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Turning block troubles
In some applications also often known as ‘foot blocks’, turning blocks are called upon to deal with fearsome loads. Their purpose is to redirect the lead of a rope – usually a sheet or running backstay – into a more convenient direction. A spinnaker, for example, might be sheeted to a point right aft, with a foot block being used to turn it forward again towards a winch. In such circumstances the static load on the block could be as much as twice (200%) – and no more than twice – the load on the sheet. Shock loads are being ignored.
To understand principle, it might help to look at the drawings (right). Figure (a) might be the sort of situation a genoa lead block would face. The sheet load is 100 of any weight unit you might choose. Here the block and sheet loads are 120º apart and it’s easy to imagine how this arrangement would distribute the loads evenly between the three loaded elements.
Now turn your mind to Figure (b) – perhaps a spinnaker sheet, taken forward to a handy winch. I’ve superimposed a vector diagram showing how the sheet loads resolve, this time increasing the load on the block to 173%. We could just as easily have come to the same conclusion mathematically by using the formula at the bottom of the page.
The last illustration (Figure (c)) shows this effect at its maximum. The sheet loads have been turned complete back on themselves (180º) and the load has maxed out at 200%. The table below shows the load factors for other angles of deflection.
The long and short of it that, if ever you are turning a rope, don’t underestimate the loads that can come with it. Years ago, crewing on a race boat, I clearly remember the shock on a crewman’s face when the running backstay he was straddling let go. The block came out between his legs, in pieces and at high velocity. He wasn’t hurt but, being young and recently married, he looked uncharacteristically thoughtful for the next couple of hours.
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Boats and gear Turning Block Troubles - why foot blocks and turning blocks can be seriously overloaded
