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Old 07-13-2013, 01:39 AM   #1
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Freshwater flush

I was on my surveyors boat on Thursday and he had rigged a freshwater flush system by tapping the top of his groco sea strainer adding a nipple, a valve and a quick disconnect garden hose adapter. Interesting idea, especially if you aren't going to be on the boat for a few weeks.

I know groco sells a whole new sea strainer with an adapter but has anyone ever rigged one?

Pro/con for flushing?
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Old 07-13-2013, 04:25 AM   #2
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I was on my surveyors boat on Thursday and he had rigged a freshwater flush system by tapping the top of his groco sea strainer adding a nipple, a valve and a quick disconnect garden hose adapter. Interesting idea, especially if you aren't going to be on the boat for a few weeks.

I know groco sells a whole new sea strainer with an adapter but has anyone ever rigged one?

Pro/con for flushing?
Well, we have installed recently a fresh water flush system on our trawler style motor yacht which has a Caterpillar 3208NA diesel.. The flush system tees into the water intake just before the raw water strainer. This has its own seacock. We connect a 3/4 inch hose to the marina water supply and turn this on when we have docked with the engine still running. Then turn off the raw water intake and run the engine on fresh water for about 10 minutes. The system has flapper valves which divert the fresh water out the transom if the valve is closed or the engine is not running. All the raw water channels in the engine are flushed through with fresh water.

We expect much reduced maintenance on the heat exchanger, on the raw water impellor and most of all on the recently replaced ( very expensive) exhaust risers.
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Old 07-13-2013, 05:48 AM   #3
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Most boats already have marine parts that can accept living in sea water .

On an aluminum IO or outboard it might be worth the effort , or on a boat where sea water is in the block it could be a help for engine longevity .

Direct sea water cooling is not common today even on IO boats.
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Old 07-13-2013, 07:17 AM   #4
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Most boats already have marine parts that can accept living in sea water .

On an aluminum IO or outboard it might be worth the effort , or on a boat where sea water is in the block it could be a help for engine longevity .

Direct sea water cooling is not common today even on IO boats.
Agreed...having toyed with idea a long time...not sure how much it really helps.

Now if you ran anti-corrosion stuff through it...maybe a different story.

Through the years I have experimented with our trash pumps we use in salvage. Fresh water washes don't seem to help as much as after I run a bit of antifreeze through them and let it sit in there. Ideally...if you could keep it full of something like antifreeze without air would be the ticket I think....then the rubber impeller would have issues I suspect (probably nitrile ones would do better).
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Old 07-13-2013, 07:41 AM   #5
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When I was dowin in Key West Fourth of July my brother was installing Groco flush fittings on strainers he was servicing that were cast bronze nipples which had a plugged tee on one side. He just replaced the hose barb on one side of the strainer with the new flush fitting. To use them you close the seacock remove, the plug and install a hose adapter. Not inexpensive but probably less than a whole strainer assembly. The fittings were heavy duty cast bronze.


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I was on my surveyors boat on Thursday and he had rigged a freshwater flush system by tapping the top of his groco sea strainer adding a nipple, a valve and a quick disconnect garden hose adapter. Interesting idea, especially if you aren't going to be on the boat for a few weeks.

I know groco sells a whole new sea strainer with an adapter but has anyone ever rigged one?

Pro/con for flushing?
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Old 07-13-2013, 07:52 AM   #6
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I have a engine and genset fresh water flush system I installed when I bought the boat. It's a simple system where I had my mechanic drill a hole into the caps of the stainer and install a ball type brass valve with a hose fitting. I open the valve to flush and close the raw water sea cock. Run the engine for about 10-15 minutes at idle. Make sure the hose doesn't collapse because the engine is sucking too much water. Don't let the water continue to run after you shut the engine down as you might force water back into the engine exhaust manifold.

There is no doubt you will have cleaner raw water components if you are consistent about flushing. The guru's on boatdiesel recommend it and it's partly why fresh water boats bring a premium. Don't you flush your O/B at season's end?
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Old 07-13-2013, 08:19 AM   #7
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I have a engine and genset fresh water flush system I installed when I bought the boat. It's a simple system where I had my mechanic drill a hole into the caps of the stainer and install a ball type brass valve with a hose fitting. I open the valve to flush and close the raw water sea cock. Run the engine for about 10-15 minutes at idle. Make sure the hose doesn't collapse because the engine is sucking too much water. Don't let the water continue to run after you shut the engine down as you might force water back into the engine exhaust manifold.

