By Peter Nielsen
A raw-water cooled engine is cooled by water drawn from outside the boat, then circulated through engine block and cylinder head.
A fresh-water cooled engine is cooled with raw water pumped through a heat exchanger, where it first draws the heat from the fresh water/antifreeze mix that cools the engine, then is injected into the exhaust hose where it cools the hot gases before being pumped overboard.
Either way, the raw water is circulated by an impeller-type pump.
Impeller pumps are simple and robust, which is what you want for such an essential item. The big flexible blades on the typical rubber (actually a synthetic compound) impeller will gobble up leaves, twigs, grit, and lots of other detritus with scarcely a hiccup. If they stop working, though, youre in trouble. When an impeller gives up the ghost, it tends to disintegrate inside the pump body. The blades from the vanes can cause serious damage, for example by blocking the tiny waterways inside a raw-water cooled engine, or by clogging up a vital artery inside a heat exchanger. With no cooling water reaching the exhaust, a fire is not beyond the realm of possibility.
Two things tend to finish off an impeller: 1) Owner negligence; 2) A lack of water.
I managed to kill an impeller by a combination of both: I forgot to open the engine intake seacock. In the aftermath, it took all day to get the bits out of the heat exchanger. Often, damage is caused by owners not bothering to drain water out of the pump body when winterizing the engine.
Some mechanics insist that you should change the impeller annually. Others say that every second year should be enough, depending on where you sail (in silty water, impellers out more quickly) and the hours you put on the engine. I once had to replace an impeller at sea, an experience Im not keen to repeat, so I figure an annual replacement at $20 or so is cheap insurance. At the very least, Id pull the impeller every spring and visually inspect it to make sure there are no cracks or splits in the blades. If it appears that the blades have taken a set, flip the impeller over when you replace it.
It takes only a minute of dry running to destroy an impeller; this doesnt give you much time to respond to your cooling water alarm (assuming you have one) and shut down the engine before the impeller self-destructs. For around the same price as a regular black impeller you can get a handsome blue Globe Run-Dry impeller, which can survive for 15 minutes without water. That quarter-hour might just be enough to get you into a harbor or anchorage where you can change the impeller at your leisure.
On some marine engines, most notably smaller Yanmar diesels, the impellers are very hard to get to. I love my little Yanmar 2GM but I have to remove the water pump in order to change the impeller, and that would be a hell of a job on a dark, rough night. So Ive fitted a Speedseal kit ($60, www.speedseal.com) that does away with the half-dozen tiny screws holding the pump cover, and will make it much easier for me to change the impeller in a hurry.
CHANGING AN IMPELLER
1. If the boats in the water, close the inlet seacock. On some engines youll have to loosen the drive belt, remove the inlet and outlet hoses, and remove the pump. Then remove the screws holding the face plate onto the pump. If you havent checked the impeller for a while, these little screws can be the devil to undo.
2. You may have to use a screwdriver to pry the face plate free from the body. Take care not to damage it. The pump cover will be sealed with a paper gasket or an O-ring. In either case, plan on replacing it. Check the face plate for wear.
3. Most impellers are push-fitted onto the central spline, but some are fastened to it with a bolt or screw which will need to be removed. You can usually get the impeller out by gripping its body (not the blades) with a pair of pliers.
4. Make sure the vanes are facing in the right direction when you fit the new impeller. Sometimes it can be a bear to bend the stiff blades enough to clear the pump body, a job you need more than five fingers for; use a plastic cable tie to hold the blades in the right position. As you push the impeller into the pump, the cable tie will just pop off.
5. Rather than fiddle about with those tiny screws again, I fitted a Speedseal cover. This has just four screws, with big knurled heads so theres no need to use tools to get the cover off. You just need to remove two of the screws, and loosen the other two; the cover is slotted, so it just slides right off. If I have a spare impeller ready to go, I should be able to replace a ruined one without having to remove the pump. Since impellers never choose a convenient time to fall apart, I look on this as a safety feature.