water impellor direction

[Destroyer]

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?

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]

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]

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]

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]

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)

[Destroyer]

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]

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

[Destroyer]

Originally Posted by mawshj

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.

[steplift20]

now im realy confused

[Destroyer]

Originally Posted by steplift20

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