There is no doubt you will have cleaner raw water components if you are consistent about flushing. The guru's on boatdiesel recommend it and it's partly why fresh water boats bring a premium. Don't you flush your O/B at season's end?
I've always flushed my outboards after EVERY use (except while cruising and the motor stays on the boat in the water)....

But after taking apart multiple outboards through the years and seeing the mechs at my marina work on hundreds more...not sure that flushing does much more than keep passageways unclogged from salt...there is always significant corrosion in them no matter what. Although newer outboards are doing better.

So the saga of life continues....experts recommend....just like doctors and people seem to still die at all different ages no matter what doctor's advice they follow (I think motors have similar genes...)...Flushing doesn't hurt (unless you fill up your cylinders after the muffler so new to flushing...be careful).....but my jury is still out in how much it helps.

I may do the same...especially as long as I sit during the summer months...and like I said about flushing with some sort of anti-corrosive stuff...might try that as well as plugging the injection to the exhaust so I can keep the system full.

As to fresh water boats bringing a premium...it's for a lot more than just a few internal engine parts...
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Old 07-13-2013, 09:28 AM   #8
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I have also considered building a flushing system, but or that past several years I have been hauling the boat several hours run up a freshwater river (Ct River) so the systems get a really good final flushing beforenstorage. I run the genset too. I also take the raw water system apart every 2 or 3 years.
As far as freshwater boats getting a premium....every boater I talk to thinks they are better...so it is a selling point to most be it justified or not
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Old 07-13-2013, 09:47 AM   #9
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Paul does have a point; can anyone conclusively prove fresh water flushing is helpful?
Someone probably can, but in the mean time it just makes sense to me that flushing with fresh water can't hurt and probably helps. After all we fresh water flush all our stainless on the boat and dive gear.
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Old 07-13-2013, 10:37 AM   #10
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Paul does have a point; can anyone conclusively prove fresh water flushing is helpful?
Someone probably can, but in the mean time it just makes sense to me that flushing with fresh water can't hurt and probably helps. After all we fresh water flush all our stainless on the boat and dive gear.
I guess the longer I'm around and the more goofy stuff I see...the harder it gets to say anything is right/wrong/black/white.

2 good examples...I was sick of seeing how much room my steel fuel tanks took up and how hard it would be to do almost anything to really get a good look at their complete condition...they were VERY rusty on the outside and very black on the inside. With the horror stories of pinhole leaks and me needing more storage room plus accepting less fuel carried around...I decided to replace the tanks with smaller plastic ones. After I cut one out and went over it carefully in the parking lot...well lets just say it might have lasted another 25 years...but the decision was made and out they came.

Second example was a broken brake line on my truck this week...perfect condition, all 11 feet of it, except for one pinhole and a rusty area the size of a dime...but it made the line worthless and repairing the line was not even in the cards.

So the same for me goes with some boat maintenance ideas....you can get carried away with ideas that may or may not extend the life of a system. Sometimes systems get changed out because of wear and tear, sometimes because of totally external damage, etc...etc..

I think flushing is generally a minimal cost and effort type of operation. If it extends the life of a component a year or so is probably worth the effort. Although again as FF pointed out, a well made cooling system is generally saltwater resistant...more likely to suffer from galvanic or stray current corrosion. The expense (pieces and parts), can be useful for winterizing, trouble shooting or emergencies...such as hooking up a salt water washdown hose to the saltwater system in such a fashion that it could help cool if the regular saltwater pump starts to or totally fails.
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Old 07-13-2013, 11:34 AM   #11
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My first cruising boat was a 30 ft sailer built for me in 1977. The engine was raw water cooled and never had a flush. Well after I sold the boat I ran across it again, in 2007 and had a chat with the owner. He had recently replaced the old yanmar with a larger one that was fresh water cooled. He reported that the old engine, on inspection, had no issues with the salt water cooling, despite 30 yrs of never being flushed, always kept in the salt. So I don't concern myself with flushing.
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Old 07-13-2013, 11:46 AM   #12
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I thought about modifying my last sailboat engine cooling system to allow for FW flush....Then I was concerned about the fact that my zinc anodes would be setting in fresh water the majority of the time, which is where one should be using aluminum anodes.....Decided to not go against the grain of decades of marine design for a "might help" benefit..... JMHO
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Old 07-13-2013, 02:03 PM   #13
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Picture of fresh water flush system

I installed something similar on my Perko strainer. This is the setup that Tony Athens recommends. There is no doubt that fresh water is less conductive than salt water and at the very least will reduce zinc usage and maybe corrosion.

