On small to midsize boats the mainsail control systems are commonly four part (4:1) purchases. The boom vang, the backstay adjuster and the mainsheet can be controlled by this arrangement of two double blocks – either standard blocks or fiddle blocks.
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 the loaded block; a six part purchase would have six lines leading to and from the loaded block.
An 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 to lower the engine in.
The handy-billy – an essential piece of kit on any boat.
Have look at the range of reinforced ball bearing blocks on the Salty John website, excellent blocks at very attractive prices.