Routine water-pump maintenance is simple but important to the health of a boat's diesel engine(s).
When we are underway aboard Easy Goin’ I tend to keep an eye on the temperature gauge. Scanning the other gauges regularly is important, but in my experience the most likely engine problem to develop is overheating. That’s because the pump that circulates seawater through the heat exchanger is subject to heavy wear due to salt, sand and other grit.
Raw-water pumps haven’t changed in decades, and perhaps with good reason. While they do require attention and service, they are remarkably reliable, considering the circumstances under which they operate.
Turning several hundred or several thousand times per minute, an impeller’s flexible synthetic-rubber blades are dragging along the inside of the pump cylinder and cover plate. The cam, an arc-shaped wedge of metal located inside the pump, is where all the action and work take place. Without the cam, no water would be transported. As each blade sweeps over the cam’s concave surface, it is deformed and creates an area of low pressure, causing water to be sucked into the void and then ejected out the pump’s discharge port. All this rotating action can go terribly wrong when the parts inside the pump begin to deteriorate. In addition to the impeller, other components are subject to wear, fatigue and outright failure; as such, they require periodic inspection and replacement.
If the cover plate wears and the fit between the plate and side of the impeller becomes too loose, water and air can squeeze past the vanes. The cover plate should be replaced as soon as wear can be felt on its bearing surface. Any irregularity that can be perceived by running one’s fingertips over the wearing surface is too much. On the other hand, visual irregularities such as discoloring are commonplace and are of little concern. An old acceptable cruising trick was to reverse a worn cover plate, thereby doubling its life. But today most cover plates are embossed with the manufacturer’s logo or part number and model number.
The cam also wears away with each pass of the impeller blades, becoming smaller and smaller. In many cases, the degree of wear a cam has been subjected to is not apparent until the boat owner compares it to a new one. Cam wear is critical in that as the cam shrinks, so too does its output. A worn cam can’t compress the vanes efficiently and the pump will lose some capacity, which causes the engine to run a bit warmer at high speeds. It’s important to carefully inspect the cam’s securing screw and gasket, as these also need to be replaced. Some pumps are equipped with a wear plate, which will be behind the impeller and on the opposite side of the cover plate. As with the cover plate, if you can feel any grooves or scoring on the wear plate, it should be replaced.
Overhaul kits are available for most pump models. A kit typically includes an impeller, a cover plate and cap screws, a wear plate, a cam, and screws and a gasket. The kits normally don’t contain parts to fix worn seals, shafts, or bearings. If your pump is leaking, you may be better off having it professionally rebuilt at a fraction of the cost of a new pump.
A good rule of thumb is to replace the cover plate, cam and wear plate (if so equipped) with new ones with every third impeller replacement. Beyond that, remain vigilant for water and oil leaks at the shaft seals. In some cases, such leaks are a result of simple wear and tear; in other cases, they are an indication of impending shaft bearing failure, which means you’re operating on borrowed time.
Don’t take chances with an engine’s raw-water supply. When a pump has seen better days, rebuild or replace it without delay.