PBO reader Oliver Chapple has been having issues changing the pump impeller on his Yanmar 4JH3E. Engine whisperer Stu Davies has this advice...


Oliver Chapple from Lincoln writes: “I have a very much loved 35ft Atlantic Ketch called Octopus, a splendid sea boat that has carried us safely to the Western Isles, Orkney, Fair Isles, Shetland, the west coast of Norway and down to Lindisfarne. It’s a fine craft but with one horrid problem – the changing of the water pump impeller.

“The Yanmar 4JH3E engine is first class and has never let us down, but when it comes to changing the pmup impeller she is a complete pig! Hanging upside down with your head in the engine bay trying to undo screws with a rolling swell outside Dartmouth and trying desperately hard not to be sick is, to put it bluntly, horrendous.

“Is there a way of changing the system to be a different type of pump – a different location or electric pump, for example – that could then be fitted in a position that would be easier to access? I’ve had PBO for over 20 years now and it’s always the best yachty read to be had.”

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PBO engine expert Stu Davies replies: “From memory from changing a pump impeller on my mate’s Jeanneau, it is towards the front of the engine but facing backward, and is not directly in front of you. Access is easiest from the front side. But it would appear that on your boat you cannot access it from there.

“Hanging over the engine and getting the screws out is challenging plus pulling the impeller out would be difficult in that position – especially if the engine is hot from recent use.

“The Speedseal option used to address situations like this but is now out of production until someone picks up the slack to continue manufacturing this practical idea where thumb screws are used to hold the end cover on.

“Instead, I’d be looking to source knurled screws to fit the pump cover. Look on the front of the pump cover to see what pump is fitted: OEM engine manufacturers usually use either a Jabsco pump or Johnson pump. They can be metric or imperial sizes.


“A helpful website for sourcing parts like this is www.parts4engines.com. Or go to a local engineering machine shop and ask them to make you some – take one of the screws with you and ask them to make a set of four with a good knurled thumb screw on them. This is essentially what Speedseal used to supply.

“Once the issue of getting the plate easily removed is solved then you need to make sure you have a threaded impeller puller or make one up. Yanmar sell one specifically for this job, I made one and if my memory serves me correctly it is threaded 18mm by 1.5 pitch thread.

“You can check this size on your spare pump impeller. I bought a sump plug for a Renault and drilled the centre to tap it 8mm for a pusher bolt. There is, perhaps, another solution – take the whole pump off. With just two hose clips and four bolts maybe that might be easier to do.

“As for going down the route of an electric pump, I wouldn’t – it’s too complicated. One last thought: is it possible to create a large enough hole in the bulkhead to the side of the engine to gain easy access? You could then give the hole a removable wooden cover plate.”

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This feature appeared in the October 2023 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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