We use blocks and tackle for lifting and moving jobs afloat and ashore. The five or six basic combinations are used over and over in many situations. Cargo is loaded aboard, and depth charges are stored in their racks. You lower lifeboats over the side with this machine. We can swing heavy machinery, guns, and gun mounts into position with blocks and tackle. In a thousand situations, sailors find this machine useful and efficient.

We use yard and stay tackles aboard ship to pick up a load from the hold and swing it onto the deck. We use yard and stay tackles to shift any load a short distance. Figure 2-9 shows you how to pick a load by the yard tackle. The stay tackle is left slack. After raising the load to the height necessary to clear obstructions, you take up on the stay tackle and ease off on the yard fall. A glance at the rig tells you that the mechanical advantage of each of these tackles is only 2. You may think its hard work to rig a yard and stay tackle when the small advantage is to move a 400-pound crate along the deck. However, a few minutes spent in rigging may save many unpleasant hours with a sprained back.

If you want a high mechanical advantage, a luff upon luff is a good rig for you. You can raise heavy loads with this setup. Figure 2-10 shows you what a luff upon

luff rig looks like. If you apply the rule by which you count the parts of the fall going to and from the movable blocks, you find that block A gives a mechanical advantage of 3 to 1. Block B has four parts of fall running to and from it, a mechanical advantage of 4 to 1. The mechanical advantage of those obtained from A is multiplied four times in B. The overall mechanical advantage of a luff upon luff is the product of the two mechanical advantagesor 12.

Dont make the mistake of adding mechanical advantages. Always multiply them.

You can easily figure out the mechanical advantage for the apparatus shown in figure 2-10. Suppose the load weighs 1,200 pounds. The support is by parts 1, 2, and 3 of the fall running to and from block A. Each part must be supporting one-third of the load, or 400 pounds. If part 3 has a pull of 400 pounds on it, part 4made fast to block Balso has a 400-pound pull on it. There are four parts of the second fall going to and from block B. Each of these takes an equal part of the 400pound pull. Therefore, the hauling part requires a pull of only 1/4 x 400, or 100 pounds. So, here you have a 100-pound pull raising a 1,200-pound load. Thats a mechanical advantage of 12.

In shops ashore and aboard ship, you are almost certain to run into a chain hoist, or differential pulley. Ordinarily, you suspend these hoists from overhead trolleys. You use them to lift heavy objects and move them from one part of the shop to another.

To help you to understand the operation of a chain hoist, look at the one in figure 2-11. Assume that you grasp the chain (E) and pull until the large wheel (A) has

The mechanical advantage equals the distance that moves the effort (E). Its divided by the distance that moves the load. We call this the velocity ratio, or theoretical mechanical advantage (T.M.A.). It is theoretical because the frictional resistance to the movement of mechanical parts is left out. In practical uses, all moving parts have frictional resistance.

and in this case,

which is the theoretical mechanical advantage.

Since the friction in this type of machine is considerable, the actual mechanical advantage is not as high as the theoretical mechanical advantage. For example, that theoretical mechanical advantage of 48 tells you that with a 1-pound pull you can lift a 48-pound load. However, actually your 1-pound pull might only lift a 20-pound load. You will use the rest of your effort in overcoming the friction.

The most important point to remember about block and tackle is that they are simple machines. And simple machines multiply effort or change its direction. You should also remember the following points:

A pulley is a grooved wheel that turns by the action of a rope in the groove.

There are different types of pulleys. Pulleys are either fixed or movable.

You attach a fixed pulley to one place. The fixed pulley helps make work easier by changing the direction of the effort.

You hook a movable pulley to the object you are lifting. As you pull, the object and the pulley move together. This pulley does not change the direction of the effort, but it does multiply the effort.

You can use fixed and movable pulleys together to get a large mechanical advantage (M.A.).