Chris Mardon writes: “I was always told that when replacing the impeller on my diesel engine raw water pump I should make sure the blades were folded in the swept back direction of rotation.

“I’ve always adhered to this impeller orientation advice but often wondered if it was based on sound evidence.

“I suspected perhaps not because when diesel engines are stopped they will often bounce back off cylinder compression causing the engine to run backwards by a few degrees – enough to force an impeller blade to reverse against the pump’s cam.

“My suspicions about the irrelevance of impeller orientation were confirmed when I had cause to replace a leaking pump shaft seal.

“You can see from my photo (top right) that on removing the cover plate two of the impeller blades were folded in opposite directions despite the care I took to ensure they were correctly aligned when I installed the impeller.

“This confirms my opinion that no matter which way you fold the impeller blades the pump’s cam will align them correctly as soon as you start the engine.

“It’s easy enough to check if other makes of pump will automatically align their impeller blades by deliberately installing them with blades reversed, open the seacock, start and stop the engine, close the seacock and remove the pump’s cover plate to see if the blades have realigned.”

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PBO engine expert Stu Davies replies: “Good PBO-ing! The first turn of the raw water pump shaft orients the lobes of the water pump vanes in the correct direction, so the advice to always ensure the vanes are oriented in the correct rotation when installing a new impeller is perhaps not quite as necessary as we’d always thought, as your photo quite plainly shows, and as I have also seen myself.

“Plenty of lubrication – which is usually supplied with a new impeller – should look after the vanes and lobes as they briefly re-orientate themselves on start-up.”

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