Raw Water Vs. Fresh Water

[Scott MacCready]

Would appreciate if someone would explain Raw Vs. Fresh Water cooling systems as it relates to my MD7A. thanks, Scott

[Carter Brey]

Would appreciate if someone would explain Raw Vs. Fresh Water cooling systems as it relates to my MD7A. thanks, Scott

Scott,

As I recall, the MD7A is a raw water cooled engine. That means that raw sea water is pumped from a through-hull, through a strainer and into the engine's cooling circuit. It leaves the engine once it has absorbed heat. Raw sea water is also used to cool the exhaust manifold.

In a fresh water cooled engine, there is a closed circuit of fresh water, or a combination of fresh water and antifeeze, which is circulated through the engine and then into a heat exchanger, which is basically a closed radiator. Raw sea water is also pumped from a through-hull, but this raw water is not used directly to cool the engine. It is pumped via a raw water pump into the heat exchanger, where it circulates through little pipes and removes the heat from the fresh water.

Once the fresh water has been cooled, it returns to the engine circuit. The raw water is pumped out through the exhaust. In other words, fresh water is used to cool the engine, and sea water is used to cool the fresh water as well as the exhaust.

The idea with the fresh water cooled engine is to reduce corrosion inside the engine.

Of course, that means you have to keep the heat exchanger cleaned out, but that's an easier task than dismantling an engine block. You simply unscrew the end caps of the heat exchanger to gain access to it's innards.

Your raw water cooled engine has the advantage of being simpler, but carries with it a greater risk of internal corrosion.

Hope that helps.

Carter

[Scott MacCready]

thanks Carter. The term "Fresh Water" was confusing me. I never considered the water in my car's radiator as being very fresh. thx

[Carter Brey]

Scott,

Yeah, it may be "fresh" compared to seawater, but I wouldn't exactly use it to make my morning espresso.

Come to think of it, I'm starting to suspect that the corner deli might have access to my coolant recovery tank.

Regards,
Carter

[Steve Laume]

I always thought sea water a more descriptive term than raw water. A "captive" water cooling system may be a more accurate description of a fresh water system. We have a captive water cooling system with a sea water heat exchanger. We always try to bring spring water from home so as not to use water from the "fresh" water tanks to make coffee, Steve.

[Warren Kaplan]

Your raw water cooled engine has the advantage of being simpler, but carries with it a greater risk of internal corrosion.

Hope that helps.

Carter

What Carter said is the true crux of the matter. But if internal corrosion is gonna' make you lose sleep at night, there is a way to keep it to a minimum rather easily. All you have to do is periodically flush your raw water cooled engine with fresh water. Its a task easily accomplished if you have decent access the the raw water intake hose and seacock.

My 26 year old raw water cooled YSM8 on my 1980 CD27 has such a set up. This is what you do. Get a 5 gallon bucket and take a jerry can (if on a mooring like me) and bring 5 gallons of fresh water out to your boat. 10 gallons if you are a belt and suspenders man.
Pour the water from the can into the bucket. Then close your raw water seacock and either;

1. remove the hose from the seacock if it will be long enough to reach the bucket full of fresh water.
or
2. Get a long piece of hose and disconnect the raw water intake hose from the engine side (after closing the seacock). Then fit the long hose temporarily to the engine side, tighten the hose clamp and put the other end in the bucket of water.

Then its a matter of starting the engine and having the water course through the innerds of the engine's cooling system, dissolving and rinsing out all the salt deposits and you're done. Takes 5 minutes. Then reconnect the hoses.

Oh yeah Kaplan...but whadda' bout when the thermostat is closed? No water will circulate through there until its hot and the thermostat opens. Cool fresh water out of a bucket hardly qualifies.
You can try it with a warm engine but the cold fresh water may just close the thermostat. Whadda' ya' gonna' do.

