Prudent preventative maintenance calls for replacing the impeller annually. Sure, it can last longer, but why take the risk? As it depends not just on hours of use but also a range of other conditions (how long it sat unused, abrasives in the seawater, temperatures, and other factors), impeller longevity cannot be easily predicted. Impellers can also suffer from extended periods of not being used. How long is too long? That depends on many factors, including temperatures and impeller size. While sitting unused for several months (through the winter, for example) should not be a problem, if the impeller has not moved in a year or more it should be replaced.

Many boaters go years without replacing the impeller, but the prudent path would be annual replacement or, at a mini- mum, annual inspection. Preventative maintenance aside, you’ll want to know how to replace an impeller in case of an overheating engine. If your engine temperature starts to increase and the strainer is clean, the impeller would be the next place to look.

Whether performing a preventative replacement or trouble- shooting an overheating engine, the first step is to close the raw water intake seacock and place a reminder tag at the key switch: You don’t want to go through this whole process only to start the engine with the seacock closed and toast the new impeller.

Next, you will have to remove the cover plate to access the gasket. This seemingly simple task can often be far too challenging. In many cases access can be shockingly restricted, leaving you to cuss the engine manufacturer or boatbuilder in disbelief.

And once you gain access, not only can the small screws be difficult to remove but they are also far too easy to drop in the bilge. Speedseal (speedseal.com) offers an aftermarket cover plate with larger knurled fasteners to facilitate easy removal.

With the cover plate removed, the next step is extracting the impeller. The fit can be snug and the impeller difficult to re- move. Prying out the impeller with a screw- driver should be avoided as it will damage the pump housing. Impeller removal tools are available for purchase, and they work much the way a corkscrew works to open a wine bottle. If you do not have the correct tool on board, you should first spray a lubricant into the housing and onto the impeller. Then bump the starter once or twice to force the impeller to rotate—the lubricant will now be dispersed between the impeller tips and the metal housing, making removal easier. Grab the vanes with a pair of pliers and pull straight out. On higher horsepower engines—roughly 300 horsepower and higher—many of the impellers have a threaded insert that allows you to use a bolt or special tool to extract the impeller from the housing.

Once the impeller has been removed, you must decide on the next step. Now that you have gone this far do you really want to put the old impeller back in, even if it looks fine? If it looks okay, the old one might make a good spare. If limited access or skills make replacement difficult, replace it. If you are heading out on a long or challenging passage, replace it.

If you want to try to reuse the impeller, a close inspection should be the next step. The impeller vanes will be bent over a bit but should return to something close to straight within a day—if not, you need to replace the impeller. The ends of the blades should be round and not worn flat. Flex the blades and look closely where they meet at the center—no cracks should be visible. And, of course, no pieces of the impeller should be missing. If the impeller passes all these inspections, the impeller remains serviceable.

If the impeller is broken or worn, you will need to inspect the rest of the system for associated issues. If pieces of the impeller have broken off, you will need to find them downstream as they will restrict the flow of the seawater cooling loop. You may find them lodged in the discharge port at the pump, at the inlet side of the heat exchanger, or at the fitting where the raw water injects into the exhaust elbow.

If the ends of the impeller are shiny or cracked and some vanes are missing, it likely ran dry at some point. If the blades are intact but have worn tips or imprints from the cam, you might have a blockage on the discharge side creating too much back pressure. Look at the inlet side of the heat exchanger for an accumulation of an- ode pieces or impeller bits.

Finally, if the impeller vanes have pieces missing from the middle of each tip, you might have cavitation inside the pump from too much vacuum on the suction side. Closely inspect the cam and pump housing for pitting and check the piping between the seawater strainer and the pump for restrictions. If this problem has happened more than once you might have to increase the diameter of the inlet pipe fittings and hose.

Once these inspections have been completed, it’s time to insert the new impeller (or the existing impeller if you have determined it is still serviceable). The re- placement impeller should match the part number for your pump and engine. Seawater pumps should use impellers made of neoprene. Neoprene maintains its elasticity for long periods of time and does not break down in salt water. (Nitrile will work if no other option is available but should be replaced with a neoprene impeller at the first opportunity.)

Like removing the cover plate, installing the new impeller can be frustrating as they fit snugly in the housing. If compressing the vanes enough to get it started into the housing is challenging, try these tips. First, identify the method by which the impeller mates with the shaft. Some rely on a key, and keeping the key in place can be tricky. A splined shaft matches up much more easily. Once you know how it will align, you are ready to install the impeller. At this point you will want to use some lubricant. Some impellers come with a manufacturer-provided lubricant. In the boatyards we use dish soap—the soap will not damage the neoprene, and once you start the engine you will be blowing bubbles out the exhaust discharge, providing visual confirmation of your success. The soap serves two purposes: It helps the impeller slide into place and temporarily lubricates it until water fills the housing. (Avoid using petroleum-based products, such as Vaseline, for lubrication as they will damage the neoprene.) Apply the soap liberally to the inside of the pump housing and to the impeller.

You can compress the blades using a hose clamp or plastic tie-wraps. Squeeze the blades and then slip the hose clamp over the end and slide it up about ¾-inch from the end. A second clamp or tie-wrap can be positioned at the opposite end. Now slide the impeller part-way into the housing and gently rotate it until it aligns properly with the key or splines on the shaft. Once engaged, push the impeller in until the first clamp stops you. Remove this clamp and slide it in further. Then remove the remaining clamp and push it the rest of the way in. The vanes will have to bend one way or the other, and there is a 50/50 chance you will have bent them the way they will eventually go. In any case, the vanes will flip over when the impeller starts rotating. But on larger impellers (those for an 800-horse- power engine, for example) it would be best to get the vanes in the right way from the start to avoid placing undue stress on them.