The Busy Shore: Block and Tackle
May/June 1973
By Jan Adkins
a creatively lazy tinkerer
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In the matter of strength, a man's a puny creature. You cannot carry a deer in your jaw, you cannot fly two thousand milks or two feet on your own, cannot antlike lift ten times your weight. Man's mind must multiply his trifling strength. But a race of men that has developed aerosol cheese spread leans heavily on its technology, and tends toward caterpillar tractors to move its garbage cans. A few people, watermen especially, rely more on clever thought and simple machines, and know that puny strength can be craftily applied to immense advantage. One of the oldest and simplest machines is the block and its tackle, an idea of such wonderfully spare reason that it seems magical, running uphill against the stream of nature. Its principle is just this: if one line suspends a weight, it bears all the load; if two lines suspends a weight, each bears half the load; if the lines are rendered movable, the weight can be moved with half the effort. There are a few cautions in using blocks. The sheaves must be of sufficient diameter and width for the size of the rope; too tight a turn will damage the fibre. keep the pivot pin oiled. Figure a safely factor of 5; the line you use should have a breaking strength of five times the expected load.
A RUNNER yields a mechanical advantage of 2x when the pull is away from the weight; but toward the weight, a single block only changes the direction of the pull.
when the GUN TACKLE is rove to advantage (when the pull is in the proper direction) its advantage is 3x; rove opposite, the mechanical advantage is 1.5x.
the LUFF TACKLE, with a mechanical advantage of 4x, is used a great deal aboardship.
a DOUBLE TACKLE is for heavy work, offering a mechanical advantage of 5x, but requiring much rope and a long haul.
the SPANISH BURTON is a specialized tackle for short hauls, used for bringing anchors, stores, and dories aboard.
Like all moving machines, friction takes its due, and each sheave demands a part of the effort. Sailors figure that 5-8% is added for each sheave, and a formula estimating a somewhat greater loss is used by fishermen (Pull on the active tine= P/W=Weight lifted / F = number of falls, or lines, from the moving block.)
A luff tackle with a tail sinnet instead of a hook is called a HANDY BILLY, and is used for many jobs aboardship, especially for setting lines taut. The sinnet is half-hitched once or twice around the line to be set, then made fast with a Rolling Hitch (see Mother # 19)
