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[Skipper Don]

The raw water impeller is one of the most important service items on a marine engine. It is also one of the most commonly neglected. A failed impeller causes overheating that can destroy an engine in minutes.

What it does. The raw water impeller is a rubber pump that draws seawater from outside the hull and circulates it through the heat exchanger to cool the engine. It is driven directly off the engine and runs any time the engine runs.

Replacement schedule. Most engine manufacturers recommend replacing the impeller every 200 to 300 engine hours or annually, whichever comes first. If you store the boat over winter, replace the impeller at spring commissioning regardless of hours — rubber hardens and develops set cracks during cold storage.

Signs of impeller failure. Rising coolant temperature is the primary sign. You may also notice reduced raw water flow from the exhaust outlet — on a healthy engine, you should see a steady stream of water mixed with the exhaust. A dribble or intermittent flow means the impeller is failing. Some impellers shed rubber vanes as they fail; the vanes can lodge in the heat exchanger and require manual removal.

Monitoring with d3kOS. The d3kOS Predictive Maintenance feature tracks coolant temperature trends over time. A gradual rise in operating temperature across multiple trips — even if it never reaches the warning threshold — is a pattern the AI flags as worth investigating. This can catch a partially failed impeller before it causes overheating.

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