Saltwater cooling and clogging

To the editor: The recent article on engines (Elements of engine cool Issue 148, September 2005) had thermostats pictured at the bottom right of page 58. A thermostat is responsible for maintaining engine temperature above a minimum level (usually 140° to 180° F).

Having had a few boats, I’ve learned about the importance of having the proper thermostat installed. A number of years ago, I owned an Ericson 29 sailboat equipped with the well-known Universal Atomic 4 gasoline engine. It came raw-water cooled and equipped with a 140° thermostat. I could not figure out why so cold a thermostat was used, and instead installed a more conventional 180° unit. We were sailing on the Great Lakes at the time, and the engine was happy and more efficient at the higher temperature. A year or so later, we moved to New England and sailed out of Narragansett Bay. A couple of years after that, we sold the Ericson, and the issue of the cold thermostat faded from my mind.

We subsequently bought a Bristol 35.5 that was equipped with a Universal 24-hp diesel and freshwater cooling. It ran at 180° to 190° and that makes diesels happy. However, a close friend had a Tartan 33 with the same engine and cooling system, and his engine ran at 140°. Inspection revealed that sure enough his engine was equipped with a 140° thermostat.

I recalled the Ericson thermostat, and that made us more than a little curious. After a bit of research, we found that at the higher temperatures, the salt in seawater tends to precipitate and clog up whatever it happens to be flowing through. In the case of a raw-seawater-cooled engine, that turns out to be the cooling passages in the block and head. No problem in fresh water.

This started us wondering about what might be happening inside our heat exchangers. During spring commissioning the next year, I removed mine for inspection and found the hot end nipple was about half plugged with salt. I reamed it out, made sure there was none inside and reinstalled it.

Since the boat came to me with the 180° thermostat installed, there was no way to determine how long the salt took to accumulate, but I had put three seasons on the engine, averaging 45 gallons of diesel per season. My friend’s heat exchanger developed a leak during its third season and was replaced. I do not recall if any salt was found when the replacement was made.

Perhaps some more knowledgeable reader can expand on the issue of salt precipitation and at what temperature it occurs.

Thanks for a great magazine.

William R. and Joy Lee Eppes are retired and, after seven years’ living on Great Diamond Island in Casco Bay, have returned to the mainland in Portland, Maine.

Contributing Editor Steve C. D’Antonio responds: You are indeed correct concerning the “cooler” thermostats specified for raw-water-cooled engines. Salt will begin to precipitate out of seawater at approximately 148° F, forming deposits on engine cooling passages, which ultimately will lead to poor water circulation and overheating. Thus, nearly all raw-water-cooled gasoline and diesel marine engines have a 140° thermostat. As far as heat exchangers are concerned, I have never had occasion to find salt accumulation during disassembly. The reason for this is the raw water that leaves a heat exchanger is only heated by approximately 10° to 15° above the temperature of the water entering the heat exchanger (if it’s too hot to hold your hand in the exhaust outlet, your engine is either overheating or on the verge of it). Therefore, the raw water should never be heated anywhere near its salt precipitation point as it passes through a heat exchanger.

Regarding the thermostats used on some Universal diesels, nearly all of the units I service specify an operating temperature range of 165° to 195° (this according to the Universal Marine Power service manual), with most running closer to the lower end of this scale. Most call for a 160° thermostat. Perhaps your friend’s diesel had been inadvertently fit with a thermostat designed for a raw-water-cooled engine.

By Ocean Navigator