You may need to do this either as a routine check or because the raw water flow through the engine has stopped. In the latter case the engine will be overheating and you will doubtless have shut it down. You’ve checked the cooling water inlet strainer and found that it’s clear of obstructions.

The next most obvious culprit is the raw water pump − or, more specifically, the rubber impeller it contains.

An impeller puller is a useful tool to have on board

Therefore, undo the screws and remove the pump’s face plate. The impeller might look intact but it pays to take a closer look to be sure. Grip one of its vanes with a pair of needle nose pliers and pull it out. Alternatively − or if all the vanes are missing − use a pair of screwdrivers to lever it out. Be careful to avoid damaging the edges of the brass casting. It helps to wrap the screwdriver shanks in masking tape to cushion them.

With the impeller extracted, examine it closely − particularly the vane roots. If you find any cracks, discard it.

It’s vital you locate and remove broken vanes

Missing  vanes

If there are any vanes missing, they have to have gone somewhere. They might have been pumped upwards towards the heat exchanger but, equally, they could have dropped back into the suction tube. Whatever it takes, hunt them down until they are all accounted for.

To draw them through a new impeller could undo all of your good work. Clean all traces of the old paper gasket from the face of the housing. Smear the new impeller lightly with washing-up liquid and slide it back onto its shaft, making sure the pin (or grub screw) engages properly with the slot and that the vanes trail backwards from the direction of rotation. Fit a new gasket and replace the face plate. Tighten the screws evenly in rotation.

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