. Discovery reports. Discovery (Ship); Scientific expeditions; Ocean; Antarctica; Falkland Islands. 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 N
. Discovery reports. Discovery (Ship); Scientific expeditions; Ocean; Antarctica; Falkland Islands. 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 I â I â I â I â I J_L Fig. 17.
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