Water-Pump Impeller: Priority One

Unless you have proof in the form of a receipt or confirmation from the shop that did the work, replacing the water-pump impeller should be one of the first things you do after bringing home your new-to-you boat. Why?  Nothing will leave you dead in the water faster than shredded water-pump impeller, and it’s important to replace it at your convenience, rather than at considerable inconvenience and expense.

A water pump with broken impeller blades, like the one on the right, can strand you and invites engine damage.

If an impeller disintegrates while you’re underway, the engine will overheat, which can lead to failed head gaskets or worse. And there is no truth to the rumor that if you keep moving, enough water will be forced through the system to keep the engine cool.

“There is not enough to be substantial,” according to Chris Leone, owner of Chris Leone Motorsports in Longwood, Fla. “At higher rpm, you may get a little bit of water, but your cooling needs are that much greater because you’re asking the engine to put out power, and power is heat — so your cooling requirements are going to be that much more.”

How hard is it to access and replace these impellers? It depends, of course, on the engine. The pump for a MerCruiser Alpha One drive, for example, is in the lower unit of the stern drive. On boats with MerCruiser Brave One and Bravo Three drives, the pump is engine-mounted and belt-driven.

The good news is that pump kits are not too pricey, and if you’re handy, you can do the job yourself without much trouble. The kit for an Alpha One is about $50. For a Bravo-drive-equipped boat, the kit runs right around $100, which includes new hardware, O-rings, the wear plate, housing, and impeller.

On larger inboard engines, an impeller remover like the one made by Jabsco shown here, helps get the old unit out without damaging the pump housing.

The Alpha One drive pump is easy to access, but the pump for a Bravo drive is mounted low on the engine, and all five bolts that hold the assembly together come in from the rear. Later-model Bravo-drive-equipped engines feature water pumps that have brass housings, which can be serviced by replacing the impeller only, unless the brass also is deeply scored. With either housing, the job isn’t much fun.

“The hard part is getting to it,” said Leone, who also has taught at Marine Mechanics Institute in Orlando. “In some applications, you can get them up out of the way and sit up on top and do it in your lap. Some of them you have to change from inside the bilge.”

Once you get it apart, inspect the condition of the impeller and housing. Ideally, the impeller should not be cracked anywhere, and it should certainly still have all its blades. If there are blades missing, you have to disassemble the cooling system and find them, so they don’t create a blockage.

The housing should be free of cracks or scratches inside. If you look inside and see scratches, which stem from the presence of sand in your cooling water, replace the housing. Also check the bridges across each water port, which can break and lead to premature blade wear.

Mercury specifies that the impeller be serviced every other year, or 200 hours, whichever comes first. However, most owners replace them as part of their annual spring fitting out. In northern climates, it’s best to install them before the boating season rather than at the end, because the blades tend to “take a set” when stored for long periods.

Because the impeller is made of fairly stout rubber, it’s difficult to get it into the housing. The trick is to coat the blades with Vaseline petroleum jelly or some other lubricant and spin it as you push it into the housing. Be sure you spin it in the proper direction, which is indicated on the replacement housing.

To make it last, follow Mother Mercury’s recommendations: don’t run your boat aground in sand, maintain at least two feet of water beneath your boat, don’t run the pump dry, and use your boat regularly.

When you’re done, you can take comfort in knowing you only have to perform this service once every two years. And if you still get stranded, at least it won’t be the water pump’s fault.

Brett Becker



  1. Mercury identifies that the impeller be maintained every other season, or 200 time, whatever comes first. However, most entrepreneurs substitute them as aspect of their yearly springtime suitable out. In south environments, it’s best to set up them before the enjoying water sports period rather than at the end, because the knives usually “take a set” when saved for lengthy stretches.