SPRENGER ratchet block

Ratchet block with ball-bearings and an aluminium sheave.
Lockable swivel shackle.
Plastic (HOSTAFORM C) housing with stainless-steel fittings.

Ratchet blocks can significantly reduce the load on the hauling part of the sheet, because the ratchet roller (in a 3-sheve block the middle roller) can be blocked during slacking.
In light wind the ratchet can be switched off to allow the block to turn both ways.
With the ratchet on, the sheet purchase has 8 times as much power as when running free.

In blocks with cleats, the angle of the cleat is adjustable.