Almost all marine engines use a flexible impeller pump for the raw cooling water system.
I have a question: Why?
The flexible impeller pumps used in marine engines typically use a neoprene or nitrile impeller, on a shaft, in a housing that is either circular with an off-centre shaft or circular with a cam on one side. This alternately compresses the blades (expelling water) and allows them to relax (taking in water). It's a straightforward, simple device.
The claimed advantage of these pumps is that they are self-priming. They are also reasonably resistant to debris; solid gunk can often pass through them without damaging the pump or clogging it.
There are, however, a few big disadvantages to these things:
- They wear out quickly, particularly if you use the self-priming capability. Run one of these pumps dry for more than 30 seconds and you're likely to damage the impeller.
- They need frequent maintenance, to the point where some boaters will modify the pump housing so that they can get to the impeller without tools. Impeller replacement is a sufficiently common task that most chandleries sell, and many cruising boats carry, a \$100 impeller-pulling tool.
- They make a royal mess when they fail, sometimes leaving shredded bits of the impeller blades inside the cooling passages.
I've never heard of one of these things lasting more than a thousand hours. There are some Yamaha outboard manuals that recommend replacing the impeller every 500 hours or 3 years; I once had a Suzuki whose pump called for annual replacement (and self-destructed anyway).
And yet we have pumps in other domains that are far more reliable.
A Hayward Super Pump, for example, is a popular centrifugal design that can crank out four hundred litres a minute and rarely, if ever, clogs when it's used to vacuum leaves and berries off the bottom of a swimming pool. I've seen many of those with 40,000 plus hours on them, never rebuilt and still going strong. The pump and strainer assembly, sans motor, is about \$150 (\$350 with a nice 1hp AC motor).
The cooling systems in cars and trucks, and the closed side of marine closed-loop systems, usually use very simple, cheap centrifugal pumps that are almost perfectly reliable. These \$40 pumps can't be used in debris-contaminated water, but it wouldn't cost much more to make a clog-resistant version.
Of course, centrifugal pumps can't self-prime when they're completely dry. Give it just a bit of water in the chamber to start with, though, and a Pacer S will prime a 7-metre vertical suction line. Trying to prime that same line with a typical marine engine's rubber impeller pump would have the impeller in shreds by the time the water reached the pump.
I think this is a case of "we've always done it that way, so why change?"
I, for one, would like to see an outboard engine with a little self-priming centrifugal impeller around the shaft in place of the rubber thing that they're all using now. It'd be one less piece of annoying, \$50-a-year maintenance that can strand you mid-lake if neglected. And it wouldn't cost more than about \$50 extra up front.
flex impeller pumps
I recall that my old British Seagull dingy outboard motor used a solid impeller, centrifigual pump on the drive shaft to pump the coolant water to the motor. I'm with you, I don't know why a centrifigual pump wouldn't work on modern outboards since the pump is below the water level except at planing speed at which time it is sufficiently primed to draw water up to it. There must be a problem with it since not one of the companies use it.
Actually 1,000 hrs is not to bad considering the environment.Unfortunately it is in an awkward location.
Run dry is a real problem for flexible impeller pumps.
A diaphragm pump would have the same pump characteristics but adds valve and may still need replacing but will at least survive running dry.
Maybe they are just simple and cheap., crude even.
Housing, shaft, shaft seal, squish element, end plate, impeller. Just don't run dry!
I am still curious.
Cheers - Paul
A positive displacement pump
A positive displacement pump is usually much smaller than an equal TDH rated centrifugal pump. for example a 1/3 HP driven centrifugal pump has a max head of 35 ft where a 1/3 HP flexible impeller pump has 60 ft of head. Since the pumps on Marine motors are usually driven from the same power source as what drives the boat, a less horsepower using pump would be advantageous.
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