METHOD OF TOWING 



489 



The machine has been tested for stability up to a speed of 16 knots. The pull on the 

 towing rope at this speed was found to be approximately 10 cwt. The thinner the towing 

 rope the less is its resistance to the water and consequently the greater the diving 

 performance of the machine (see p. 463). Yet a good margin of safety must be main- 

 tained, for the machines are costly. 



I will describe the equipment employed in 

 towing the recorders from commercial steamships 

 across the North Sea. Whilst the ships co-operating 

 do not attain speeds of more than 14 knots, and 

 usually run at 11 or 12 knots, it must be expected 

 that the pull on the rope in rough seas may at times 

 exceed the calculated 10 cwt. for 16 knots. A 

 breaking strain of 3 tons was decided on ; but by 

 selecting an aircraft cord which is made of a much 

 higher tensile steel than ordinary commercial rope, 

 it was possible to keep down its size to a diameter 

 of 0-255 m - An unforeseen failure occurred. On 

 a third run across the North Sea the rope broke 

 and a machine was lost. The broken end was ex- 

 amined in the laboratories of Messrs British 

 Ropes, Ltd., who were able to determine that the 

 fracture was due not to overloading but to the 

 development of fatigue through vibration. To 

 overcome this difficulty three measures have been 

 taken : 



(1) Messrs British Ropes have made a special 

 rope for this work which is the same aircraft cord 

 as before but with the addition of a fibre core 

 which has the effect of damping down vibration 

 and so reducing the development of fatigue. The 

 detailed specification of this rope is as follows: 

 Drawn galvanized plough steel wire rope, 6/19 

 construction with a main fibre core, each wire 

 0-018 in. diameter; breaking stress 60 cwt. 



(2) A special shock absorber in the form of a 

 rubber compression block within a steel cylinder, 



/NCHES 

 2 



l.i.l 



I ■ I ■ I ■ I ■ I 



J_L 



Fig. 17. Longitudinal section of the shock 

 absorber or vibration damper placed between 



illustrated in Fig. I 7 and described below, was the towing eye and rope. For letter references 



made to be fixed between the end of the rope and see text. 



the machine. This has a further effect of damping down the vibration in the rope at the 



outboard end where it is likely to be greatest and where fatigue had previously developed. 

 (3) Between the towing rope and the shock absorber is shackled a double bridle 



(2 ft. 6 in. long) of the same specification as the towing rope. This gives double strength 



