Block & Tackle
Pulleys are used for their ability to redirect force, such as hoisting a flag to the top of a flagpole. A pulley at the top saves us the trouble of climbing the pole. Onboard ship we refer to pulleys as blocks.
Anatomy of a Block
The shell of a block may be of wood or metal construction; only metal may be used with wire rope. The sides of the shell are called cheeks and their height, in inches, is how a block is sized. The cheeks of a block contain the sheave, a grooved wheel, over which the line is rove, or passed. A small metal ring on the bottom of a block, to which the standing part of the tackle is spliced, is called a becket.
Some blocks have multiple sheaves, and this combined with the height of their cheeks is how the block is referred to. For example, you may have a single 6” block or a double 9” block.
How Tackles Work
Grabbing and pulling sideways on a line will give you much more mechanical advantage, or purchase, if you do so in the middle of a line than if you do so at one end of a line. This is essentially how blocks provide mechanical advantage. When you introduce a second block you now have a tackle. This is where their true advantage comes because the effects of mechanical advantage are multiplied by the number of sheaves used. Simply: the more sheaves you have, the more power you have.
There are two ways that a tackle may be rigged: to advantage or disadvantage. In any tackle arrangement there will be one block that is suspended and doesn’t move. This is the stationary block. The other block moves towards the stationary block as the two blocks are drawn together through the action of hauling. This is the movable block.
A tackle is said to be rigged to advantage when the hauling part is rove through the movable block. This also means that the movable block is affixed to the object you are trying to move. Therefore, during operation, the movable block takes the greatest strains.
The part of a line that passes from one sheave to another sheave is said to be the “fall” of the tackle, and most tackles have multiple falls. This is the simplest way to calculate the mechanical advantage of your tackle arrangement; you count the falls. If there are five falls, then you have a 5:1 power ratio. If your tackle is rove to advantage, then you add one. Now, with five falls rove to advantage, you have a 6:1 power ratio.
Sizing for Use
When sizing blocks for use with rope, the cheek length of a block should be in inches about three times the height of the circumference of the line it is intended to be used with. Wire is much stiffer and requires a greater bending radius, sheaves for use with wire should be 20 times its diameter.
Operation of a Tackle
When the tackle is hauled completely together, it is said to be “two-blocked,” or “chock-a-block,” and they may jam as a result. Separating the two blocks from this, or in preparation for use, is called overhauling.
Types of Blocks
Snatch block – hinged to allow the admission or removal of line without having to reeve its entire length through the block. This makes a snatch block very useful as a fairlead.
Handy billy – a small light tackle with blocks of steel or wood for miscellaneous small jobs is called a handy billy.
Made block – made from several pieces of material attached together to form a block. Almost all blocks these days are made blocks.
Morticed block – a block fashioned from a single piece of wood.