapping in two) in gale conditions. For this to occur (such was surmised in the case of the USS Cyclops) the wave length would have to equate with the length of the vessel.

We know that the Waratah was significantly overloaded, placing undue stress forces on her hull. Could she have experienced a similar fate off the Wild Coast?

[hide]Conditions Necessary for a Fully Developed Sea at Given Wind Speeds, and the Parameters of the Resulting Waves
Wind ConditionsWave Size
Wind Speed in One DirectionFetchWind DurationAverage HeightAverage WavelengthAverage Period and Speed
19 km/h (12 mph)19 km (12 mi)2 hr0.27 m (0.9 ft)8.5 m (28 ft)3.0 sec 9.3 ft/sec
37 km/h (23 mph)139 km (86 mi)10 hr1.5 m (4.9 ft)33.8 m (111 ft)5.7 sec 19.5 ft/sec
56 km/h (35 mph)518 km (322 mi)23 hr4.1 m (13.6 ft)76.5 m (251 ft)8.6 sec 29.2 ft/sec
74 km/h (46 mph)1,313 km (816 mi)42 hr8.5 m (27.9 ft)136 m (446 ft)11.4 sec 39.1 ft/sec
92 km/h (58 mph)2,627 km (1,633 mi)69 hr14.8 m (48.7 ft)212.2 m (696 ft)14.3 sec 48.7 ft/sec
http://en.wikipedia.org/wiki/Wind_wave

The above chart gives a better understanding of the factors contributing to wave length. According to the data, a gale would have to be blowing at roughly 75 km/hr (47 mph) over a sustained period of at least 42 hours in order to create a wave length approximating the length of the Waratah - 465 ft. (141.7 m). Could this have been possible off Cape Hermes late 27 July, 1909. The simple answer is 'no'. The Captain of the Harlow clearly stated in his account that the storm approaching from the southwest had not materialized off Cape Hermes by that time.

If, for arguments' sake, the Waratah had continued on course for Cape Town and run into the 'storm of exceptional violence'. Within the first 24 hours of the storm, it is not possible, given the data above, that the wave length could have achieved 141.7 m. 

However, shorter wavelengths, in a turbulent, stormy sea create significant forces against an already strained hull. The excessive rolling and pitching forces could theoretically cause failure of a compromised hull. It stands to reason Captain Ilbery, under the circumstances, made a decision to turn back for Durban. One thing is clear, whatever presented the final straw for the Waratah's hull integrity, could not have been in the form of a 'back-breaking' wave length.

Important update:

http://waratahrevisited.blogspot.co.za/2016/06/is-there-alternative-to-poenskop.html

See:

http://waratahrevisited.blogspot.co.za/2016/04/needed-12000-tons-cargo-for-stability.html



                                                       Liberty Ship, SS Quartette









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