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.