Block (sailing) Block (sailing) In sailing, a block is a single or multiple pulley. One or a number of "sheaves" are enclosed in an assembly between "cheeks" or "chocks". In use, a block is fixed to the end of a line, to a spar, or to a surface. A line (rope) is "reeved" through the sheaves, and maybe through one or more matching blocks at some far end, to make up a tackle. The "purchase" of a tackle refers to its mechanical advantage. In general the more sheaves in the blocks that make up a tackle, the higher its mechanical advantage. The
Block and tackle output force), formula_5 is the ideal mechanical advantage of the system (which is the same as the number of segments of line extending from the moving block), and formula_6 is the mechanical efficiency of the system (equal to one for an ideal frictionless system; a fraction less than one for real-world systems with energy losses due to friction and other causes). If formula_7 is the number of sheaves in the purchase, and there is a roughly formula_8% loss of efficiency at each sheave due to friction, then: This approximation is more accurate for smaller values of formula_7 and formula_8. A