Overheating Issue on maiden voyage.pics

[Lawrencemd]

We just launched our 28 ft home built cabin cruiser and are having some overheating issues.
To start off, I will outline the the cooling system route. It is an enclosed system using 50/50 water and antifreeze. There is approx. 25 ft of 1" cooling pipes under the boat. The cooled water runs from the cooling pipes, through the transmission cooler, into motor (250 chev 6 cyl), out of motor into cooling tank, then on to marine exhaust manifold, then back out to cooling pipes.

My question is whether this is the correct order in which to cool the different systems. Should the motor be cooled before the transmission cooler??Should the exhaust manifold be cooled first? What order should the systems be in the flow of cooled water from the pipes?

Thanks

[mark775]

Your order is fine. What are your coolers made of? What pump do you have?

[baeckmo]

Sorry, have to correct you there!

A) During "heating-up" period, there is no coolant flow through the xh manifolds with your setup, resulting in a massive heat build-up. The thermostat is in closed position. In most engines today, this means that the flow is recirculated thru' an internal shunt; nothing trough manifold and cooler.

B) On top of that, you have about 0.5 m2 of cooling area exposed to the sea, which may cater for the cooling of ~50 hp, depending on flow velocities outside and inside the pipes! Your engine would produce more than that, unless throttled. A working rule of thumb is 100 hp/m2 for diesels and ~85 hp/m2 for gassers (lower thermal efficiency > more heat to reject)!

Manifolds should have a parallell cirquit, but you may use "fiddlers recipy"; drill a leak hole, dia ~4mm in the thermostat valve. Heating period will be longer, and the thermostat will often allow wider temp variations in the block, but try it, you may live with it! Don't forget to install a vent pipe (~5 mm) from highest point in the "hot branch", probably the manifold, directly to tank, in order to dissipate gas and vapour. This prevents vapour locks that choke the centrifugal circulating pump!

In order to prevent cavitation in the pump, it should get the coldest water, so from cooler first into engine water inlet! Then from engine to oil cooler (transmissions often run too cold!).

[tkk]

Isn´t the antifreeze ratio a little extreme? Water is better than antifreeze when it comes to heat transfer.

Unless you are operating an ice-breaker in arctic conditions, you would need antifreeze only for corrosion-prevention and 20% should be enough.

We use 50-50 in our cars, that gives you freezing point around -35C.

[Lawrencemd]

I should have also mentioned that there currently is no thermostat in the motor, in other words coolant is flowing all the time.

The coolant is pumped through the system with the original engine water pump.


Im very interested in your statement, "Don't forget to install a vent pipe (~5 mm) from highest point in the "hot branch", probably the manifold, directly to tank, in order to dissipate gas and vapour. This prevents vapour locks that choke the centrifugal circulating pump!". Do you mean another hose from manifold to tank? This is what we were worried about and had trouble with, vapor locks in the system.

I am thinking of changing the flow to the following; Cooled water comes from keel coolers directly into engine (water pump housing), out of engine down to trans cooler, out of trans cooler into exhaust manifold, out of exhaust manifold and into expansion box (highest point in "hot branch") and then out of expansion box and back out cooling pipes. What do you think?

[mark775]

Baeckmo is right. I missed the part about the cooling tank. And transmissions often run cold, as he said. But your routing is obviously not the cause of the overheating. Yes, a smaller antifreeze/water ratio would transfer heat better but we're still not finding the overheating culprit. The pump should pump water first, before the cooler, to have to draw less. There's a big fiddler's recipe now!

[tkk]

What kind of temp metering do you have?
Can you measure the temp of coolant coming from the cooler pipes?

That would help determine if the problem is in circulation or cooling capacity of your cooler.

[wardd]

get a infrared thermometer to check various points in the circuit, i bought one for about $50 from pep boys auto store and it seems fairly accurate, accurate enough for what you need

by the way, cool get out on the water thing you've built

[Lawrencemd]

We have an electronic water temp gauge connected to the motor. I will see if we can record the temp leaving the pipes.

What I found was interesting, is that after it overheated, and we let the temp come back down to around 180, we would start the motor and chug along for quite a long time before the temp would start to rise again. You would think the temp would start to rise gradually, but no, after 20 mins of motoring between the 180-190 degree temps, all of a sudden it would start to rise very quickly. We would then shutdown and let it cool.

By the way, thanks for your quick resonses.

[tkk]

Is your system properly pressurized? Maybe your engine boils the high-antifreeze coolant and you get gas bubbles in the system that prevent proper circulation

[Lawrencemd]

It was properly pressurized on land but that is what we are going to test today.

[tkk]

Is it possible that some air was trapped in the system all the time?

I think someone already mentioned a small pipe from some high spot, preferabley above cyl head into the expansion tank. That should take care of both.

[Lawrencemd]

That is what we were thinking, that there was air in system. When we first set it up, we had a lot of trouble getting the air out.

