Pulleys allowed people to build large sailing ships to explore the world. Without the pulley, the force of the wind on large sails would have been too big to be countered by sailors. So, the early seafarers would not have been able to set their sails to explore the world!

When exactly the pulley was invented, and who the inventor was, no one knows. Popular theories reason that the pulley most likely came from Eurasia, although the basic building blocks for the pulley were known in other areas of the world. Archimedes is credited with developing the first documented block and tackle pulley system.

A pulley is a wheel with a groove along its outer edge, to hold a rope or cable wrapped around the wheel. The wheel allows us to change the direction of an applied force. For example, the pulley allows us to apply a horizontal force, which the pulley converts to a vertical force, to pull downward on a billowing sail, or to lift or move an object.A pulley allows the rope to travel through with next-to-no friction. A basic pulley has a simple spindle that allows the pulley to turn. But pulleys that incorporate bearings of some kind, to reduce friction, are more effective because the pulley offers less resistance to the applied force.

Pulleys differ in size, the materials used to construct them, the method used to reduce friction and the way they are applied. A number of pulleys can be grouped in a single pulley unit, or "block".

Pulley types

Fixed pulley

A "fixed pulley" is stationary. The pulley is fixed or anchored in place, allowing the tackle to act around it to redirect the force applied to the rope.

A fixed pulley cannot reduce the required force on its own, and therefore has a mechanical advantage of 1.

Movable pulley

A "movable pulley" is not anchored to a fixed position. It has a free axle that can move around in space.

A movable pulley can be used to change the force required to move an object. A movable pulley, used on its own, has a mechanical advantage of 2.

For example: If one end of the rope is anchored to a fixed position while the pulley is attached to an object, the force applied to the loose end of the rope will be doubled.

Compound pulley

A "compound pulley" is a system of pulleys, combining fixed and movable pulleys to perform a task.

A compound pulley can greatly increase the applied force, resulting in a mechanical advantage of more than 2.

Pulleys are usually used together, in configurations of two or more, designed to reduce the amount of force needed to lift a load or move an object. A compound pulley makes it easier to move heavy objects by trading distance for effort. A compound pulley block and sheet system reduces the amount of force needed to move the sail, but the end of the rope has to travel a longer distance through the set of pulleys.

The magnitude of the required force is reduced, but it must act over a longer distance to perform the same amount of work. So, by making use of compound pulley systems, a sailor can exert a huge force, without having to be a bulging muscle man! But he/she will still be performing the same amount of work, because the smaller force has to act over a longer distance.

For example: With a compound pulley system using two pulleys, a sailor would only have to apply half the force to tighten or set the sail, but he/she would have to pull the sheet twice as far. And using a three-pulley configuration, the required force would be one third, but the tackle travels three times as far.

The effort needed to lift a load is roughly the weight of the load divided by the number of wheels in the compound pulley system. So, the more wheels there are, the smaller the force required to move the object - in a perfect world. But in real life, the more wheels there are, the bigger resistance there will be against the applied force and the less effective an additional pulley will be.

When two or more pulleys are used together, to reduce the force needed to lift a load or move something, they are collectively called a "block". The rope or cable used in the block system is called the "tackle".

Sailing pulleys

A sailboat may have a number of different pulley systems to do different things. Pulleys can be used in the boomvang, cunningham, downhaul and mainsheet. Some pulleys are more suitable to marine applications, such as sheet blocks on sailboats, because their components do not corrode as easily as other pulleys. When a pulley gets used aboard a sailboat, it is usually known as a "block", not a pulley. But the term "block" is normally used to refer to two or more pulleys incorporated into a single unit.

You don't need to know how to calculate the forces in a pulley system to sail your boat. But you do need to know what makes a pulley work properly, so that you can adjust your rigging to use your sailing blocks effectively. Sailing pulleys make it easier to sail a boat. They make sure that you don't have to break your back adjusting the sails in a strong wind.

The pulleys on your sailboat are your friends. Just remember to keep your fingers and hair clear, so they don't get caught up in the workings!