water impellor direction - Page 2

THEFERMANATOR · #11 07-24-2012, 04:52 PM

Quote:

Originally Posted by Destroyer

Ferm, I know that you know your stuff, and everything you said it true,
but just for the sake of arguement, let me put forth the following. Both of my Seagulls pump a ton of water when they run. One is a 2 1/2hp and the other is a 4hp. Both have solid impellers similar to the kind found on just about any pedestal sump pump. I have a pedestal sump pump, and looking at it I find I was wrong, it's a 1/2hp motor, not the 3/4 I thought it was. Regardless, it still pumps to a 25 foot head. At 3 feet of head it will pump about 4000 gph. According to it's specs, it's designed to pass up to 3/8" solid debris, which is far more than any rubber impeller will.
You say that rubber impellers are self priming by nature, but if submerged, so is a pedestal pump. You would probably have to redesign the lower unit a little to get the pump impeller under the water intake ports, but other than that the self priming feature is pretty much a wash.
As stated above, the solid impeller will pass debris like sand and seaweed about the same as a rubber unit... perhaps a little better. Finally, my pedestal pump impeller is direct coupled to the motor via a shaft, turns at 1725 rpms and pumps 4400gph at zero head. That's a lot of cooling water at idle speed.

I know that this is all just conjecture, but it would seem to me that there's really no good reason to not use a solid impeller, especially given the fact that the British Seagull engines, (considered by many to be the best, most reliable small outboard engine ever built), use a solid impeller in their design. Really, I cannot see why they could not be used on larger engines. I wish some engineer could give me some hard facts documentation as to why the rubber impellers are used. (Other than the fact that it's just good business to make a unit that needs yearly replacement at about $35 per unit)... Multiply that by how many hundreds of thousands of outboards in use and that's really big business.

Your leaving out one KEY component in your equation that REALLY changes things, and that is speed. Your small engines are designed for engines that don't run that fast, and probably never experience cavitation while running. Now lets say you have that solid impeller and hit a wave and suck air through your water pickup. Now you've lost prime and quit pumping water, yet your still going on plane. Now you have to come off plane for it to pick up water again, and then you get that cold water hitting the hot cylinders that were just left dry for a few seconds and you get that nice steaming and stressing of the metal. I see what your point is, but in a high HP planing application the rubber impeller is king.

spareparts · #12 07-24-2012, 08:03 PM

keep in mind, you need to drop the lower unit every couple of years just to grease everything up, if you don't, when you really need it to come off, it won't

Destroyer · #13 07-25-2012, 02:23 AM

Quote:

Originally Posted by THEFERMANATOR

Your leaving out one KEY component in your equation that REALLY changes things, and that is speed. Your small engines are designed for engines that don't run that fast, and probably never experience cavitation while running. Now lets say you have that solid impeller and hit a wave and suck air through your water pickup. Now you've lost prime and quit pumping water, yet your still going on plane. Now you have to come off plane for it to pick up water again, and then you get that cold water hitting the hot cylinders that were just left dry for a few seconds and you get that nice steaming and stressing of the metal. I see what your point is, but in a high HP planing application the rubber impeller is king.

Actually I had thought about speed but was hoping with a redesign of the lower unit that the impeller pickup could be placed somewhere where a wave/blowout would not result in anything more than the normal shot of air that even rubber impellers get occasionally. I'm sure there are good sound engineering reasons for using the rubber impeller over a solid type. (But I'd still love to see factory data)....

Looking at it from a different angle though, here's a thought/question I've always had. At speed, how much water does a rubber impeller really pump? Think about it. At low speed an impeller pumps whatever water is in the pickup. But at high speed, when you're up on plane and skimming along the surface like there's no tomorrow the water that's under your boat is hitting your lower unit water pickup with more pressure than a fire hose. Does the impeller continue to pump water at all, or does the extreme pressure flex the vanes out of the way and go directly to the engine all by itself? ... or maybe it is a combination of the two...pumping and pressure?

Quote:

Originally Posted by spareparts

keep in mind, you need to drop the lower unit every couple of years just to grease everything up, if you don't, when you really need it to come off, it won't.

Excellent point also.

THEFERMANATOR · #14 07-25-2012, 02:31 PM

Without the pump, I guarantee you no water would go through the engine even at speed. The water going by actually pulls teh water away from the pickup with the current designs we have, hence why a regular lower unit becomes just about worthless above 60 where it will cavitate from the water rushing by so fast.

Blue_Runner · #15 07-25-2012, 03:24 PM

I'm betting they could make them out of a wear-resistent rubber that would not be so apt to wear out. It is 2012.

billymoca · #16 07-25-2012, 04:52 PM

Quote:

Originally Posted by Blue_Runner

I'm betting they could make them out of a wear-resistent rubber that would not be so apt to wear out. It is 2012.

Ah yes, but keep in mind - designed obsolescence. THEY have to put their kids through college.

spareparts · #17 07-25-2012, 05:35 PM

barco made "blue impellers" that were not suposed to burn up, hte EPA put them out of business. I rarely see a worn out impeller, usually i see a burnt one from running it dry. The small diameter mercs are bad about knocking the blades off though, especially if it sat up for a while. The harbor taxi has 2000 hours on their impellers when they let me change them(they were just over a year old)

SkunkBoat · #18 07-25-2012, 08:14 PM

Quote:

Originally Posted by spareparts

keep in mind, you need to drop the lower unit every couple of years just to grease everything up, if you don't, when you really need it to come off, it won't

You beat me to it...best reason for needing to change it is the act of changing it.

Destroyer · #19 07-25-2012, 10:37 PM

Quote:

Originally Posted by THEFERMANATOR

Without the pump, I guarantee you no water would go through the engine even at speed. The water going by actually pulls teh water away from the pickup with the current designs we have, hence why a regular lower unit becomes just about worthless above 60 where it will cavitate from the water rushing by so fast.

Now that's interesting. I didn't know that. Thanks for that info. I always just figured that as the water rushed by faster and faster the water pressure in the pickup just increased..

mawshj · #20 08-03-2012, 12:08 PM

the correct direction is obtained by rotating the impeller in the normal direction of shaft travel and let the blades curl back. the blades are designed to act as a displacement pump at low speeds and to flex under pressure at high speeds and act as a centrifical inpeller and thus relieve some water pressure. a sump pump only runs at one speed. I have rarly seem an impeller worn out unless someone was plowing with it for a long time. the cost in fixing a water pump is the labor ,so always replace it when you have the engine open