Raw water cooling systems

Having a 30 year old raw water cooled diesel, I'e got a few choices.

1. Do Nothing. Just keep an eye on the temperature and hope the heavy old block and water jackets last a few more years.

2. Change over to a fresh water cooled system with a Heat exchanger and hope it lasts a few more years.

3 Install a keel cooled system.

4. Leave it raw water cooled but circulate fresh water through the engine when back at the dock before shutting down. (and hope it lats a few more years)

I'm leaning towards the 4th option, as it is minimal cost and may reduce the ongoing corrosion rate by.... 1% / 90% -... I don't know.

It will leave fresh water in the water jackets for 95% of the overall time;
but - is the corrosion mainly happening during run-time, or is it equally rotting out while my engine is sitting filled with sea water at the dock ?
Any thoughts on this?

To your last question, hot salt water is more corrosive than cold for sure.

I think a lot depends on your engine and its prospects for continued mechanical life, parts availability and so on. The freshwater cooling (H/E or Keel) would give an added benefit of being able to run a proper 180 thermostat and get better operation and less soot.

My Perkins 6 (another boat) although freshwater cooled, had a saltwater cooled exhaust manifold and turbo adapter. One of these parts became unavailable and not wanting this engine to go unrepairable for want of this one part; I converted it to all FWC. Now I don't have to worry about when those parts are going to fail and where the water is going when they do. As a bonus I get faster warmups and better temperatures at idle/trolling speeds with less cold smoke. On this boat I also have my water jacketed exhaust elbow set up to drain out each time the boat is stopped. I don't know if this will extend its life but at least it won't drain into the turbo & engine when it fails.

Untreated fresh water will corrode the cooling passages as thoroughly as hot salt water.

You don't say what the engine is but if it is a little one on sailboat and hasn't rotted away already, I would look at installing a home made keel cooler made of a few feet of copper pipe down near the garboard. Fill the circuit with treated coolant, never pure fresh water.

The (real) guru's on BoatDiesel recommend your Option 4: a fresh-water flush with fresh water left in the system back at the dock. Of course there is the annual preventative steps that should be taken too, like maintaining engine zincs and cleaning out heat exchangers. It doesn't sound like you are seeing higher-than-recommended engine temps at this stage but you could also pre-emptively use one of the 'BarnacleBuster/EcoTrac' type products and circulate those for 3-6 hours to clean pretty thoroughly to give you a good starting point.

I would add, that we here also have some "real gurus".

Thanks for your thoughts guys.

I'm still no convinced in the value of part time fresh water flush.

The engine is a Volvo MD17D, so yes - parts are becoming scarce. Still only 2000 hours on it, and running well with no temperature issues. Being raw water cooled I run a low temp thermostat. Running at full operating temperature would be nice. I do have an issue with exhaust fumes at times, and running hotter would probably clean the exhaust up a bit.

RickB - Keel cooling sounds so simple - I wonder why more boats don't use it. What are the disadvantages? other than a bit of drag, and potential damage to tubes. The bilge keel stabilizers on my hull would protect any cooling tubes nicely.

Being raw water cooled I run a low temp thermostat.

All raw water cooled engines usually have a 140F or lower operating temperature, to keep the engine from plugging with salt.

These are excellent engines and the cost of a used heat exchanger and a simple sea warer pump is no big deal.

Both Volvo and Johnson sell dual pumps that simply mount where your old sea water pump is located.

If you thermostat for 180F circ water , place a second gasket in the Hot Fresh water circ pump.

And remember if you have a circ FW heater , the water will be MUCH hotter!!

Hotter circ water will make the engine more "efficient" but probably at 3/4 GPH will you notice??

AusCan said:

RickB - Keel cooling sounds so simple - I wonder why more boats don't use it. What are the disadvantages? other than a bit of drag, and potential damage to tubes.

Click to expand...

Some folks freak out at the idea of a couple more through-hulls. Most have just never seen or used a keel cooler system since builders of production recreational boats prefer to just drop in a complete power package. If they offered keel cooling they would have to invest in more man hours to install more parts themselves. They prefer to let you invest in heat exchangers and pumps.

You can buy a new or used factory made keel cooler or you can make your own very easily if you enjoy doing that sort of thing. Look around at commercial fishing boats, many of them use keel coolers for dependability and economy.

FF said:

All raw water cooled engines usually have a 140F or lower operating temperature, to keep the engine from plugging with salt.

Click to expand...

That should add to the angst of those who lie awake nights worrying about the catastrophic effects of low loading leading to their engines operating too cool.

AusCan said:

Having a 30 year old raw water cooled diesel, I'e got a few choices.

