Rigging blocks are indispensable on board a sailing boat. They provide mechanical advantage to allow you to move large loads with modest effort 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 top of the mast to haul up the sail. This type of ‘turning’ block crops up all over the boat – directing halyards and sheets being the most obvious application.
By combining blocks into sets that work together as a team you gain mechanical advantage. And that’s a lesson for life, if I may wax philosophical.
The most common examples of block combinations on a boat are the systems for controlling the mainsail: the mainsheet, the boom vang and the backstay tensioner.
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.
You can tell 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. A three part purchase would have three lines leading to and from it and a six part purchase would have six lines.
On small to midsize boats the mainsail control systems are most commonly four part (4:1) purchases. Another extremely useful four part purchase is the handy-billy. Equipped with snap shackles at each end it can be used for all manner of things: Clipped between boom bale and toe rail as a boomvang it provides the most effective way to hold the boom down when on a run; it can help hoist the dinghy, the outboard or even a MOB. I’ve used my handy-billy to hoist the cooker out of the cabin and the beer supplies in.
To get even greater mechanical advantage you can use one purchase to haul on another purchase – a compound system such as this gives 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.
Quick or easy? A downside of block systems is that the higher the purchase the slower the work is done. You have to choose between effort and speed. Very high mechanical advantages involve hauling considerable lengths of line through multiple-sheave blocks, moving the load at a snails pace. For this reason it’s unusual to see individual block systems of higher than 6:1 purchase.
Friction is also a formidable enemy in block systems so choose ball bearing blocks which will keep the loss at each block to less than 3% rather than 10% or more for blocks with sleeve bearings.
There you have it – use blocks to make light work of a tough job.