Turning block troubles
The loads on turning blocks are not always understood. This article explains why
I recall it well. In brisk conditions, reaching under rather too large a spinnaker, a friend of mine was standing astride the sail's sheet when there was a loud cracking noise and the turning block rocketed forward between his legs. Recently married, he remained in a subdued, rather thoughtful mood while we completed the race.
In some applications turning blocks, also often known as ‘foot 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 direct 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 the principle, it might help to look at the illustrations here.
The illustration above shows a block turning a rope through 120° – one third of a circle. The load of 100 (it could be pounds, kilos or any other
unit) is shared equally. Next....
Here the load on the rope remains the same but the load on the block is now 173. Let's move on....
Here the static load on the block reaches its maximum and it's easy to see why. If this were a tug-of-war it would be a two against one situation. Excepting shock loads (which can be considerable) this should be the maximum endured by the turning block.
Finally, for those who fancy doing the sums, the following formula for calculating the load on any angles might be of interest. I think!