FWIW - 140 degree thermostats and raw water cooling

[ericinga]

I have always wondered about the reason for 140 degree thermostats in raw water cooled engines. Fresh water cooled engines typically use a 165 degree thermostat. This is closer to the automotive temp of 180 degrees.

Well, the answer was in my ABYC study materials. Raw water cooled engines are developed to operate in salt and fresh water. Starting at 160 degrees, salt begins to fall out of solution and crystallize. You can imagine the problems created by salt crystal build up in an engine block. So, fresh water motors can run a high temperature thermostat if needed. Another consideration is exhaust system temperature. Systems components exceeding 200 degrees F require shielding to protect electrical components and crew members.

Thought this was an interesting piece of information.

[Tuggle]

Good info, thermostats are on my list of things to change, my port engine runs significantly hotter than my starboard. Being in fresh water, Lake Lanier, what would the optimum temp to run?

[ericinga]

I run 140 degree thermostats. Given the age of your motors, I would pull the exhaust manifolds and clean them out. You can take them to a shop for cleaning or DIY. I used a power washer with the 0 tip. It blew all kinds of crap out of the passages. The output side of the riser was almost completely clogged.

One sign of clogged riser or manifold passages is water or rust seepage lines on the manifold below the riser.

As an side, you might want to pull all the cooling hoses and look for impeller pieces. I found an old impeller blade resting on top of the thermostat during the port engine tear down. The current impeller is fine. There were no other pieces in the system. These pieces can reduce or block water flow.

[EWRice]

There is a second reason for a lower temp in a raw water cooled engine. This is, in my opinion, even more important than salt and sediment.

A closed cooling system builds pressure as it heats up raising the boiling point of the coolant. With a raw water system, there is no pressure, keeping the boiling point at 212. Just because the average coolant temperature at the t-stat (and sending unit) is running around 140 or 160, doesn't mean that it is that low against the top of the combustion chamber or around the cylinder walls. Running a lower temp on a raw water engine keeps the gap between desired coolant temp and boiling point at a safer distance. If water boils against the hot iron of the head or cylinder walls, it will begin to errode, and you will get sediment precipitation onto the cooling surfaces. This is more noticeable in engines with wet liners (ie diesels) but it happens in gas engines too. I pulled apart a 350 chevy marine engine that was run hard with a 195 t-stat in it. The heads looked worse than any salt water engine, and the block had pin holes through the cylinder walls. This was a fresh water Michigan boat.

The engineers know what they are doing, sometimes.

[javalin390]

As FastJeff once said, steam makes a terrible coolant !

[ericinga]

I've always run the 140 degree thermostats. The engineers had a reason to put them in the motor.