Get another piece of hose. The raw water intake setup stays just the same as above. Then disconnect the exhaust water hose where is comes out of the engine on its way to going out of the boat. Attach the second hose to that and lead the free end back into the bucket where the raw water hose is drawing water into the engine. When you start the engine water will be drawn into the engine from the bucket, go round and round inside the engine, then head for the exhaust hose, and now, it will go back into the bucket instead of out of the boat. So it goes round and round, and each time it passes through the engine the water temperature heats up. Eventually the thermostat will open and the water will go round and round through those channels and rinse that all out.

The beauty of this is with this virtually closed circuit you are not limited to 5 or 10 gallons of water. Its an endless supply and you can rinse for an hour if you feel the need to.

By the way, when winterizing your engine, use the closed circuit setup but replace the fresh water (after you've rinsed the engine out for winter storage) with marine antifreeze. The antifreeze will heat up and open the thermostat (if it didn't stay open from the water rinse) and you've effectively put antifreeze into you entire engine. (The only thing in winterization that is different is to connect the real exhaust hose at the very end and then circulate a little antifreeze thru it.)

Sorry for being long winded but these techniques are a snap and they don't take long. I do it maybe once a season but others can do it every few weeks if you are worried about corrosion.

[Stan W.]

Would appreciate if someone would explain Raw Vs. Fresh Water cooling systems as it relates to my MD7A. thanks, Scott

Scott:

Cape Dory offered "fresh water cooling" as an option on MD 7A equipped CD 28s and CD 30s. The heat exchanger was made by a company named Sendure. Sendure is still in business and probably could help if you want to retrofit but it may not be worth it if your block has had salt water coursing through its veins for 25 years.

The main advantage is that you can keep rebuilding a fresh water cooled MD 7A into eternity, or at least until Volvo runs out of parts.

[Oswego John]

Hi Warren and all,

Agreed, that's one way to flush the salt out of the block. I'd like to add to your thoughts.

If at all possible, try to pony up to a pier that has fresh running water available for the flush. This way, you can run the engine without removing the discharge hose from the block.

The reason I say this is because the majority of engines of this type use a "Wet exhaust". The warmed water from the block blends with the very hot exhaust gasses and tempers the exhaust to a somewhat safe temperature before being discharged overboard.

Your pail system has merit. I'm wondering if a design could be worked out to allow at least a trickle or so of water into the exhaust to temper the very high heat that is produced by the exhaust gasses.

Any other thoughts out there?

O J

[Oswego John]

Warren and all,

Duh, pardon the senior moment. I'm not the schmarty pants that I thought myself to be.

You flush your engine at the pier. You disconnect the garden hose, rehook up the engine water intake and open the seacock. After the dockmaster looks at his watch for the umteenth time, you cast off and motor to your mooring.

Your engine is now again filled to the max with Oyster Bay Harbor variety salt water. This briny mix will rest in the water passages until next weekend, when you will recycle the brine for fresh brine.

It is very clear to me, at this point, that all of you salt water sailors should move your boats to the Great Lakes Fleet area and sleep much more soundly at night.

We shall discuss the commuting bit at a later date.

Best regards,
O J
Hope to see many of you at the GLF party on March 11, '06

[Warren Kaplan]

Hi Warren and all,

Agreed, that's one way to flush the salt out of the block. I'd like to add to your thoughts.

If at all possible, try to pony up to a pier that has fresh running water available for the flush. This way, you can run the engine without removing the discharge hose from the block.

O J

John,
Its far better to do it at a dock with an endless source of fresh water, as you suggested. I was describing a technique to do at a mooring, when endless fresh water is not available.

Given enough time the thermostat will open, as it does in its normal use when a raw water cooled engine is constantly drawing in cold water from the outside, passing it through the engine just once, never to be recycled, and then out with the exhaust.

So, as John suggests, if you can bring a hose into your boat and put it in the bucket, you will not have to do anything with the exhaust water hose, but it may take a little longer for the thermostat to open with cold water always being drawn into the system. The difference in time is not all that much, so I would do it John's way if I had the option.