Whether hauling a motor off your boat and into storage in preparation for the end of the season or lowering an outboard motor into place during the spring fitting-out, a block and tackle offers you the ability to raise and lower heavy weights in a controlled manner. Because the device reduces the amount of effort you have to use to move substantial objects, a block and tackle with two single-sheave blocks, called a "gun tackle," requires only a third the effort that would be necessary to manhandle something heavy into place.
- Skill level:
- Moderately Easy
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Things you need
- 2 single-sheave blocks
Reeve the rope through one of the blocks. This will be the stationary block if hung from a fixed point, or the movable block, if you attach this block to the load you wish to move. "Reeving" is the technical term for inserting the rope through the shell of the block, onto the pulley.
Reeve the end of the line through the second block.
Carry the end of the line back to the bottom of the first block. On the bottom of the block -- the end opposite the hook -- there's a ring called a "thimble." Securely tie the end of the line to the thimble.
Tips and warnings
- When raising a weight above your level, rig the tackle to disadvantage; when raising a weight to your level, rig the tackle to advantage.
- To calculate the mechanical advantage, count the number of "parts" supporting the movable block. If you haul the rope from the stationary single-sheave block of a block and tackle, the block has a "part" of rope going into the sheave and one coming out of the sheave. The mechanical advantage is two-to-one and it's rigged "to disadvantage." If you haul from the single-sheave movable block, the block has a part from the thimble to the stationary block, another coming into the sheave, and a third from the sheave to your hands. The mechanical advantage is three-to-one and it's rigged "to advantage."
- A pair of single-sheave pulleys is called a gun tackle. The only change needed to turn a gun tackle from being rigged "to disadvantage," to being rigged "to advantage," is to turn the entire gun tackle over.
- Multiply the diameter of the rope by 9.42 to calculate the size of the block you need.
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