What is an impeller? Function and when to replace it

3 min read

The Impeller – The Heart of Your Inboard Engine's Cooling System

If you have a boat with an inboard engine – especially a Volvo Penta or Yanmar – you've probably heard of the impeller.
But what does it actually do, and why do all boat mechanics talk about it needing to be replaced on time?

Simply put, the impeller is the engine's little "heart" in the cooling system.
It's a small rubber propeller located inside the raw water pump, ensuring that cold water is pumped in from the sea or lake,
so the engine doesn't overheat.
Without the impeller, the engine would quickly get too hot and be damaged.

The impeller is therefore one of the most crucial wear parts on your boat – and precisely for this reason, it needs to be checked and replaced regularly.

How does an impeller work in an inboard engine?

An inboard engine is usually not cooled by a closed system like in a car, but by raw water.
Water is drawn in through an intake under the boat and then passes through the raw water pump where the impeller is located.

As the impeller spins, it pushes the water further into the engine's cooling channels, ensuring the engine maintains a consistent and safe operating temperature.

The impeller is made of a flexible rubber material that can withstand pressure and seal against the pump housing.
But because it constantly rotates, flexes, and is exposed to water, heat, and sometimes air, it gradually wears down.
That's why it doesn't last forever.

In short: The impeller is small, but without it, the cooling system won't work – and without the cooling system, your engine won't work.

When should you replace an impeller?

There are two schools of thought here, but both agree on one thing: don't wait too long.

  • General rule of thumb: Replace the impeller once per season, preferably in the spring before launching.
  • Alternative rule: Replace every 100 engine hours if you run it frequently.
  • Visual inspection: Remove the impeller and look for cracks or wear.

 

Factors affecting impeller lifespan

  • Winter storage: When the impeller is idle all winter, the blades can stiffen and crack.
  • Saltwater: If you primarily operate in the sea, the impeller will wear out faster than in fresh water.
  • Dry running: If the engine is started without water, even for just a few seconds, the impeller can burn out immediately.

Replacing the impeller in time is a cheap insurance against major problems.
A new impeller only costs a few hundred kronor, while an overheated engine can lead to repairs costing tens of thousands of kronor.

Why choose an original or Orbitrade impeller?

To ensure correct fit and durability, it is recommended that you choose an original impeller or an
Orbitrade impeller adapted for your engine model. They are made of high-quality rubber that can withstand both heat and salt water,
and provide optimal performance season after season.

At Ohooj.se you'll find impellers for most Volvo Penta and Yanmar engines:
Find the right impeller for your engine at Ohooj.se

Summary

The impeller is a small but vital component in the engine's cooling system.
By replacing it on time, you avoid overheating and costly engine damage.
Replace it every season or according to the engine's running hours – it's a simple measure that ensures a safe and trouble-free boating summer.