This series 30 halyard block is used for direct attachment to the headboard plate via the 8mm shackle pin to the mainsail of yachts up to approximately 45 ft loa.
The mainsail halyard then passes through the 30mm st.steel sheave and is then dead ended at the top of the mast.
The 2:1 mechanical advantage reduces the effort required to hoist the sail by nearly half.(there is small loss due to friction).
The block body is machined from Al.alloy and anodised black.