Changing a raw water impeller, before it changes your plans
The rubber impeller in the seawater pump is a short job on a calm mooring and a miserable one in a seaway. How to change it, and what to do with the old one before it goes in the bin.
In this piece
The raw water impeller is a small rubber wheel with flexible vanes, inside the seawater pump on the front or side of the engine. It draws seawater in through the seacock and strainer and pushes it through the heat exchanger, where it cools the engine’s own coolant, and then into the exhaust mixing elbow, where it joins the exhaust gas and leaves the boat with it. On a directly cooled engine, usually an older or smaller one, the seawater runs through the block itself. Either way, when the impeller fails the flow stops, the exhaust runs dry and hot, and the engine overheats within minutes.
Changing one is a short job on most engines after the first time: close the seacock, take off the pump cover, lever out the old impeller, push in a new one, fit a new gasket, open the seacock. The details below are what make it go right, and the one that matters most is what you do with the old impeller after it comes out.
Signs the impeller is failing
- Less water coming out of the exhaust. Get to know what yours looks like at idle and at cruising revs, because a weaker, spitting flow is usually the first sign.
- A temperature alarm, or a gauge sitting higher than it normally does.
- Steam from the exhaust, more than the wisp you see on a cold morning.
- A different exhaust note: drier, harsher and louder, without the wet splutter of water in it.
- A smell of hot rubber, or an exhaust hose too hot to keep a hand on.
Any of these means stop and look. Throttle back, and if you can do so safely, stop the engine. If you need it to stay off something, keep it at idle and watch the temperature while you get sail up or gain sea room. A diesel that overheats badly can warp its cylinder head, and the rubber exhaust hose and plastic waterlock downstream of the elbow can be damaged long before the engine is.
How often to replace the impeller
Start with the engine maker’s word. The Yanmar 2YM15/3YM20/3YM30 Operation Manual, on page 44, says to replace the seawater pump impeller every 1000 hours, and to inspect it when less seawater comes out of the exhaust. Other makers give their own intervals, some in hours and some in years, and your engine’s manual is the place to look.
Most cruising boats never come near 1000 hours a year, and rubber ages whether it turns or not. Vanes left bent in one position over a winter take a set, and they crack at the root with age. So many owners, us included, change the impeller every year at the start of the season whatever the hours, and always after the engine has run dry for any reason: a seacock left closed, a strainer packed with weed, a plastic bag over the intake. Less than a minute of dry running can be enough to wreck one, because the water is also what lubricates and cools the vanes.
Whatever interval you settle on, carry at least one spare, better two, with the gasket or O-ring for the cover. An impeller is small and not expensive, and it tends to fail at a bad moment: leaving an anchorage, or motoring into a harbour entrance against the wind.
Tools and spares
- The right impeller for your pump. Check the part number in the engine’s parts catalogue or on the pump itself; impellers that look alike differ in width, in the way they fit the shaft and in the number of vanes.
- A new cover gasket or O-ring. Most pumps use one or the other, and a reused gasket is a common cause of drips and air leaks.
- An impeller puller made for your pump, or two flat screwdrivers, or long-nosed pliers. Wrap the screwdriver tips in tape so they do not score the housing.
- A lubricant the impeller maker approves, usually glycerine or a silicone grease; the packet will say. Never petroleum grease or oil, which attacks the rubber some impellers are made of.
- Spare cover screws, a magnet on a stick, and a cloth to spread under the pump. The screws are small and the bilge is deep.
- A torch and a small mirror, for pumps mounted where you cannot see the front.
How to change an impeller, step by step
- Make the engine safe and close the seacock. Engine off and the key out; some people hang a tag on the key that says the seacock is shut. Close the raw water seacock. If it will not close and the pump is below the waterline, stop here and deal with the seacock first.
- Take off the pump cover. Put the cloth or a small container under the pump to catch water and screws. Undo the screws a little at a time and lift the cover away; it may need a gentle tap sideways. Peel off the old gasket or O-ring and clean both faces without scratching them.
- Note the vane direction. Before anything else, look at which way the vanes are bent, and take a photo. They trail behind the direction of rotation, and the new impeller needs its vanes bent the same way as the old ones.
