water impellor direction
 

-I changed my water impellor today on my 150 mercury o.b. 1988, im not 100 percent sure its in the right direction, I started it and it pissed out good my question is this, if its in wrong how would you know? the ribs facing the wrong direction, how would you know, thanks

don't worry, once you fire it up, it will flip them the correct direction, the large diameter water pumps can stay the wrong direction, but the small diameter mercs tend to flip in the correct direction

Not to hijack this thread, but I still wonder WHY we have these flexable impellers that we need to change all the time. (Aside from the obvious expense that equals profits for the makers).
I have 2 British Seagull outboard motors... both of them have solid impellers like those found in a pedestal sump pump. They work perfectly. So why do we put up with these rubber impellers? My pedestal pump has a 3/4 hp motor on it and it pumps to a 25 foot head. And thinking about it, a regular car water pump also has a solid impeller, and they do just fine circulating the water around the engine. So WHY do we have rubber impellers?????

Probably the same reason we don't have permanent light bulbs. :head:

Did you know there's a light bulb that has been burning almost continiously since 1901 in a firehouse? It's in Livermore, Ohio and has been the subject of several researches, investigations, and documentations. It started out as a 60 watt bulb and now burns as a low (4) wattage bulb. It doesn't give out much light, but even still... 110 plus years......
(And I have a 6v high intensity night light on my night table that I put a 12v automobile bulb in...that bulb has been on continiously for the 27 years we've been living in the house. It's not a lot of light, but works just great as a night light). Obviously the trick to long life for a lightbulb is the burn it at a lower wattage than it was designed for.)

That's pretty cool. My point was there would be lots of lost revenue in the aftermarket parts industry if we had impellers that lasted for the life of the engine. Seems like everything these days is made to wear out after a certain amount of time. I don't think it is a coincidence and in a lot of cases I don't think it is just because it would be cost prohibitive to make stuff more durable.

Rubber impellers are used because the yare self priming by nature, and are forgiving to debris going through them like sand, and provide good output at low RPM's. There are other optons, but the rubber impeller is still the best overall option for reliability, longevity, and durability.

I'm still musing over the fact that Destroyer still sleeps with a night light:you:

Ya,.. I got tired of stubbing my toes against various items in the dark.. The glow of a little light works just great. Beside, it lets me see other "business" in the dark. :hide:

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.

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

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?

Excellent point also.

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.

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.:sly::sly:

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)

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

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..

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

Exactly, it's that centrifugal action that got me thinking about this in the first place. A sump pump impeller IS a centrifugal impeller. So if the rubber impellers are designed to flex under pressure and become centrifugal impellers anyways, why not just start out with them like that in the first place. The speed of the impeller really doesn't matter, as long as it doesn't run faster than whatever it was designed for. In the case of my British Sterling outboards they run anywhere's from 700 to 4500 rpm. About the only thing I can even think of that would be a logical arguement against using a centrifugal impeller is the displacement properties of the rubber impeller at idle speeds. A displacement impeller provides a positive fluid flow, whereas an impeller at low speeds looses some of it's head pressure. The question is how much pressure do they loose at say....what?....500rpm? At higher speeds they actually would provide more fluid flow since the faster you turn them the more they pump. Which might also be a problem, but I'm sure that the water passages could be designed for improved water flow. After all, an automobile's engine uses a centrifugal impeller and it does just fine at varying speeds. And for that matter, I've seen some inboard and I/O engine's that use a raw water pickup use exactly the same type of impeller also.

now im realy confused

Naaaa, it's easy. Just remember the toe bone's connected to the foot bone, the foot bone's connected to the ankle bone, the ankle bone's connected to the leg bone, the leg bone's connected to the knee bone, the knee bone's connected to the thigh bone, the thigh bone's connected to the hip bone, the hip bone's connected to the back bone, the back bone's connected to the neck bone, the neck bone's connected to the head bone. See?... Easy :hide: