Why is the impeller not made out of metal?

[ZmOz]

I've never changed one, so please excuse me if this is a stupid question. If the impeller in an outboard needs to be changed so frequently and easily breaks, why the heck don't they make them out of a hunk of steel?

 

[Paul Moir]

Re: Why is the impeller not made out of metal?

Those rubber impellers are actually pretty cool: <br />At low speeds, the vanes contact the outside walls. Since the driveline is off-center, the volume between a pair of vanes on one side is less than the other. So it works like a displacement pump. The benifit there is that they're self priming, and can develop a lot of pressure at low rpms. <br />As the rpms pick up, the vanes actually fold back, and the impeller effectively becomes like a centrifical pump impeller. The housing acts like the volute. The benifit is little impeller wear and high volume pumping.<br /><br />I think there were some older outboards that actually used a submerged (non-flexible) impeller. I have no idea how they worked. Anyone else?

 

[roscoe]

Re: Why is the impeller not made out of metal?

I would like to see them offset from the driveshaft, gear driven, and with an access cover in the side of the midsection. This way, you could open the cover and pop in a new one in about 5 minutes. But it would require more room, moving parts that would need lubrication, and lots of extra cost.<br /><br />Or how about an access hole, and a 2 piece impeller that could be fitted around the driveshaft, then slid down into the housing?<br /><br />Wishfull thinking.

 

[Chinewalker]

Re: Why is the impeller not made out of metal?

Many Pre-WWII outboards had variations on the metal water pump. Some were plungers, some were centrifugal, and some were even force-fed from forward motion.<br /><br />The problem with a metal pump is sand. Motors with metal pumps would bind up if they ran through silty water. The metal would wear down and wear out. Also, there was the problem of lubrication. Water is not a very good lubricant for metal surfaces. In order to properly pump, you had to have close contact with metal on metal. You can't lubricate it with oil if it's exposed to water.<br /><br />The force-fed pumps were fine in the days before gear shifts. When neutral was added to a motor, you no longer had guaranteed forward motion, and reverse was another matter entirely.<br /><br />The rubber impellers, for motor motors, are relatively easy and inexpensive to change. Just don't get me started on the old Bail-A-Matic Scott-Atwaters or the Auto-Tranny Mercs...<br /><br />Thus concludes today's history lesson...<br />- Scott

 

[JB]

Re: Why is the impeller not made out of metal?

Like, Scott, I have messed with several old engines that had metal vane constant displacement pumps. <br /><br />One that I particularly remember is a 50s Elgin 7.5. It worked well as long as it was used in clear water and never got into silty or even muddy water. It didn't take much sand to trash all three brass impeller blades and the housing.<br /><br />I, too, would love to see an outboard with a quick change water pump impeller, but think it would add too much weight and cost.

 

[nightstalker]

Re: Why is the impeller not made out of metal?

What's the cost of having an impeller replaced by a marina's mechanic?

 

[andrewkafp]

Re: Why is the impeller not made out of metal?

It's about $120AUD and $40-100 for parts depending on whether it needs base plate and outer housing.<br />$220AUD Or $150US is quite cheap...

 

[Chinewalker]

Re: Why is the impeller not made out of metal?

Assuming no salt or corrosion issues to deal with, a simple waterpump change shouldn't run more than an hour ($50.00 to $75.00 in my area) and shouldn't cost more than $50.00 in parts for most motors...<br />- Scott