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Old 13-10-2012, 20:34   #1
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Cooling Explantion

I asked this question in a different way in the general forum, but am curious if someone could explain the difference between a fresh wayer and raw water cooled engine. In a Yanmar 3-cylinder. Are parts of the raw water cooled engine "sacrificial." How is the corrosive raw sea water kept from corroding the parts of the engine it touches? And is it just touching "cooling" hoses and pipers or getting into the engine proper. Forgive my ignorance, but sailing is learning enough let alone engines too for all love!
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Old 13-10-2012, 20:48   #2
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Re: cooling explantion

Around the cylinders and head of water cooled engines are water passages.

In a raw water cooled engine - water is drawn in from the sea, circulated through the passages then expelled overboard - usually through the exhaust system where is helps cool the exhaust and muffle the engine sounds.

In a "fresh water" or "closed" cooling system the water passages are plumbed to a cooler and back to the engine and the coolant (coolant can be a mix of coolant and fresh water) is circulated by a fresh water pump just like your car.

In your car the "cooler" is a radiator and the air flowing past the radiator takes the heat out of the water.

In your boat the cooler or "heat exchanger" is "pack" of cooling pipes in a housing. Raw water is drawn in from the sea and circulated around the pack of cooling pipes, then expelled overboard, usually through the exhaust system as above.

Corrosive elements exist in both fresh water and raw water systems. Salt and scale are the primary drivers in raw systems. Rust is the driver in fresh systems. Keeping the right mix of fresh water and corrosion inhibiting coolant ensures a very long life for the fresh water system on the boat.

Raw water systems can be considered more reliable - fewer parts. But closed systems have the advantage that impeller failures or other debris can't enter the engine block as they are separate systems and a properly maintained closed system should never (never say never) have internal engine corrosion issues.
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Old 13-10-2012, 22:03   #3
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Re: Cooling Explantion

Raw water cooled systems work as they run at a lower temperature, about 130 degrees F. This to keep the mineral buildup at bay. The closed loop / fresh water system runs at about 180 degrees. F, as mineral buildup is much less a problem and then diesels work better at the higher temps.

The raw water side of a closed loop system still runs below 130 degrees F which greatly reduces mineral buildup. Most of the mineral buildup happens over time, mainly after the engine shuts down and the cooler raw water heats up and minerals start dropping out of solution.

No real sacrificial parts, other then zincs (in raw water and some freash water engines).
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