Tuesday, 27 December 2016

BILGE KEELS AND HANGING IN A LIST.

Waratah, despite stability limitations, was the latest in technology for 1908. The following extract from A. Hoehling's book describes an important innovation:

“fitted with Bilge Keels, which make them exceptionally steady at sea'

Hoehling, A.. Lost at Sea: The Truth Behind Eight of History's Most Mysterious Ship Disasters.

Bilge keels, long fins of steel welded along the length of the steamer (190 ft. (40%) amidships in the case of Waratah - see image of model below) at the turn of the bilge, on either side, increase the hydrodynamic resistance (creating inertial forces) when a vessel rolls, limiting the amount of roll a vessel has to endure. This is known as a passive stability system. Bilge keels absorb roll energy through the viscous-eddy effect - interrupting the flow of water around the chine of the hull (sharp change in angle in the cross section of a hull), a low pressure created behind the fin which in turn absorbs energy of motion. This effect is velocity-sensitive, and the faster the steamer travels, the stronger the damping effect. The effect of bilge keels can be so pronounced that in conditions of a following sea and the deep slow rolls generated, the steamer, as in the case of Waratah, might hang in a list - i.e. the inertia generated by the bilge keels make it harder for the steamer to right herself from heavy rolls. Bilge keels were introduced primarily for the comfort of passengers and to reduce the incidence of sea-sickness. They did not improve the safety factor of a steamer's stability. In the case of Waratah, which was significantly tender / top heavy during her first three voyages, the tendency for long, slow rolls was accentuated by the bilge keels which in turn in all probability contributed significantly to the Waratah's tendency to hang in a list, compounded by windage of course and enhanced by a following sea, with or without adverse weather conditions.  












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