- Pull the old impeller. Use the puller, or work it out with the two screwdrivers on opposite sides, levering a little at a time so it comes out straight. On a tired impeller, long-nosed pliers gripping the vanes close to the hub also work. On some pumps the impeller is also held by a screw, a pin or a circlip, so look before you pull.
- Count the vanes. Lay the old impeller on the bench, count the vanes and look at each one. If a vane, or part of one, is missing, it has gone downstream and you have to find it. Pieces usually travel as far as the first narrow passage: the inlet end of the heat exchanger, or the end cap of an oil cooler if one comes first in the circuit. Take off the end cap and pick the pieces out until the old impeller is complete again on the bench. A vane left in there partly blocks the flow, and the engine will run hot with a brand new impeller.
- Check the cam and the wear plate. Inside the pump body is the cam, a crescent-shaped block that squeezes the vanes as they pass; make sure its screw is tight. Look at the inside face of the cover, or the wear plate if your pump has one, for a groove worn by the impeller. A deep groove lets water slip past and the pump loses suction. Some covers can be turned over, others have to be replaced. If the pump has a weep hole, check it for drips: water there means the shaft seal is going.
- Fit the new impeller. Lightly lubricate the vanes and the inside of the housing. Bend the vanes the way the old ones lay, line up the hub with the shaft (splines, a flat or a slot), and push it in while turning it the way the engine turns, so the vanes fold in the right direction. A cable tie round the bent vanes holds them tucked while you start the impeller into the housing; cut it off once the impeller is part of the way in.
- New gasket, cover back on. Fit the new gasket or O-ring, refit the cover and tighten the screws evenly in a cross pattern. Do not overtighten; the screws are small and the threads in the pump body strip easily.
- Open the seacock. It gets its own step because it is the one people forget. Take the tag off the key.
- Run it and check the flow. Start the engine and watch the exhaust. On most boats water should come out within half a minute or so, once the waterlock has filled. If nothing comes, stop the engine and find out why before the new impeller cooks. Check the cover for drips at idle, and again once the engine is warm.
Marine engine overheating: what to check, in order
An overheat alarm is not always the impeller. This is the order we work through, roughly from the quickest and most likely to the slowest.
| Check | What you are looking for | |
|---|---|---|
| 1 | Raw water strainer | Weed, plastic, jellyfish, shell. Clean it, and check the lid seals: an air leak here stops the flow as surely as a blockage. |
| 2 | Seacock and intake | Seacock fully open and nothing over the intake outside. If the strainer sits below the waterline, water should rise strongly in it with the lid off and the seacock open. |
| 3 | Impeller | Missing or cracked vanes, or vanes so set that they no longer seal against the housing. |
| 4 | Heat exchanger | Impeller pieces at the inlet end, scale or silt in the tubes, and on some engines the remains of a zinc anode. |
| 5 | Exhaust mixing elbow | On older engines, carbon and scale build up where the water meets the exhaust until little water gets through. |
| 6 | Thermostat | Stuck closed. Take it out and watch it in a pan of water as it heats: it should open. |
| 7 | Coolant level | On a freshwater cooled engine, check the header tank with the engine cold. |
| 8 | Belt | Slipping or broken. It drives the coolant circulating pump on most engines, and on some the raw water pump too. |
The first three cost nothing but a few minutes and catch most overheats. If the exhaust is spitting plenty of water and the engine still runs hot, the problem is more likely on the freshwater side, or in a heat exchanger that is passing water but no longer passing much heat.
Read the pump pages before you need them
The interval, the impeller part number and the cover gasket are all in your engine’s manuals. If you have them on board, read the seawater pump pages now, on a calm evening. If you do not, our guide to finding your boat’s manuals covers where to look. And since an impeller can go at the worst moment of a passage, it is one more reason every leg of a passage plan needs a bail-out bay you could sail into.
SafeBoat’s BoatGPT can answer this one from the manual. Ask it how often to change the impeller on a Yanmar 3YM30 and the answer comes from the Yanmar 2YM15/3YM20/3YM30 Operation Manual, naming page 44 with a link that opens the page, so you can read the maker’s words yourself. When an answer comes from general knowledge rather than a manual, it says so. It needs a free account.
Tonight’s job, if you have not done it: find the spare impeller, check its part number against the parts list for your engine, and put it in one bag with a cover gasket, the puller or screwdrivers and a few spare screws, somewhere you can reach from the engine compartment without emptying a locker.