Rigging blocks provide mechanical advantage and they also redirect the lead of a line to make it more convenient to pull on.
A single block at the masthead with a halyard running through it is a simple one part purchase. It provides no mechanical advantage, but it does redirect the lead of the line to the base of the mast so that you can conveniently haul on it. Without the block you’d have to balance on the top of the mast to haul up the sail.
By combining blocks into sets that work together as a team you gain mechanical advantage. How much mechanical advantage is gained is known as the ‘purchase’ of a set of blocks – three to one purchase, four to one purchase, and so on – and this is determined by the number and configuration of blocks in the system.
The more times the line runs through the loaded set of blocks the greater the advantage. The most common examples on a boat are the mainsheet system, the boom vang and the backstay tensioner. On small and mid-size boats these are most commonly four part purchases – a 4:1 mechanical advantage.
You can tell this is a four part purchase because the loaded block – the one that moves with the load – has a total of four lines leading to and from it.
The downside of block systems is that the higher the purchase the slower the work is done – so you have to choose the right balance between speed and ease of effort. Bigger boats with heavier loads will require greater multiplication of effort – or gorillas for crew – and that means more line to haul and more time taken to do it.
Friction is also a formidable enemy in block systems so choose ball bearing blocks which will keep the loss at each block down to around 3% rather than 10% or more for blocks with sleeve bearings.
To get even greater mechanical advantage you can use one purchase to haul on another purchase – a compound system. This provides huge mechanical advantage because the efforts of each system are multiplied, not added together: A three part purchase pulling on a four part purchase gives a 12:1 advantage, not merely 7:1.Or you can use a winch to haul on a block system to even greater advantage.
Blocks are a metaphor for human teamwork; a single block achieves very little on its own but working together with others the results are almost magical.