LESSON 2: BLOCK AND TACKLE
 
Target Ages: Middle School & Up
   
Introduction: In the age of sail, naval ships like the Providence carried a large crew to fight in battles. Many of the crew-members were needed to shoot the cannons, and few sailors could be spared to sail the ship. Likewise merchant vessels carried small crews to keep costs down and carry more cargo. To maneuver the vessel with a few sailors the crew depended on the mechanical advantage of blocks and tackles to lift sails, anchors, boats and cargo.
   
Materials: Block & Tackle
Whip tackle
2 50lbs 'Cargo' bags
Block with cheek removed
Pulley Machine
Spring Scale
Ruler
Preventer luff tackle
Secondary luff tackle
   
Lesson Objectives: Block and tackles allow sailors to lift heavy objects through mechanical advantage giving them 2, 3 even four times their normal strength. Also show how the distance pulled is 2, 3, 4 times farther. I use the whip tackle and a two-fold purchase to teach how block and tackle multiplies effort and distance pulled making it easier for fewer sailors to lift heavier objects.
   
Problem: The captain needs cargo loaded aboard the ship. How are you going to move this heavy cargo?
   
Answer: By block and tackles
Lesson Sequence

First briefly explain that a block and tackle is a system of "pulleys and ropes" used to lift and pull heavy objects. Use the block that is missing a cheek to point out the major parts of a block and tackle and some of the various uses of blocks and tackles on board a boat. Show how a whip alters the direction of pull, however it does not alter the force needed to pull or the distance pulled. Then set the pulley machine as a two-fold purchase.

Explain that the effort has now increased 4x's and the total distance of line pulled to move the block has also increased 4x's. Now set up the 'cargo' on the whip tackle and the two-fold purchase hanging from the quarterdeck Let the kids lift the cargo by hand with the whip, and finally the two-fold purchase. Let the kids two block each of the tackles and measure how much line is used each time.

Show how the line has to be pulled four times as far to lift it, yet it is not 4 times as easy. This is due to the friction in the sheaves and pins on each line.

* If time (and the students are old enough) measure the effort (E) to pull the weight (R for resistance) up on a whip line with the spring scale and measure the distance. Figure out the mechanical advantage (MA) of the whip by this formula: MA = R/E

Then repeat the process to measure the effort and distance pulled for a gun tackle, luff tackle and two-fold purchase. The effort will decrease as the number of falls is added. Thus the mechanical advantage will increase proportionately.

**Dockside alternative/little kids/stall technique: Block and Tackle Tug o' War
 
Explain how a block and tackle works.

Use preventer luff tackle. Set up a line to running block place a majority of the kids on that line. Fix standing block to a sturdy piling or post. Place several students on the hauling part of line. Make sure that the moving block has more students on it, however with the 4:1 ratio the hauling part will easily beat the moving block team.
Also be sure and mark where each team started and finished to show how far the hauling part had to pull to move the moving block.

***If there is an amazing amount of time that needs to be filled set up a luff on luff tackle. Attach the moving block of the secondary tackle to the hauling part the primary luff tackle. Fix the standing block of the secondary tackle to a sturdy piling or post.

A luff on luff tackle will generate a high mechanical advantage. For example the primary luff tackle has a 3:1 ratio, the secondary luff tackle has a 4:1 ratio, therefore the overall mechanical advantage is 3 x 4 or 12:1. With a luff on luff tackle you should be able to place the several of the smallest kids on the hauling end of the secondary luff tackle and the rest of the group on the moving block of the primary tackle and the small kids will be able to pull the rest of the class!
 
Vocabulary
Blocks - A pulley to a landlubber
Falls -The lines of the block and tackle
Tackle - The whole system of blocks and falls
Shell - The entire outer part
Cheek - The side plate to the block
Sheave - The slot which the line is reeved through
Mass - The "weight" of an object
Effort - The force taken to move an object
Purchase - A mechanical device used for lifting or pulling a heavy object
Whip - A 1 to 1 ratio, a fixed single block and fall, i.e. a fair lead
Gun Tackle - A 2 to 1 ratio, a fixed single block and a movable single block, i.e. throat jig
Luff Tackle- A 3 to 1 ratio, a fixed double block and a movable single block.
2-fold purchase - A 4 to 1 ratio, a fixed and a movable double block, i.e. main sheets