How is a water Cooled Outboard Motor Cooled?

[cpostis]

I've been thinking of the recent exhaust stack problems for someone's F225 that has been numerously posted. Last year I had my first overheat alarm on my SX150. I considered what I did , and the drastic improvement in performance.

So, I thought I'd put together what I thought about how an outboard cooling system works. I also thought of how valuable an infrared thermo gun is worth in determining the performance of a cooling system.


I know different Yammies are "piped and sensored" differently, but would appreciate some input to what I have put together so far.

This page one of a draft. If some of you want to comment by placing your comments in red via quoting this, I would appreciate it.

What is an outboard motor cooling system?
An outboard water cooling system is a once through “heat exchanger”. That is, it exchanges excessive heat from the engine to a place where it is unobjectionable.

How does an outboard cooling system work?
Water is drawn through the water pump from the lower unit to the cooling passages in the heads and block to the “thermostat valves”. If the valves are closed due to being below “opening temperature point” water will not pass through the heads and block and by-passed through the lower unit and exhausted (some outboards use pressure control valves to prevent excessive pressures).
If temperature rises above “opening temperature point”, water will pass through the cooling passages of the heads and block. Heat will be transferred between the surfaces within the heads/block and the once through cooling water. The heated cooling water will be expelled through the lower unit. Once enough heat is removed, and drops the temperature of the cooling water below the “closing temperature point” of the thermostat valve(s), the thermostat valve(s) will close again.

Why is an outboard motor not cooled sufficiently?
The above cycles will continue keeping the temperature of the engine within acceptable tolerances under various conditions. However, there are two reasons why an outboard motor is not cooled properly. Generally, they are insufficient cooling water flow (several reasons) or fouled heat exchanger surfaces one reason).
Insufficient cooling water flow
Reasons for insufficient water flow can be:
1. Poor water pump performance
2. Clogged water passages
3. Improper thermostat valve operation (closed or partially closed)
4. Leaks in cooling water supply to cooling passages
5. Pressure control valve stuck open
Reason for fouled heat exchanger surfaces can be:
1. Accumulation of deposits on heat exchanger surfaces (note that increased heat exchanger surfaces temperature will increase the ability of deposits to “stick” to the surfaces, as will low flow).

What do water temperature switches do, and how do they work?

[fairdeal]

Looking to reading more about this, no doubt Boscoe will contribute.
Rodbolt may or may not be interested....


I'm sure water flow varies model to model.

I've looked at the F225 , trying to get a handle on it -
impressively complex arrangement of passages through the various castings.

If the valves are closed due to being below “opening temperature point” water will not pass through the heads and block and by-passed through the lower unit and exhausted

I believe - not at all sure -

that some of the water entering the block exits around the exhaust "manifold" -

without contact with, and regardless of the state of, the thermostats.

Once the thermostats are open, additional water exits via a different route.

Here's an annotated view of the underside of the F225 -

have I "guessed correctly" ?

[HarriesDGH]

Thermostats

This is what I found at the end of this season. Despite regularly flushing with fresh water, all four thermostats (twin 115hp V4 2-strokes) were badly salted-up.

[boscoe99]

[boscoe99]

Something to think about.

Aluminum has a nominal melting point of about 1220 degrees F.

Exhaust gas temperatures can be up to ruffly 1600 degrees F.

Why doesn't the aluminum normally melt?

[pstephens46]

This is what I found at the end of this season. Despite regularly flushing with fresh water, all four thermostats (twin 115hp V4 2-strokes) were badly salted-up.

Those look to be many years old. I inspect them every year. Prob change every three on average.

[cpostis]

Something to think about.

Aluminum has a nominal melting point of about 1220 degrees F.

Exhaust gas temperatures can be up to ruffly 1600 degrees F.

Why doesn't the aluminum normally melt?

Now, that brings me to another question or two.
My SX150 two stroke has an exhaust plate. Hot exhaust gasses are directly exited from exhaust ports into/over the plate. On the opposite side of the plate are cooling passages, and then and exhaust cover.

I wonder if the cooling water is sufficient to keep the temperature of the aluminum way below melt point.

