Block

A marine pulley assembly containing one or more sheaves over which a rope or wire runs.

Blocks change the direction of a line and, when combined as a tackle, can provide mechanical advantage. Parts may include the shell, sheave, pin or bearing, cheeks, becket and attachment such as a shackle or swivel.

A block must suit the line diameter, construction, load, lead angle and environment. Side loading, damaged sheaves, worn bearings and undersized attachment points can sharply reduce safe working capacity.