Cooling System Generic Info

[Alumarine]

A friend asked a question the other day which prompted me to ask it here.
When running our I/O's or most other motors on land we always use muffs which forces the water through the cooling system.

When running kickers out of the water we use muffs if possible but with some motors we use a garbage can.
For 2 different small Honda's the manual says to have the water level 2" above the cavitation plate the other says 4".
This is to ensure the water level is above the pump according to the manual.

His question was, if it's important to have the water level above the pump, how does the pump work when the boat is on plane?
I thought it was a reasonable question.

 

[racerone]

The pump is not self priming.-----Therefor the impeller must be submerged.-----When boat is moving the water is forced up to the impeller and the pump was primed on start up.-----

 

[Alumarine]

The pump is not self priming.-----Therefor the impeller must be submerged.-----When boat is moving the water is forced up to the impeller and the pump was primed on start up.-----

I understand it's not self priming.
What forces the water up to the impeller when under way?

 

[racerone]

???----Atmospheric pressure.

 

[dave8311]

Notice the design around the water inlet holes?

 

[flashback]

???----Atmospheric pressure.

And the foil shape of the lower unit where the intake is causing pressure as the water hangs on the foil..

 

[Scott Danforth]

I understand it's not self priming.
What forces the water up to the impeller when under way?

when the boat is stationary, the impeller is about 8-10" under the water level. this does the initial prime

the fact that the lower unit inlet holes are pointed forward means that water is forced in as the propeller pushes the drive forward. that forward movement keeps the impeller primed. just like when you are water skiing and you fall backwards while traveling forward forcing water up your nose.

hit a plastic bag, carp, or chunk of seaweed and that prime can be lost. do that at 40mph and you can toast the impeller before your temp gauge even moves a micron.

 

[Alumarine]

I thought maybe Atmospheric Pressure as well or the water was forced in through the intake.
I'll have to look at the intake on my mercruiser but on one of the honda kickers it doesn't really look like water would be forced in.

 

[racerone]

???----Water is forced in by atmospheric pressure !!------Stick a clear plastic hose in the bath tub.------Suck on the hose.----It is atmospheric pressure that brings water up the hose..

 

[04fxdwgi25]

At 65 mph, open window (any window) on your car and stick your hand out.

You will instantly see how the water being forced into the intakes works.

 

[Alumarine]

Finally had a chance to look at my outdrive as well as a honda 50.
On the water intakes, both of them have plastic grates with a bunch of tiny scoops.
I guess that's how it works.
I am wondering as others suggested pressure differential comes in to play as well.

 

[flashback]

Think about the shape of the leg, it's a foil when it pushes the water to the sides the water immediately wants to go back where it was. Atmospheric pressure. Put a pump intake in the path of this water that you just pushed out of the way and the water will go there like stink on fish..