Towline Loading

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One of the major concerns during a towing operation is how much load is being put on the towline and the fittings used to secure it. There is a way of approximately calculating this – and it will at least give an indication of when the tow line is getting close to or has reached its safe working load.
Maximum safe working load is usually taken to be 50% of the rated strength of the rope. The rated strength or break load being the load at which the line will probably part. Any rope manufacturer or supplier should be able to give not just the rated strength of a rope, but also its ‘elongation at break’ or ‘stretch at breaking’.
For example

  • Polyester 3 strand 20 mm Diameter - break load 6300kgs - elongation at break 30%. Using the above example a cheap & easy ‘eyeball’ method of gauging loads on the towline can be set up. 
  • If the elongation at break is 30%, then the elongation at the ropes max safe working load (50%) will be approx 15%.
  • A preset length is measured and marked on the towline – in the illustrated example 2m. 15% of 2m is 300mm so the when the rope is at its max safe working load those two marks on the towline will no longer be 2m apart but 2.3m apart.
  • To give a visual indication of what is going on, a light easily seen line is attached (usually by stitching & whipping) to the towline at the marks. The light line should be attached such that there is 2.3m of it between the marks. 
  • With no load on the towline the light line will hang loose, as the load and hence stretch increases, the light line will become tighter. At max safe working load the light line will be stretched tight.
  • The light line should be small enough that if it were to catch on any fittings during the towing operation it will not snag but simply snap – this also ensures that a broken line will be a clear indicator if the max safe working load is exceeded.

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