Question:

[Destroyer]

Originally Posted by THEFERMANATOR

Lets say your running along and get some grass on your lower unit, not enough to cause you to come off plane, but just enough to allow the water pickup to suck air in. With a rubber impeller no big deal as it will continue to pull in the aerated water mixture, BUT your centrifigal pump could stop pumping all together as soon as some air hits it. I understand your way of thinking, but I still feel there is a reason for sticking with a tried and proven design. And it comes down if it aint broke, don't try and fix it. I'm willing to bet others have tried running centrifigal pumps on outboards, and I would bet they found out real quick they didn't work well on engines that can ingest an air water mixture at speed.

I don't necessarily disagree with you Ferm, but think about it. ANY pump, at speed, is going to have gallons and gallons of water forced into it simply by the movement of the engine through the water. So just like a rubber pump collapsing (and not doing any pumping at high rpm's) a centrifugal pump will not suffer loss of pressure if it ingests some air, because the pressure is going to be supplied 100% plus from the engine moving thru the water. In fact I'd be willing to bet that over pressure might be a bigger problem. Might have to have some kind of pressure relief valve built into the cooling system.

And there are other pump designs also.. Like gear pumps that don't suffer loss of suction with air ingestation, or moyno pumps or screw pumps. I just find it hard to believe that after 108 years of building outboard engines nothing better has been found. Sure, the materials may have changed from rubber to silicone blends, etc, but the overall basic style of the pump hasn't changed at all... and that bothers me and makes me wonder why.

[phatdaddy]

i am glad that you need to change it annually, that insures you can get the lower unit off when you need to.

[smokeonthewater]

There are a LOT of things that don't change over time.

If it works it works.... As for the impeller sure it's a good idea to change yearly but I had a '59 Evinrude 7.5 25 years ago and a friend has it now... To the best of my knowledge it has the original impeller in it as my school teacher who gave it to me said they had never done anything to it. The impeller in it is at least 30 years old now and my friend is still using it.

They aren't THAT bad of a design....

As for the flow of water... The water isn't picked up from the movement of the lower unit through the water... It COULD be if the intake were designed differently but only at high speed... Low speed operation which doesn't necessarily mean low rpm would always require the work of the pump.

[Destroyer]

Originally Posted by phatdaddy

i am glad that you need to change it annually, that insures you can get the lower unit off when you need to.

Now there's an argument that I can agree with 100%.

[Destroyer]

And lets take this a step further... WHY is it necessary to regulate the water flow pressure? I mean, think about it.. does it really matter what the pressure is, as long as the volume is sufficient to cool the engine? We're talking about metal engines, and I seriously doubt that additional water pressure will cause any harm. So you build up a higher head pressure with a centrifugal pump. Who cares???? The temperature of the engine will still be regulated by the thermostat, regardless of what the water pressure is. Automotive applications (like your car) have used centrifugal pumps forever, and they seem to run just fine with them. So why the big whup over higher pressure in an outboard? What would it hurt?

[smokeonthewater]

Higher head pressure is higher parasitic drag, less mpg and less power to the water.

[THEFERMANATOR]

Contrary to what you might think, higher head pressures CAN cause problems inside the engine. The gaskets and such for the cooling system would have to be completely re engineered to handle pressures in excess of 20 PSI. The only sealing surfaces meant to hold higher pressures are the sealing ring for the head gasket in the cylinder area only. Also higher pressures can have a negative effect on cylinder liners not only from abrasion, theres also cavitation, and micro bubbling at the cylinder liners from the engines combustion process. You are going to get micro hot spots which form tiny air bubbles along the cylinders in the cooling jackets, and these air bubbles under pressure can act like a snad blaster when a combustion event occurs. It is like a hammering action when the cylinder fires, and that hammering action acts upon the cylinder wall in the cooling jacket. As pressures increase, this event is worsened. Believe me, there have been attempts at finding something better, but if it existed don't you think somebody would have come out with it by now. The engines you are talking about with hard impellers osund more like tender engines used on non planing hulls where ingesting air at speed, high RPM useage, and other factors are not a concern, hence why they can use what they use.

And engineers have come up with a pump that can do multiple applications, it just so happens to be the rubber impeller that has proven itself for over a century now.

[Destroyer]

Originally Posted by THEFERMANATOR

Contrary to what you might think, higher head pressures CAN cause problems inside the engine. The gaskets and such for the cooling system would have to be completely re engineered to handle pressures in excess of 20 PSI. The only sealing surfaces meant to hold higher pressures are the sealing ring for the head gasket in the cylinder area only. Also higher pressures can have a negative effect on cylinder liners not only from abrasion, theres also cavitation, and micro bubbling at the cylinder liners from the engines combustion process. You are going to get micro hot spots which form tiny air bubbles along the cylinders in the cooling jackets, and these air bubbles under pressure can act like a snad blaster when a combustion event occurs. It is like a hammering action when the cylinder fires, and that hammering action acts upon the cylinder wall in the cooling jacket. As pressures increase, this event is worsened. Believe me, there have been attempts at finding something better, but if it existed don't you think somebody would have come out with it by now. The engines you are talking about with hard impellers osund more like tender engines used on non planing hulls where ingesting air at speed, high RPM useage, and other factors are not a concern, hence why they can use what they use.

And engineers have come up with a pump that can do multiple applications, it just so happens to be the rubber impeller that has proven itself for over a century now.

Ok, I'll go along with what you say as far as the re-engineering of various gaskets to take a higher pressure. So you'd have to use an automotive type of gasket. (Or some kind of pressure regulator elsewhere in the engine) But the cavitation and micro bubbling you speak of I have my doubts about. After all, you would have the same effects in a car, and I don't recall ever reading about it in and of my shop manuals.

You are spot on about the seagulls.. they are low speed engines for sailboats and other non-planing hulls. Still not convinced about the rubber impeller though.

[SkunkBoat]

Originally Posted by phatdaddy

i am glad that you need to change it annually, that insures you can get the lower unit off when you need to.

BINGO!!!

Also, in a car, you've got a rubber belt to self destruct if the impeller jams.

[Destroyer]

Originally Posted by SkunkBoat

BINGO!!!

Also, in a car, you've got a rubber belt to self destruct if the impeller jams.

Perhaps, but the towing service that comes and hauls your car away for you if that happens doesn't cost you hundreds of dollars, or maybe legally takes your car as salvage. I agree with the bolt removal thing, because it is important
to keep things accessible.