Water pump impeller: how to avoid breaking your boat's engine?

Jun 25, 2023

"Stop the engine, stop the engine!"

The alarm screams. Georges looks over the side and sees that the engine exhaust is no longer spitting water. He stops the machine. "Le croque soleil is at anchor. The barometer had dropped, the wind had shifted, and he was now exposed to a swell that was pushing him towards the beach. Having left to take refuge in the harbor with Janine, a slight setback immobilizes them there.

- We're out of engine cooling, Janine! - Yes, and quickly, quickly. The engine temperature alarm is sounding and there's no more water in the exhaust! Open the water pump housing and let's check the impeller. I'll throw out the spare anchor in the meantime.

Georges dismantles the pump cover. The impeller is well out of order, almost melted, the tips of the blades in pieces. No doubt about it! Let's change it. Not easy to extract when there's so much damage and the boat is dancing on the waves.

Janine pokes her head inside: - What's up? - He's H.S. All right, let's not get out of hand, shall we? - No, it's fine for now. - I've almost replaced it, so get ready to start. Extracting it from this corner of the engine is already a feat, but I can't tear the new gasket here. - Are you done? Hurry up, will you? It's blowing up there! - Yes, it's okay.

Janine starts up and looks over the side to see if the engine is cooling down nicely again at the exhaust outlet.

"Stop the engine, stop the engine! It's no good, there's still no water coming out of the exhaust. - Gee, wait a minute, let me see if the seawater filter is working. - What's up? - I've found the problem: it's completely blocked.

Once the filter has been cleaned of algae, "Le croque soleil restarts again.

VRROOOUUUUMMMMM

The seawater is sucked through the strainer, passes through the valve, reaches the seawater filter for filtration, then is finally ready to meet the blades of the raw water pump. There, it is ejected into the cooler, meets the exhaust elbow, relaxes in the waterlock before returning to the sea. Now that the engine has cooled down, the ship can continue on its way to shelter in port.

A water pump impeller that falls apart is an all-too-common problem. When the impeller stops performing its function of cooling the engine, there's a risk of breakdown. If precautions aren't taken in time, the risk of breakdown can be irreparable.

The engine must be stopped immediately, to avoid further damage, if the engine cooling alarm sounds, if the temperature sensor shows an abnormally high temperature, or if no water is coming out of the exhaust.

To work on the raw water circuit, first close the sea inlet valve.

If it's never used and no longer works, there's no point in forcing the handle. It could make the situation worse. This is where pinoches come in handy. One should be quickly inserted into the suction pipe of the raw water pump to stop the water ingress.

The pump cover can then be removed.

Once the impeller is accessible, you can visually check that there are no missing blades and that it is in good condition. If damaged, it should be replaced.

To extract it, a pair of pliers is often the best solution, and is usually part of the on-board toolbox. On the other hand, it is not advisable to pry out the seal with a screwdriver, as this will deform the pump body and create a leak.

For reassembly, first clean the sealing surfaces of the pump body and cover to ensure maximum pump tightness, especially at high speeds.

Then, to ensure that the impeller comes out easily at the next replacement, it's a good idea to apply grease to the shaft and impeller assembly. This grease is supplied in the replacement kit, as it is specially designed to ensure the long life of this part. Caution: do not lubricate with any type of grease. Finally, fit the new gasket and close the lid.

If there's no engine cooling on the seawater circuit, the first thing to think about is the impeller. You can check it to make sure it's in good condition. If it is found to be damaged, it is still necessary to ensure that the cause of the failure is not a clogging of the upstream circuit, such as a seawater filter clogged with algae, or a strainer clogged with shellfish. Because if the engine cooling circuit is clogged, replacing the broken impeller won't get it running again. In fact, the impeller would heat up and deteriorate rapidly, putting the engine in alarm once again.

First and foremost, the impeller must be replaced as a preventive measure, in accordance with the manufacturer's recommendations, generally every year.

Then, when fairing, it's important to check the engine's sea inlet strainer. It's the first component in the engine cooling chain, and the most difficult to check when sailing, as it's submerged. Secondly, regular inspection of the seawater filter is the best way to ensure the long life of the deckhouse. If your boat doesn't have a seawater filter, it's a good idea to install one and clean it regularly.

Finally, it's important not to forget to have a complete spare kit, so that you can change it at sea if necessary. The kit should contain the impeller, new gasket and grease.

Maintaining your engine cooling system and knowing how to intervene quickly can save a crew from many catastrophic sailing situations.