49° 



DISCOVERY REPORTS 



at the point on the rope where fatigue is likely to develop. In the North Sea survey 

 these double bridles are replaced by new ones after every crossing. 1 Their cost, eight 

 shillings, is a small premium to pay for safety when the machine costs close on a hundred 

 pounds. After three runs the outboard end of the main rope is shortened by a fathom, 

 so that a possibly fatigued end is discarded ; after a further three runs the rope is turned 

 end for end, so that the inboard end not hitherto subjected to fatigue may now be used ; 

 after the next three runs the rope is again shortened by a fathom, and finally, on com- 

 pletion of twelve runs the rope is discarded. 



Since these precautions have been adopted recorders have run for over 30,000 miles 

 without further loss due to fatigue. 



WATER LINE 



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30 



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36 FEET 



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 / 



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10- 



20- 



30 



40 



50- 



60- 



66 FEET 



10 



-20 

 7qJ metres 



FEET 



Fig. 18. Graph kindly prepared by the School of Mine Design, H.M.S. 'Vernon', Portsmouth, showing the 

 depths taken up by the Type II Recorder with different lengths of towing rope of 0-255 ' n - diameter at speeds 

 of eight to sixteen knots. 



The shock absorber or vibration damper may now be described in detail. It is shown 

 in position in Figs. 3 and 4, X and in section in Fig. 17. It consists of a steel barrel XA 

 closed at one end and fitted with a screw cap XB at the other. The jaw XC, which is 

 secured to the towing eye of the machine, has a pin formed on it which passes through 

 the end of the cap and is secured internally by a nut which is tightened up to a shoulder 

 on the pin ; the end of the thread is riveted on to the nut. The end-cap is screwed into 

 the main barrel and prevented from turning by a locking screw XD. Inside the main 

 barrel is housed a cylindrical rubber compression buffer XE against the end of which a 

 plate XF bears ; the towing bolt XG, formed into an eye at its otuer end, passes through 

 a hole in the closed end of the barrel, through the centre of the rubber buffer and through 

 a hole in the plate XF against which it is secured by a castellated nut and split pin. 

 The towing strain thus compresses the rubber between the plate and the closed end of 

 the barrel; adequate clearance is allowed between the buffer and the barrel and the 

 buffer and the bolt for the lateral spread of the rubber when under compression. 



From trials carried out at speeds of 8-16 knots H.M.S. 'Vernon' has kindly provided 

 1 Except on the short Hull-Rotterdam line when they are replaced after every two runs. 



