Your cruising speed?

A couple years ago, this flushing topic was kicked around a bit here:

http://www.cbrats.com/viewtopic.phpt=18 ... sc&start=0

To my eye, with a dealer input, I conclude engine flushing depends on
1. Your specific engine - Mfg, model - not all use same flush protocol
2. Read your Owner's manual and follow the instructions for your engine
3. What we did years ago may not apply now (hence 2. again)

Aye.
 
First, what is CBrats.com vs c-brat.com? Foggy's link was a new one for me.

I had a water pump fail on an almost new Honda kicker (8 hp) while flushing the engine with mouse ears. It was covered under warranty, but I was surprised to see the vanes in pieces. Now, I will only use my flush bag on the kicker.
 
The link which Foggy posted is spam. This has nothing to do with either C Dorys or C Dory dealerships.

I have no idea what force it would take on the prop to turn over a 90 hp engine--but it would be huge. Try turning the pro with your hands, when the engine is in gear.

On larger boats--I swung 24" to 28" diameter props in my motor sailers with pitch about 24", and 3:1 reduction gears. Even with the free wheeling, you had to be going at least 6 knots knots to get the prop turning (and that was not with any resistance from the engine--just the gear box.

Can it be done on a Honda 90--perhaps, if the boat is at planing speed? Has anyone done it? Extremely unlikely to had any movement at displacement speeds, as when one was fishing or no wake zones.
 
I've trolled many, many hours on both my previous 22 and my current Tomcat on one engine running and one in the water off and in neutral. Not a problem. I like leaving them both down since if for some reason I need to fire the other one up quickly, it can be done.
 
That's how we troll, for hundreds of hours and never a problem. I leave them both down so the drag of the other dead engine slows the boat down to trolling speed. The steering takes a lot more force to move with one motor up as well. When I switch motors like I usually do every hour or so, its a lot less of a procedure to just turn the key, rather than raising and lowering motors

As for cruising speed, I don't really have the patience to go slow. We're loaded pretty heavy most of the time and cruise at about 4500 rpm and 15-17 knots unless its rough out. In that case we slow down to about 12 knots.
 
This is not spam. It's off this website.

Hence (again):

http://www.c-brats.com/viewtopic.php?t=18028

The other reference was a cut'n paste error.

There has been no explanation for, or reason for,
seawater turning a prop which cranks the stopped
engine and I don't know why it was offered as an
explanation of "reverse flow" water damage.

Sorry for any confusion.

Aye.
 
Sea Wolf":32mkzbby said:
I think the problem they anticipate is that with
the motor down,
the gear shift in reverse, and
enough force to turn the motor backwards,
the water pump will turn backwards, damaging the vanes.

install_imp.jpg

Joe. :teeth :thup
Click to expand...
Foggy:
It seems to me, rather than incoming lower unit water pressure turning the
prop which turns over the engine or vane damage in the water pump, the
damage comes from the "reverse flow" of water entering where it shouldn't be
via the exhaust/cooling system which is not pressurized with a stopped engine.
Apparently, this can happen (only ?) with a stopped engine, gearshift in reverse,
and the boat moving forward.
Click to expand...

There has been no explanation for, or reason for,
seawater turning a prop which cranks the stopped
engine and I don't know why it was offered as an
explanation of "reverse flow" water damage.
Click to expand...

At the low speeds, there is no way that the engine will rotate due to water pressure on the prop blades. The water pump is a displacement pump. The pump is fixed to the output shaft of the engine--and if not moving acts as a barrier to water flow up the normal passages. So I don't see any way that water can get up thru the normal water passages. Yes, running the pump backward will damage the flexible vanes of the pump.

If the boat was moving backward fast enough, --then there it is possible that water can be forced up thru the exhaust, and bast the exhaust valves. But that does not seem to be what the manual implies.

Foggy, thank you for fixing the link to the erroneous spam site. It is always a good idea to hit the "preview" before submitting a post, so you can check to see if a link works, or if a photo you linked to appears.
 
Is it possible that the concern is that if you left one engine down at speed(probably much faster than idle speed of the other engine), that you could get enough positive pressure through the water intake/s that in the non running engine you could get water back up past valves in to the combustion chambers through the midsection? In a Mercury I rebuilt, there is some bypass exhaust volume in the midsection, which at idle is released through the exhaust exterior port. At higher speeds, greater proportions were exhausted through the exhaust tube and then through the prop. In this scenario there would be no concern of turning the impeller.
 
The only way I could see water getting back up thru the exhaust, would be if the boat was backing. Going forward, there is just no place for the water to be pushed up thru the exhaust.
 
But could water be coming back in through the water pick up points on the side and just below the cavitation plate on the lower unit? I think that it is physically possible if there were enough positive pressure at those points. At least on some motors. I took a look at a schematic and it looks like that might be possible.
 
Since the water intakes are on the side of the lower unit, I would think that if the pump is not running you could end up with negative pressure on the water intakes (low pressure over the ports from the moving water). This would tend to suck the water out of the engine, but this is likely stopped by the vanes on the impeller blocking the flow.

If water is coming in through the water pickups and the motor is not turning, the pump will probably prevent the passage of water to the rest of the engine (the pump cannot free wheel).

If you have a bad impeller things could be different.
 
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