Updated:
Original:

Impeller Etiquette

Get to grips with an often over-looked but vital part of an engine cooling system in this step by step procedure

A raw-water cooled engine is cooled by water drawn from outside the boat and circulated through its engine block and cylinder head.

On a fresh-water cooled engine, the “raw” water is pumped through a heat exchanger, where it draws the heat from the fresh water/antifreeze mix that cools the engine, and then is injected into the exhaust hose, where it cools the hot exhaust gas before being pumped overboard. In both cases an impeller-type pump circulates the raw water.

Impeller pumps are simple and robust, which is what you want such an essential item to be. The big flexible blades on the typical “rubber” (actually, it’s a synthetic compound) impeller will gobble up leaves, twigs, grit, and other detritus with scarcely a hiccup. When they stop working, though, you’re in trouble. When an impeller gives up the ghost it tends to disintegrate inside the pump body, and the shards from the vanes can cause some 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 getting to the exhaust, a fire is not beyond the realm of possibility.

Two things tend to finish off an impeller—owner negligence, or a lack of water. I managed to kill one by a combination of the two—I forgot to open the engine intake seacock. It took me all day to get the bits out of the heat exchanger. Often, impeller 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 every second year should be enough, depending on where you sail (silty water wears impellers out more quickly) and the hours you put on the engine. I once had to replace an impeller at sea, an experience I’m not keen to repeat, so I figure an annual replacement at $20 or so is cheap insurance. At the very least, I will 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 only takes a minute of dry running to destroy an impeller; this doesn’t give you long 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 I’ve fitted a Speedseal kit ($60) that does away with the half-dozen tiny screws securing the pump cover. This will make it much easier for me to change the impeller in a hurry.

Changing an Impeller

1. If the boat is in the water, be sure to close the inlet seacock. Impellers can be hard to access. On some engines you’ll have to loosen the drive belt, remove the inlet and outlet hoses, and remove the pump. Then remove the screws holding the faceplate onto the pump. If you haven’t tried to open the pump for a while, these little screws can often be the devil to undo.

2. You may have to use a screwdriver to pry the faceplate free from the body. Take care not to damage it. The plate will be sealed with a paper gasket or an O-ring. In either case, plan on replacing it. 

a. Check the 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 channel-lock or needle-nosed pliers. You could also buy a special impeller removal tool.

4. Make sure the vanes are facing in the right direction when you fit the new impeller (opposite to the “direction of rotation” mark on the pump faceplate). Sometimes it can be a bear to bend the stiff blades enough to fit them into 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.

a. When you push the impeller into the pump, the cable tie will slide off. Grease the pump body lightly with Vaseline or water pump grease; this will keep the impeller lubed until the water starts flowing.

5. Rather than fiddle about with those tiny screws again, I fitted a Speedseal cover.

6. This cover has just four screws, with knurled heads so there’s 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 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. Speaking of which—don’t forget to open the seacock! 

Related

Rob-Spets---10-19-21

Kiteboarder Sets New Jamestown Record

Rob Spets is the new record holder for the around-Jamestown circumnavigation aboard his foiling kiteboard Skellinger. His lap took 50 minutes and 48 seconds, improving the previous record set by Jason Carroll by 1 minute and 14 seconds. Spets completed the course in an 18-knot ...read more

03-IMG_0590

Bill Tilman’s Simple Sailing

Like an ostrich on a bad day, I’m head-down in the lazarette of Nellie, my Beneteau First 42, dealing with the propane tank. My wife taps me on the shoulder, and I rise to see a pair of foiling catamarans accelerating onto their carbon-fiber wings. As the blood drains from my ...read more

01-LEAD-210801_PM_Tokyo20_22825_5540

Olympic Sailing: Where to Now?

It’s official, not only is the United States no longer an Olympic power when it comes to sailing, it’s fast beginnings look like an also-ran—albeit an also-ran with loads of potential. What other conclusion is there to draw from the fact that for the second time in three ...read more

244526945_394104495768313_1401658800642145082_n

Return of the Annapolis Boat Show

After a hiatus in 2020, the United States Boat Show in Annapolis, Maryland returned in full force last weekend. “Pent up demand” was the name of the game for visitors and exhibitors alike. Queues to get in each morning stretched around the block, and the docks were congested ...read more

Untitled-1

Sailing Hall of Fame Inducts Class of 2021

This weekend, the National Sailing Hall of Fame has inducted eleven new members to make up the class of 2021. “The remarkable achievements of this year’s class exemplify excellence and an unwavering dedication to our sport,” said National Sailing Hall of Fame president Gus ...read more

ed3b8ae9-b65d-2941-47ec-cd0277bfcbe8

Mirabaud Voting Open to the Public

Photos from the industry's top photographers are in, and the 12th annual Mirabaud Yacht Racing Image competition is underway. An international panel of judges has selected this year's 80 finalists, which have been published online. The panel will also select the winner of the ...read more

P1320232-copy

Annapolis’ Boat Show is Back

After a year off in 2020, the United States Boat Show in Annapolis is back. From the diminutive Areys Pond Cat 14 XFC to the massive Lagoon Sixty 5, many of the SAIL’s 2022 Best Boats Nominees are on display for the public to get a firsthand look at, and SAIL’s Best Boats panel ...read more

05-Squall-in-the-ITCZ

Close-Hauled to Hawaii

The saying “Nothing goes to windward like a 747,” is one of my favorites. I actually once took a 747 upwind, retracing my earlier downwind sailing route across the Pacific. I’ve also done a fair bit of ocean sailing to windward. The 747 was a lot more comfortable. But then ...read more