Question:

[THEFERMANATOR]

Originally Posted by Destroyer

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.

The micro bubbling is more an issue with diesels due to the higher cylinder pressures, but it is present in gas engines as well. Do a search on INTERNATIONAL 7.3's(not the powerstroke, but the IDI version), and you will find plenty of problems with cavitation and cylinder erosion due to the micro bubbling and it's effects. As pressure increases, the effects are normally worsened. Outboards already use automotive grade gaskets or better. Most automotive engines rarely see more than 16-18 PSI whereas in an outboard that is common pressures, and 20-25 can be seen. I understand your thinking, but your over thinking it. I know many think they can build a better mouse trap, but i nthe end the tried and proven design normally wins out. This is one of those situations.

[Destroyer]

Originally Posted by THEFERMANATOR

The micro bubbling is more an issue with diesels due to the higher cylinder pressures, but it is present in gas engines as well. Do a search on INTERNATIONAL 7.3's(not the powerstroke, but the IDI version), and you will find plenty of problems with cavitation and cylinder erosion due to the micro bubbling and it's effects. As pressure increases, the effects are normally worsened. Outboards already use automotive grade gaskets or better. Most automotive engines rarely see more than 16-18 PSI whereas in an outboard that is common pressures, and 20-25 can be seen. I understand your thinking, but your over thinking it. I know many think they can build a better mouse trap, but i nthe end the tried and proven design normally wins out. This is one of those situations.

Not looking to build a better mousetrap, just wondering if one has already been built but they want to sell us new impellers instead.

[RidgeRunner]

Popcorn... Must remember the popcorn...
My flats boat with the jack plate all the way up puts the cav plate (anti-vent plate) nearly even with the surface of the water. At idle I do think there might be a problem with prime on a centrifugal pump.
Not that it matters much, they make positive displacement pumps with harder vanes than the rubber impeller we see in the modern outboard. Jabsco makes a hard vane puppy pump that I like for fuel transfers. Debris MIGHT be the only spoiler and I know it will move a lot more debris than you would ever want in the water jackets of your outboard.
I am not arguing, The question has no correct or incorrect answer!

[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.

[spareparts]

fixed vane impellers don't suck well, just like my pool pump. You have to have a head pressure to get it to pump. Sliding vane impellers work well, but they don't like any kind of contaminant. the one I use for waste oil get stuck up all the time with particulates that get by my 10 micron filter. Gear pumps require too much lubrication and don't make enough volume. roots type pumps move a lot of volumes, but take a lot of horsepower to operate. I squash plate type pump runs too critical clearances adn would cost too much to implement. Enter the floppy vane impeller, moves enough volume, makes enough pressure, is tolerable to running to dry to an extent, able to self prime, very forgiving in tolerances. Cheap to make, easy to repair, works through out a wide variety or rpm ranges. SO why not?

Next we enter the housing aspect. Mercury used to use a small diameter stiff blade impeller in a plastic housing with a stainless sleve. Very good pump, but don't let it sit still for a long time and absolutely don't run it dry, even for 10 seconds. It will ruin everything. OMC large diameter floppy vane, low pressure, high volume. Stainless sleeve in a plastic housing. Run it dry long enough and it will melt the plastic housing. Does OK. Yamaha copied the same design but with a few differences, same issues. Suzuki stepped up with a cast stainless steel housing on the same type of impeller, you can smoke three or four impellers and the housing don't care, but run it in enough sand adn you'll be crying about how much that housing cost. Enter Mercury for round two, large diameter floppy vane with a stamped stainless housing, cheap to make, just as resilient as the zuki,easy to replace. Mercury wins with teh best design water pump assembly. BTW, I probably replace 100-150 impellers a year. I/O and Inboard pumps are a whole nother subject