fresh water vs raw water cooling

Chris-380

Chris-380

Well-Known Member
Jan 20, 2019
2,435
Pontoon Beach IL
Boat Info
2000 380 Sundancer
Engines
7.4L Horizon w/V drives
Hello all, should be an easy answer for a lot of people on this forum

I have a 2000 380 DA with the 7.4 Horizons. Fresh water cooled, always on a lake in middle USA

What does the freshwater cool, and what does the raw water cool?

Curious what, if anything, i need to plan for items like oil coolers for a boat that is in lake water...

Thanks in advance

Chris
 
Freshwater cooling draws raw water through a heat exchanger, then out through the exhaust. Antifreeze circulates through the engine and the heat exchanger. This keeps the raw water out of the engine (and trans, and oil, etc). Generally only your manifolds see any raw water in this configuration. In fresh water I really don't think manifolds ever need to be replaced.

Raw water has no heat exchanger. Raw/lake water circulates directly through the engine, out the exhaust.

I had a trans heat exchanger fail on a prior boat (fresh water). With boats this age it's probably a good idea to replace the heat exchangers before they fail. I bought a set from MrCool. Highly recommend...pretty easy DIY.
 
Stee6043 said:
Freshwater cooling draws raw water through a heat exchanger, then out through the exhaust. Antifreeze circulates through the engine and the heat exchanger. This keeps the raw water out of the engine (and trans, and oil, etc). Generally only your manifolds see any raw water in this configuration. In fresh water I really don't think manifolds ever need to be replaced.

Raw water has no heat exchanger. Raw/lake water circulates directly through the engine, out the exhaust.

I had a trans heat exchanger fail on a prior boat (fresh water). With boats this age it's probably a good idea to replace the heat exchangers before they fail. I bought a set from MrCool. Highly recommend...pretty easy DIY.
Click to expand...
I have a 08 freshwater and the raw water flows through the trans, oil and power steering cooler.
 
This a great one to share. Many folks think that closed cooling systems have no raw water going in on the side. I wish for mine!
 
Last edited:
My boat is freshwater all its life and is raw water cooled. Still on the original manifolds and risers, which is common on the great lakes. I probably will crack the risers off maybe this summer to take a peak.
 
Spark600 said:
I have a 08 freshwater and the raw water flows through the trans, oil and power steering cooler.
Click to expand...

Yeah, my post may not have been the most clear. But with fresh water cooled each of those systems should have a heat exchanger. No direct raw water flowing through the engine, trans, etc. Raw water flows through heat exchangers ie. coolers.
 
Stee6043 said:
Yeah, my post may not have been the most clear. But with fresh water cooled each of those systems should have a heat exchanger. No direct raw water flowing through the engine, trans, etc. Raw water flows through heat exchangers ie. coolers.
Click to expand...

this was the info I was looking for, I am curious what the life is with coolers and heat exchangers in lakes (I am trying not confuse it with “freshwater” for non salt)
 
Have closed cooling/fresh water cooled, the heat exchangers for the manifolds and risers are original, and the boat has been in salt all its life (2007). There are also oil coolers for the transmission, again, original equipment. The fuel pumps have raw water going through them and both have been replaced at least once. so, I know that doesn’t answer your question, but maybe helps someone else or at least let’s you know they last awhile. Manifolds, risers, elbows done once, risers will be done at the end of this year. Mechanic scoped them, they are fine now, but should be replaced in the next year to be safe.
 
Chris-380 said:
this was the info I was looking for, I am curious what the life is with coolers and heat exchangers in lakes (I am trying not confuse it with “freshwater” for non salt)
Click to expand...

Well I can tell you that my freshwater 340 had a trans cooler fail at 16 years old. Hence my comment above. Cheap replacement and I’d imagine 15-20 years is pretty well max life expectancy.
 
Here's the closed cooling system diagram.
Solid arrows = Antifreeze flow
Outline arrows = sea or lake water


CC1.jpg
cc2.jpg
 
JimT said:
Don't we mean closed water cooling not freshwater?
Click to expand...
Probably, but for some stupid reason, the term most salties use for closed cooling is "freshwater" cooling. I know you know this, but for the benefit of newbies:

Closed cooling / freshwater cooling (and there is full and half closed cooling. Both use antifreeze circulating in the block, but full also cools the manifolds with coolant. Half, just the block)

Raw water cooling / sea water cooling (water from the lake/ocean circulates through the whole system, including the block)
 
kap2 said:
Here's the closed cooling system diagram.
Solid arrows = Antifreeze flow
Outline arrows = sea or lake water
View attachment 85048
Click to expand...
This is a half closed system. The manifolds are cooled with raw water in this example picture. A full closed cooling would have coolant going into the manifolds and then back to the heat exchanger.
 
Stee6043 said:
Well I can tell you that my freshwater 340 had a trans cooler fail at 16 years old. Hence my comment above. Cheap replacement and I’d imagine 15-20 years is pretty well max life expectancy.
Click to expand...
That's good advice. I think I may put that on the list of things to do over the next year. Looks like OEM part is about $400 per side.

My buddy had a v-drive transmission fail last summer on his early 90's 310DA (mechanical, not cooler related) and it was CAD$8,000 to replace, including an engine R&R (marina did not want to play around with an in boat replacement. They said it is faster for them to just pull the engine.).
 
Creekwood said:
That's good advice. I think I may put that on the list of things to do over the next year. Looks like OEM part is about $400 per side.

My buddy had a v-drive transmission fail last summer on his early 90's 310DA (mechanical, not cooler related) and it was CAD$8,000 to replace, including an engine R&R (marina did not want to play around with an in boat replacement. They said it is faster for them to just pull the engine.).
Click to expand...

Mr Cool sells drop-in replacements. Made right here in Michigan. I highly recommend...and less than half the cost of OEM when I did it...
 
On mine (350 07), the salt water goes into the elbow, then out the exhaust. There are block off gaskets between the riser and the elbow, replacing the elbows this winter...
 
For the 2000, 7.4L Mercury/Mercruiser only offered the Half Closed System as show in the diagram. Their engineers claim a Full Closed System would not be dissipate the necessary heat from the exhaust manifolds. With that said, Monitor Product does offer a Full Closed System with an updated heat exchanger that will fit between the exhaust tubes on a v-drive set up.
 

Attachments

  • ClosedCooling.pdf
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Great info, I am not in salt water, but guessing a fully closed cooling system would eliminate winterizing the motors.... which I would love to be able to boat earlier and later in the season without fear of freezing

kap2 said:
For the 2000, 7.4L Mercury/Mercruiser only offered the Half Closed System as show in the diagram. Their engineers claim a Full Closed System would not be dissipate the necessary heat from the exhaust manifolds. With that said, Monitor Product does offer a Full Closed System with an updated heat exchanger that will fit between the exhaust tubes on a v-drive set up.
Click to expand...
 
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