1. Do Nothing. Just keep an eye on the temperature and hope the heavy old block and water jackets last a few more years.

2. Change over to a fresh water cooled system with a Heat exchanger and hope it lasts a few more years.

3 Install a keel cooled system.

4. Leave it raw water cooled but circulate fresh water through the engine when back at the dock before shutting down. (and hope it lats a few more years)

I'm leaning towards the 4th option, as it is minimal cost and may reduce the ongoing corrosion rate by.... 1% / 90% -... I don't know.

It will leave fresh water in the water jackets for 95% of the overall time;
but - is the corrosion mainly happening during run-time, or is it equally rotting out while my engine is sitting filled with sea water at the dock ?
Any thoughts on this?

Click to expand...

A couple of questions.

Does the engine have zincs and are they holding up?

If not you can buy a easy to connect Grouper zinc that has a 12 ft cable you hang over the side.

Does the raw water only go through the manifold or the engine block?

If the raw water only goes through the manifold, its not that hard expense to take the manifold off and have checked tested. Local radiator shops probable can do that. On the salt I would do that every 5 years.

The Eagle gen set the raw water runs though the manifold, that I have a grouper zinc connected to, the manifold has a drain plug, and is above the water line so it also self drains. In the salt every 5 years I take off the manifold and have checked

Usually what goes first is the mixing elbow where the raw water and the exhaust mix as it rusts/corrodes through which allows raw water to back flow into the engine. So at least check the mixing elbow. I had a custom SS elbow make at auto exhaust place.

I would do some checking and preventative maintense first!

"You can buy a new or used factory made keel cooler or you can make your own very easily if you enjoy doing that sort of thing. Look around at commercial fishing boats, many of them use keel coolers for dependability and economy."

Our 50 Ft USN Utility has a simple system consisting of 2 - 21 ft long 1 1/2 galvanized pipes as coolers ,with a 2 elbows and a short nipple to make the 180 deg turn.

Even running hard in FL the system over cools the 6-71 DD set to 180HP.

The temp gauge reads 180, engine thermostat opens to let the water out , the temp drops to 160F , and the reading climbs in a few minuets to 180F .

Will eventually install a bypass thermostat in the keel cooling circuit ,( pilots love "perfect") but the DD took it in Maine with 34F deg water with no problems.

Rick is right , the biggest downside to a DSKK (dry stack keel cooled) setup is the extra work for the boat assembler.

Remember if the sell the boat with fuel boxes , instead of marine fuel tanks (that can easily be MAINTAINED) to save a buck, what is not demanded by the first purchaser will never exist on a Coolie/cookie production hull..

Or on an entry level USA build either.

While every boat, every engine, and every situation is different, as a single data point we have a friend who has a 28' Richardson that their father bought new in the very early 1950s. It has two Gray Marine gas engines that are raw-water cooled. For its whole life the boat has been kept in salt water during the summer and fresh water during the winter.

These fifty-plus-year-old engines are still in fine shape and run well. The brothers who inherited the boat from their father are concerned about what condition the cooling passages might be in and have been talking for years about trying to find the same kind of engine but without the hours and the salt water usage to install in place of the original engines. But they have not pursued the search very vigorously. Meanwhile they continue to use the boat.

When it is moored in salt water they do not flush the engines with fresh water after every use. They just shut them down and go home.

I know pretty much nothing about Gray Marine engines, but they were very popular in boats in the first two-thirds of the 20th century and many of them used raw water cooling. So it may be that these engines were designed with this in mind with larger cooling passages, thicker walls, etc. I don't know. So comparing something like a Gray Marine gas boat engine to the typical diesel's used in cruising boats like ours may not be a valid comparison.

I know two people who had keel coolers in their boats. One is a friend in the San Juans who for several years operated a barge service with a surplus LCM landing craft. The LCM is powered with two 6-71s. When he and his wife bought the LCM they overhauled the keel coolers but the system was trouble-free during the several years they operated the barge on an almost daily basis.

The other person is Carey who used to participate in this forum. His 36' custom lobsterboat was originally powered by a Sabre engine that was keel cooled. The Sabre seized and destroyed itself not long after they bought the boat. They had it replaced with a Cat. The Cat uses a "conventional" cooling setup with a heat exchanger and the cooling water goes out with the exhaust.

They've had no real trouble with this although one time when we were out together they sucked up a huge amount of eelgrass that completely clogged the long raw water hose run between the through-hull and the sea strainer. After the engine had cooled down they idled back to the dock where Carey spent the next hour fishing the eel grass out of the hose.

But he wishes (he had expressed this long before this incident) that when he'd had the Cat installed that he'd gone with the keel-cooler and a dry stack exhaust.

The only downside to a keel-cooler system are the holes required in the bottom of the boat. But our boat, for example, has eleven holes in the bottom as it is. So four more more to accommodate keel cooling, two of which could be the existing raw water intake through hulls, would be no big deal.

The only downside to keel cooling I can imagine is the potential for damage. But in both the LCM and Carey's lobsterboat, the keel coolers sat in recesses in the hulls. So the chances for damage were very minimal.