Why are impellers....fragile?

[Don S]

Re: Why are impellers....fragile?

The impeller on the Cobra outdrive is relatively easy to access. Remove the rear cover and it's right there. Don't have to drop a lower end.

So! Should everyone change to a Cobra drive that isn't manufactured anymore.

Guys, with a bit of preventive maintenance, and an alarm system, and watching the gauge, the impellers are fine. They are not that fragile. Many run them for 10 years and brag, actually they are just lucky. but the do not often fail for no reason, and most plastic bags just slow down the water flow and may overheat the engine but not destroy the impeller.
The whole idea that it's fragile is crazy, it puts up with a lot, but it is also inexpensive compared to the alternatives.

 

[DavidW2009]

Re: Why are impellers....fragile?

"So! Should everyone change to a Cobra drive that isn't manufactured anymore."

No. I was just pointing out a fact. Also, if I read the manual correctly, the King Cobra doesn't even have a rubber impeller. Just the water pump and raw water flow.

 

[achris]

Re: Why are impellers....fragile?

The reason it's made of rubber is because only rubber (and some of the newer synthetics that weren't around back in the 60s when this system was designed) will give the properties needed by this particular pump... This pump is quite unique in that it must operate over a wide range of speeds, At low speed it must be a displacement pump, and that creates the need for the blades to touch the housing, and be flexible; and at high speed the blades fold in and it becomes a centrifugal pump.

The reason it's buried in the centre of the leg is (apart from historic) that it uses the drive shaft as it's motive force, without the need to engineer more shafts and seal, all sources of failure. And it helps to be underwater to be able to better prime. Yes, I know about engine mounted raw water pumps, but they also need to be primed, and they are also dual mode pumps, with all the same drawbacks, and advantages.

As Paul said, it doesn't take long to pull a leg, split it and service the pump. It also gives the person who's doing the job a chance to have a good look around and service anything else before it causes major failure, as already stated by The Oracle. And most importantly, it forces you to change the leg oil!

If you really don't like the leg mounted pump, remove it and go for an engine mounted 'Japsco' type pump.

Chris.......

 

[paulspaddle]

Re: Why are impellers....fragile?

Thanks for chiming in guys. My original question was based on inexperience and spending more time reading than doing. Bought my first i/o in October, have yet to put a wrench to it.

Spending time here with clean finger nails in the middle of winter can create a motor hypochondriac. Thanks for entertaining the original question (I admitted in my OP I felt like a ridiculous noob for asking) and offering an educated answer.

Appreciated, I learned a bunch....