marine neophyte's engine questions...

[duns227]

Hi All. I was thinking about keel cooling systems and a couple of random questions occurred to me. I was hoping someone could edu-ma-cate me I'm fairly familiar with automotive system but am just now learning about marine systems so I apologize if these questions are simplistic.

If you had a boat using a closed cooling system with a keel cooler, would it be possible to damage the engine with coolant that is too cold coming from the keel cooler?

For example, theorize that a boat is running at full throttle for an extended period of time in water that is really cold, and the keel cooler is way over-sized. The engine thermostat is generally on the output side of the engine cooling system right? Is it possible that the cold water coming into the hot engine from the keel cooler could actually damage it?

Also, since the keel cooling system is generally much longer in flow length than a standard radiator, do keel cooling systems need a bigger or auxiliary water pump to help circulate the water or would standard water pumps be sufficient?

Depending on how badly I get smacked around in this thread I may have other questions...

[FAST FRED]

YES inlet overcooling IS a concern.

There are thermostats that can be built into the return water system that tempers the water to about 20F below the departing water , same as on a vehicle.

Many boaters don't bother with the expense and simply use a bypass valve to control the temperature of the returning water.

Since there is great latitude on returning temps , it is only a seasonal adjustment , or a relocation adjustment.

The tiny tubing in a std radiator is more resistance than 40 ft of 1 1/2 or 2 inch pipe and a couple of elbows.

Good thinking

FF

[CDK]

Not a real issue.
The coolant is pumped around within the engine until the thermostat opens just a bit, then cold water from the keel is mixed with it. A thermostat is not bipolar, at least it shouldn't be.

[duns227]

Originally Posted by CDK

Not a real issue.
The coolant is pumped around within the engine until the thermostat opens just a bit, then cold water from the keel is mixed with it. A thermostat is not bipolar, at least it shouldn't be.

So the coolant actually circulates inside the engine before the thermostat opens?

[gonzo]

Yes it does.

[duns227]

Oh wow. I know less than I thought.

So what makes it divert outward when the thermostat opens? Is there just less resistance to flow outward than to recirculate through the engine? The thermostat doesn't act like a Y-valve (to my knowledge at least).

[gonzo]

There are several circulation patterns. Some have a small bypass at the thermostat itself. Others have a bypass hole or pipe. Usually, the thermostat acts like a restriction and when it opens, most of the coolant goes to the radiator, heat exchanger or exhaust.

[duns227]

Huh, learn something every day.

So what I'm taking away from this is that the introduction of too-cold water from a keel cooler won't really matter on a thermostat controlled motor (unless it's extremely cold maybe??) and that the length of the keel cooler run isn't really an issue for the water pump to be able to pump the coolant through.

Does that sound correct? Thanks for the all the input.

[thudpucker]

I had just such a problem on an old boat with a 25 Hp Gray Marine engine and two lengths of 3/4" pipe (40') for a keel cooler.
I put an old Faucet on the output of the engine coolant, to regulate the amount of hot water escaping back into the cooler.
It worked just fine.
The Temp gauge would rise up for a bit, then cool back down a bit. Always stayed in the green though if I had the water regulated correctly.

[gonzo]

The thermostat will take care of the engine's temperature. As long as the coolant is not frozen solid, it doesn't matter.

[baeckmo]

Like Gonzo says, the built-in thermostat in all modern engines is controlling engine temperature, no matter what the cooler temp is. This is accomplished by making the thermostat into a three-way shunt (instead of the straight open-close function in old engines).

You find the principle in the attached sketch. Please note the leak-hole trick in order to provide cooling to the exhaust manifold from start-up, and the air-vent to the expansion tank.

[Carteret]

I agree with Gonzo & Baekmo: To be on the safe side. If freshwater cooled exhaust manifolds are used (engine coolant cooled not raw water) you must have some bypass through the manifolds or the coolant in the manifolds can be boiled out before the engine thermostat fully opens. You should also install a bleed valve on top of the thermostat housing to bleed off entrapped air