History
Tackles are systems of blocks and lines that increase pulling or lifting power. Cranes using single-pulley wheels were invented 3,000 years ago and compound tackles, with several wheels, date back to 400 B.C.
It is said the Greek mathematician, Archimedes, devised a compound pulley capable of hauling ships ashore! Today's sailors use tackles in many sail adjustment systems, including vangs, cunninghams and mainsheet.
6:1 and 8:1 Tackles
To determine a tackle's mechanical advantage, count the number of short line lengths in the tackle, or the number of times the line goes over a movable block. For example, a single block with becket divides the line into three parts for a mechanical advantage of 3:1. Ignoring friction, this lets you pull 90 lbs (41 kgs) with only 30 lbs (14 kgs) of effort. To determine the correct number of purchases for your system, calculate your line load and divide by pulling power, 50 to 60 lbs (23 kg-27 kg) for an average adult.
Remember that the more purchases you have, the more line you will need to move the block. (For example, to haul the sheet 1 foot (30.48 cm), you must pull 3 feet (91.44 cm) of line). Also remember that the more times your lines run over a sheave,
the more friction increases. Harken ball bearing blocks with large diameter sheaves help keep friction to a minimum and direct more pulling power to the task at hand. When increasing purchase, a slight reduction in line diameter also reduces friction.
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