Archimedes boasted “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.” An application of the same mighty principle of leverage is found in the blocks and tackles that are used to lift weights on board ship and in the construction industry. “Give me a rope long enough, and I shall lift the world.”
“Mechanical Advantage” is defined as the ratio of the output force to the input force applied to a mechanism. If a device has a mechanical advantage of 5 (5:1), for example, applying 100 pounds of input force c generate an output force of 500 pounds. As increasing the length of a lever enables a small force to generate a much greater force, so extending the length of line looped through block-and-tackle arrangements translates minimal effort into significant power.
Mariners are familiar with standard block-and-tackle arrangements such as two-fold, three-fold, gun tackle, single-luff and double-luff, as well as the ways these can be rearranged or paired to be even more effective weight-lifters. But what is the procedure for reeving a block?
A USCG examination question recently asked where would one start to reeve a right-angle three-fold purchase.
|To reeve a right-angle three-fold purchase start with the _____|
|A. left sheave bottom block|
|B. left sheave top block|
|C. middle sheave top block|
|D. right sheave bottom block|
How to Reeve
Here is a description of the procedure as found in the “Construction Manual for Building Structures.” In reeving a simple tackle, the blocks should be laid a few feet apart. The blocks should be placed down with the sheaves at right angles to each other and the becket ends pointing toward each other.
To begin reeving, lead the standing part of the falls through one sheave of the block that has the greatest number of sheaves. If both blocks have the same number of sheaves, begin at the block fitted with the becket. Then pass the standing part around the sheaves from one block to the other, making sure no lines are crossed, until all sheaves have a line passing over them. Now, secure the standing part of the falls at the becket of the block containing the least number of sheaves, using a becket hitch for a temporary securing or an eye splice for a permanent securing.
With blocks of more than two sheaves, the standing part of the falls should be led through the sheave nearest the center of the block. This method places the strain on the center of the block and prevents the block from toppling and the lines from being cut by rubbing against the edges of the block.
Falls are generally reeved through 8- or 10-inch wood or metal blocks in such a manner as to have the lower block at right angles to the upper block. Two, three-sheave blocks are the usual arrangement. The hauling part must go through the middle sheave of the upper block or the block will tilt to the side and the falls jam when a strain is taken. If a three- and two-sheave block rig is used, the method of reeving is about the same but, in this case, the becket for the dead end must be on the lower, rather than the upper, block.
You must reeve the blocks before you splice in the becket thimble or you will have to reeve the entire fall through from the opposite end.