[wardd]

if any part of the circuit goes up then down that may be it's own high spot and may need a vent that can be opened, it would act as a trap for air much as a trap under a sink acts as a trap, for air the u would be up and for water the u would be down

[tkk]

You might need to open some hose connections slightly to let the air out while filling up the system. Once filled, the small pipe from the big hose coming from the cyl head should be enough

[Lawrencemd]

Currently, the highest point is the expansion box. What if I put a small bleeder (like a rad bleeder) in the rubber hose going into the expansion box. Then when filling it up, I'll open it and let the air out.

[baeckmo]

Holy grail, how often do we have to tell people NOT to take the thermostat out of the engine!!!!????? When there is no thermostat, the shortest loop for the water is directly back to the engine through the internal shunt; ergo NO COOLING!!!!

Drill the leak hole as I told you, put the darn thing back where it belongs and off you go......

[baeckmo]

Normally the cooling circuit in the engine is as follows:

1) Water from cooler coming in at pump suction side.
2) After the pump, water is distributed trough engine block and top, ending up at thermostat housing.
3) When the engine is too cold, there are two methods;
3a) the thermostat is directing all water straight back to the pump inlet via a small bore pipe "internal shunt". Often this channel is hidden in the casting.
3b) older engines without shunt stop the flow totally. This method has been obsolete for ages.
4) When block and top has normal operating temp, the thermostat is balancing the flow to cooler/shunt to keep temp constant.

The shunting allows full flow through the engine all the time, preventing local overheating due to pockets of low flow. It also lets the centrifugal pump work close to its design point, preventing cavitation and aeration.

So, removing the thermostat means that you have no balancing restricion that forces cooling water via your cooler when the engine has reached working temperature, neither is there any water circulating through your manifold, which is quickly boiling the slug of water standing there.

And: The water must NOT circulate through the tank, because you will most certainly have a free surface there. With the turbulence of the moving fluid, air will be drawn into the cirquit, causing expansion troubles plus increased corrosion due to the free oxygen available! The tank should be a "dead end", connected to the pump inlet, and with a vent from the highest hot spot, as mentioned before! The ideal is to have the water degassed as completely as possible, so any contact with a free surface is to be avoided!

[baeckmo]

For a better understanding, see attachment! Top sketch shows "as built". If you remove the thermostat valve, the water will take the easy way = the shortest way, hence no cooling, neither to engine nor to manifold/oilcooler.

Without the shunt, as in older engines, the thermostat (if fitted) would be closed during warmup; still no cooling to the manifold!

Lower sketch shows a reasonable compromize, note the connection of tank with vent. This arrangement needs the leak hole in the thermostat, otherwise no manifold cooling during warmup.

Normally in marine engines, the cooling water to the xh manifold is taken from block before the thermostat (bypassing it), and then connected to the thermostat outlet to cooler via a calibrated restriction.

And,finally: even with this corrected and with a lower mix of antifreeze, your cooling surface is still not enough for more than ~50 hp!!

[kapnD]

Dont run it without a thermostat! If you're still getting air after you install the thermostat, you could have a leaking head or turbo gasket.
Drilling the hole in the thermostat flange sounds like a good trick, as the manifold does get real hot fast.
Some marine engines have a bleed hose off the top of the manifold to the tank.
Some motors need to have the coolant filled very slowly to avoid trapping air pockets, use a funnel with a small opening to regulate fill rate.
Good luck!

[Brent Swain]

I have always used 1.5 sq ft per 10 hp. No problems. I prefer to run the water from the engine to the cooler, and would l cool the tranny, then the exhuast water jacket after the tranny. That way you are not warming the water before it hits the engine.
I set up my header tank so I can jam a plastic hose barb in the bottom, outlet to the cooler, and attach it to a presure tank (ss pop can ) filled with antifreeze and water, Then I pressurise the tank and force the coolant into the system to force any air bubbles out.
This also lets you flush the sytem by hooking it up to a dock hose and letting the hose flush it out.
Brent

[Lawrencemd]

OK, today we rearranged the coolant flow, matching it up to another fishing boat here, which had the same engine transmission combination. After very slowly filling it with coolant/water, the temp stayed down at 185 for 35 mins at idle. Much better than yesterday.

I was puzzled that this worked, and was initially thinking that there had to be air in the system yesterday. However, after thinking about the new flow route, we are now moving coolant through the water cooled manifold, in the opposite direction from yesterday.

Yesterday, we were moving liquid through the manifold, from front to back. Today it is running back to front (which is the opposite of what I would think). The other fishing boat, and another boat online here, are setup the same way.

I am thinking that maybe running coolant through the wrong direction was causing turbulance, and making it much harder to get the air out of the coolant lines? nd yes we are installing a thermostat.

Does this sound plausible?

[baeckmo]

I have always used 1.5 sq ft per 10 hp. No problems.
Brent

.....that equals 72 hp per square m; ie slightly more conservative than my 85 hp/m2. That would allow a power of 41 hp with the cooling area Lawrencemd has built in. So, back to basics as I have explained!

[hartley]

If your cooling system is in order perhaps it is time to look further, is your engine overloaded,what gear ratio,what size prop? .Remember props move boats not engines ......cheers Hartley