They might sound similar, but their purpose and function are vastly different from one another. For example, both are used to operate a boat but are quite opposite in their functionality. A propeller is a type of fan that propels fluid by pushing against it, converting rotational force into a linear motion. An impeller is a type of rotor that is typically part of a pump and creates a sucking force, i.e. drawing liquid into a vehicle or engine. Let’s take a deeper dive (yes, a boat pun) into what each of these components are and their functions.

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Impeller

Simply put, an impeller is a rotor that creates an intrinsic sucking force, as part of a pump set up. An impeller in the case of a boat is made up of a series of flexible rubber vanes attached to a central hub. This hub rotates around an axis inside a stainless-steel liner that is housed within the pump. Cold water is pumped by the impeller into the boat’s engine in order to cool it down while under load. Marine engines have an impeller in the water pump, and that is used to bring water from outside the boat into the plumbing system.

There is an inlet on the impeller that gives water a path inside the boat, and the rubber vanes push the fluid through the inlet. The impeller is the centrifugal pump’s rotating component, and it pushes the fluid away from the center of rotation. As a result of this, energy is transferred from the pump to the water, this creates velocity in the water and causes it to move outward from the pressurized pump. The Archimedes screw can be looked at as a very early and basic version of an impeller.

High, medium and low solidity impellers

Impellers can be sorted in three different categories: “high”, “medium” and “low” solidity types. What this means is that high solidity impellers have rather large square shaped blades that are mainly used for the dispersion of gases, as well as for blending high or medium viscosity fluids. The specific design and size of these blades stop gas from just going around them, which would happen a lot easier if the blades were of a more narrow shape.

Low solidity impellers possess much thinner blades than their high solidity counterparts. They are generally used for blending low viscosity fluids and solid suspension. Medium solidity impellers are a middle ground between high and low, naturally.

Cross-section of an impeller within a high-pressure blower. Source: [Surasak/Adobe Stock]

Propeller

A propeller is a fan that pushes a fluid away from it, converting rotational motion into linear motion (extrinsic). Again, in the case of a boat, a propeller works as a fan the creates thrust and therefore generates power for the boat to move. Typically, propellers consist of three or four blades that rotate around an axis to create forward movement. While the blades are rotating, the water pressure behind each blade increases, moving the boat in the water.

This is made possible as the blades stick out from the hub at an angle, allowing water from the front of the prop to the rear without pushing the water to the side. Generally, the more blades that are on the propeller, the more power that can be generated for a vehicle. An increase in blade number can also reduce the amount of drag that occurs. The thrust created by propellers can be explained by either Bernoulli’s theorem or Newton’s laws of motion.

Boat propellers operate in water to generate thrust and move the boat in the water. Source: [Denys/Adobe Stock]

What are the main similarities between impellers and propellers?

• There are many ways in which impellers and propellers are similar:
• They both use rotational energy to move fluid in a certain way
• They both rely on an external motor to power them
• Ongoing maintenance is required by both to keep them running at optimal performance
• Impellers and propellers each utilize water to generate energy, whether it is propulsion or pressure
• Regular lubrication is needed by both to stop their parts from wearing out in a short amount of time
• Referring back to our boat example, they both are vital to a boat’s operation, without propellers a boat will not move, and without impellers a boat cannot keep their engine cool

What are the main differences between impellers and propellers?

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The main difference between the two is their purpose and function. Impellers are typically a main component in a cooling system, where they have to draw in water into an engine to keep it cool and at the right temperature while it is in operation. Propellers are a main part of a vehicles propulsion system; they generate movement in vehicles such as boats and airplanes. The faster the propeller blades turn, the more speed and acceleration is generated.

Even with this major difference between the two, there are still other minor difference that can be drawn between both components:

• Impellers are contained within a pump housing, propellers are not
• Impellers utilize vanes to draw in fluid, propellers use blades to push fluid away
• Impellers create forward motion, propellers generate clockwise rotation
• Impellers create pressure, whereas propellers create water propulsion

When impellers and propellers were initially created and used, they were fundamentally different in most ways. Now, as time has passed and with the advancement of technology, they both use aerofoil technology for their designs, which is derived from Bernoulli’s principle. So, their principle of use is now more similar than ever.