Originally posted by: hidro
Hey guys there seems to be mixed ideas about steam coming from the exhaust of inboard engines cooled with raw water. What is causing this and is it a problem? Thanks G
The answer is: it depends.
How much steam are you talking about and under what conditions?
Raw or closed cooling systems both dump the cooling water into the exhaust so that it flows overboard. The cooling water was warm to start with (having just removed that heat from the engine) and when it gets mixed with the super hot exhaust the water heats up even more. Some of the water could flash to steam.
When this hot water flows out the exhaust it will sit on the surface of the lake/ocean (warm water is lighter than cold water) where the air will blow across it.
I am sure you have seen steam/fog rising off the water on a cold morning. This is because the water is warm enough and the air cold enough to create the fog. When you dump really warm water into the lake it will tend to do the same thing.
On a really cold day it would not be unusual to create a mini fog bank at the back of your boat from this.
If the amount of "steam" that you are seeing is an unusual amount that would not be explained by the air temperature then there is a good possibility that you are not getting enough water flow from your raw water pump. The engines need to have a certain amount of heat removed in order for them to operate properly. This heat can be removed by heating up a large amount of cooling water a little bit or by heating a little bit of water up a large amount.
Keep in mind that the cooling water has to first cool the engine, which will want to be operating at a fixed temperature, usually 160 to 180 degrees. If the water flow is low then the cooling water could warm up to nearly the temperature of the engine core. Next this "warm" water is used to cool the exhaust manifolds (this is different than an automobile). The exhaust manifolds can warm the water up a lot more. Again, if the water flow is low it can raise it to almost boiling. Now the water is dumped into the exhaust where it mixes with really hot exhaust gases. If the water was near boiling to start with it could flash to steam.
If you are not sure if the steam is because the air is cold or because the water is really hot try sticking your hand into the flow of water out of the exhaust. If it is scalding hot you should worry about it. If the water is more like a hot tub then it is fine.
If in doubt, change the impeller on your water pump.