I drilled and tapped the cover for 1/2" NPT and threaded a street el, a couple of close nipples, a 1/2" ball valve, a 3/4x1/2" bushing and a female hose adapter. Parts cost was about $25.

To use it, you run the engine in idle, connect up a water hose and with the thru hull valve open you open the fresh water flow. The theory is that the fresh water displaces the salt water and only fresh water gets sucked up into the engine.

Well, with our low slip fresh water flow it didn't work that way. After idling for 5 minutes I pulled a zinc and the water still tasted brackish (our water is always brackish). Maybe it would work better with stronger flow. And it would certainly work if I shut off the thruhull, but I am hesitant to do that.

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Old 07-13-2013, 05:33 PM   #14
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I installed something similar on my Perko strainer. This is the setup that Tony Athens recommends. There is no doubt that fresh water is less conductive than salt water and at the very least will reduce zinc usage and maybe corrosion.

I drilled and tapped the cover for 1/2" NPT and threaded a street el, a couple of close nipples, a 1/2" ball valve, a 3/4x1/2" bushing and a female hose adapter. Parts cost was about $25.

To use it, you run the engine in idle, connect up a water hose and with the thru hull valve open you open the fresh water flow. The theory is that the fresh water displaces the salt water and only fresh water gets sucked up into the engine.

Well, with our low slip fresh water flow it didn't work that way. After idling for 5 minutes I pulled a zinc and the water still tasted brackish (our water is always brackish). Maybe it would work better with stronger flow. And it would certainly work if I shut off the thruhull, but I am hesitant to do that.

Here is the picture:
greysailor makes a good point...if you don't use aluminum "zinc" anodes are you really doing a disservice to your cooling system metals if you flush?
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Old 07-13-2013, 06:48 PM   #15
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greysailor makes a good point...if you don't use aluminum "zinc" anodes are you really doing a disservice to your cooling system metals if you flush?
Hardly, zinc and aluminum are so close together on the galvanic table that it makes no difference in the application described.

I put this issue in with the "what should I worry about when I have absolutely nothing else to do or think about on my boat" class of self induced concerns.
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Old 07-13-2013, 10:42 PM   #16
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Hardly, zinc and aluminum are so close together on the galvanic table that it makes no difference in the application described.

I put this issue in with the "what should I worry about when I have absolutely nothing else to do or think about on my boat" class of self induced concerns.
Is that true for external zincs too? (shaft, rudder, tabs, etc)
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Old 07-14-2013, 06:16 AM   #17
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I drilled and tapped the cover for 1/2" NPT and threaded a street el, a couple of close nipples, a 1/2" ball valve, a 3/4x1/2" bushing and a female hose adapter. Parts cost was about $25.

Most of the top covers I have seen are fairly thin. I would either add a nut inside or braze the street el in place.
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Old 07-14-2013, 07:13 AM   #18
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Is that true for external zincs too? (shaft, rudder, tabs, etc)
The OP mentioned not using the boat "for a few weeks." Even if it were for a few months it would make little difference. There are plenty of boats that move from salt to fresh water all the time, they don't change anode material for each transition as it is really really silly.

Look around Lake Union at the fishing fleet and the coastal freighters. Few of them use an impressed current protection system, they use zincs just like the rest of us. They have steel hulls, fiberglass hulls, aluminum hulls, wooden hulls, engines that use zincs, engines that don't use zincs, some use shaft zincs, some don't ... they all share one thing in common - they don't haul the boat and change from zinc to aluminum or magnesium anodes or back after passing the locks. They don't dissolve at the dock either.

Those who are really concerned about mooring in fresh or brackish water for extended periods can use a magnesium version of the fish or grouper style zinc on a wire hanging over the side.
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Old 07-14-2013, 08:16 AM   #19
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Most of the top covers I have seen are fairly thin. I would either add a nut inside or braze the street el in place.
FF made a good observation. In my case the mechanic had to use a backing nut and I treat the whole fixture gently.
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Old 07-14-2013, 08:42 AM   #20
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Most of the top covers I have seen are fairly thin. I would either add a nut inside or braze the street el in place.
I was concerned about the thickness and was expecting to have to braze it. But I found that the cover was about 1/4" thick near the edge and I caught about 4 threads. I made sure that the threads were full depth and at the beginning of the taper so it made up tight. It feels solid.

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