Now, are we sure both my exhaust plate and the exhaust stack in an F225 are made from aluminum/alloy? Wish a had put a magnet to my plate while it was removed. I guess seeing that the edges of the plate are exposed between the block and exhaust cover, I could see if a ferrous metal may be indicated by use of a magnet?

Now, in both my motor and an F225, cooling water is not sufficiently passed over either, will both the plate and exhaust stack fail. My plate appeared brand new after removing carbon deposits.

Boscoe, do you happen to have a diagram like the F225 you posted for my SX150, can you post? Sure would like to see it if you do.

Thanks!!

[fairdeal]

Something to think about.

Aluminum has a nominal melting point of about 1220 degrees F.

Exhaust gas temperatures can be up to ruffly 1600 degrees F.

Why doesn't the aluminum normally melt?

Well, an ordinary propane torch is stated to be 1995 F

copper stated to melt at 1984 F

but when you are sweat soldering pipe, you'll never melt the copper.

And if you've got even a little water standing in the pipe,

its hard to melt the solder - which only takes 450 F

[rodbolt17]

but remember aluminium WARPS at a much lower temp than it melts.
that is why certain parts like the oil pan,ex guide and muffler get supplied water reguardless of T stat operation.
and yes I have seen muffler tubes melt into slag particles.
mostly on the z300 and z 250.\
I have seen heads warp so bad you had to take .020-.025 off to make them flat again.
oil pans and ex guides do the same thing.
get them hot and they change shape but the gasket does not.

[cleddau1]

good question boscoe

Temperature and heat are different things, a gas may be at a high temp but it's density and hence thermal mass is low and also Al is a good conductor and any heat will dissipate. A furnace melting rocks pours molten rock ie lava through water cooled copper nozzles over water cooled copper spinners to make rock wool insulation.....

[99yam40]

Temperature and heat are different things, a gas may be at a high temp but it's density and hence thermal mass is low and also Al is a good conductor and any heat will dissipate. A furnace melting rocks pours molten rock ie lava through water cooled copper nozzles over water cooled copper spinners to make rock wool insulation.....

I learned something new today
I can coast the rest of the day now

[cpostis]

I am asking questions bit by bit, please bear with me.

Boscoe, others;

Please look closely at the diagram Boscoe posted.
If I read this correctly, if the thermostat is closed no water is flowing through either the "In-bank exhaust" or the block. That is flow is stopped at the thermostat , yet flow is not interrupted through the "Exhaust pipe" (it continues). Water constantly flows through/over (not sure if which/both?) the oil pan and the exhaust pipe.

Only if the thermostat opens beyond opening point will water flow be established through the heads/block. Kind of like an automobile radiator and thermostat, except not recirculating. Are there passages to allow some water flow through the heads/block at all time or otherwise?

[boscoe99]

Temperature and heat are different things, a gas may be at a high temp but it's density and hence thermal mass is low and also Al is a good conductor and any heat will dissipate. A furnace melting rocks pours molten rock ie lava through water cooled copper nozzles over water cooled copper spinners to make rock wool insulation.....

From listening to the so called news and weather channel meteorologists you would never know it.

Another thing that seems to be not so well understood is that hot relates to temperature. Not heat.

They talk about the "heat index" and then use the term degrees. Which relates to temperature and not heat.

And then there are engine makers that refer to a closed switch as be "on" and an open switch as being "off".

Hay caramba!

[boscoe99]

I learned something new today
I can coast the rest of the day now

Don't be slacking off on us now. Get out of the wagon and put your shoulder back to the wheel.

[boscoe99]

I am asking questions bit by bit, please bear with me.

Boscoe, others;

Please look closely at the diagram Boscoe posted.
If I read this correctly, if the thermostat is closed no water is flowing through either the "In-bank exhaust" or the block. That is flow is stopped at the thermostat , yet flow is not interrupted through the "Exhaust pipe" (it continues). Water constantly flows through/over (not sure if which/both?) the oil pan and the exhaust pipe.

Only if the thermostat opens beyond opening point will water flow be established through the heads/block. Kind of like an automobile radiator and thermostat, except not recirculating. Are there passages to allow some water flow through the heads/block at all time or otherwise?

Yamaha thermostats (and most I believe) have a passageway so that a bit of water can get across the thermostat at all times.