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Old 11-03-2016, 15:49   #1
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Salty freshwater tanks mystery

Hey guys and gals,

I have a serious mystery on my hands that is making me feel like a real dummy. I'm getting very salty water in the starboard freshwater tank of my Lagoon catamaran. My watermaker system has been working like a champ for the year that I've had this boat. My watermaker system has a manual selector valve - either port tank, starboard or right into a hose that I use to fill water bottles. The watermaker is producing wonderful product, and I can fill and empty my port tank no problem. The mystery is that my starboard tank water is extremely salty to the point that a shower is like jumping in the briny blue! I've of course emptied the tank and refilled it no fewer than 10 times and it maintains the same salinity!!!

I even had a watermaker specialist take a look in case somehow my brine was getting into the freshwater tank. It is possible that I filled with bad water but it seems way too salty to be from bad dock water.

I thought I would throw a post up here to see if anyone has any suggestions for troubleshooting or has had a similar issue? It is so strange because this happened somewhat suddenly.

Any ideas?? I'm hoping it is a simple "wow I'm an idiot" issue!

Thanks all!
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Old 11-03-2016, 16:24   #2
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Re: Salty freshwater tanks mystery

Have you followed the watermakers brine discharge line to make sure it is on it's own overboard thru hull? I've seen some pretty sketchy plumbing by DIYers for this line. If it is on it's own thru hull, as it should be then the watermaker is not your problem. Check your tanks air vent line as well. Do you have a salt water deck wash down pump?


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Old 11-03-2016, 16:31   #3
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Re: Salty freshwater tanks mystery

Yes I have a deck wash. I would be surprised if it was the water maker since we have not made any changes to the piping of the system since the time it was working properly. Also we can refill the port tank fine from the water maker and the tank has no issues.
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Old 11-03-2016, 16:36   #4
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Re: Salty freshwater tanks mystery

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Yes I have a deck wash. I would be surprised if it was the water maker since we have not made any changes to the piping of the system since the time it was working properly. Also we can refill the port tank fine from the water maker and the tank has no issues.

I'm just shooting in the dark here. But each tank has it's own air vent. I have seen people "T" into the air vent with the brine discharge hose because they wouldn't install a new thru hull. Again, I would follow the brine discharge hose from the watermaker from the beginning all the way to the over board discharge location.


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Old 11-03-2016, 16:52   #5
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Re: Salty freshwater tanks mystery

Which Lagoon? Are your tanks in the keels?
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Old 11-03-2016, 16:58   #6
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Re: Salty freshwater tanks mystery

Hey there,

you have a hole in one of your mini keels. Very easy to verify - drain the tank in question and sail for a few days - if it comes from the outside it will refill itself slowly (or quickly, depending on the size of the hole...)

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Old 11-03-2016, 17:25   #7
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Re: Salty freshwater tanks mystery

You need to prove if you are turning fresh water salty via a contamination in the tank or are you replacing the fresh water with salt water by ingress, or are putting salty water into the tank by mistake.

If you fill the tank with good fresh water does it instantly start flowing salty?

If this answer is yes does it happen if you fill with water not from water maker eg: Dockside tap?

Leaving the tank empty for a few days should prove if you have sea water getting in if so, then you need to find the source.
From memory the Lagoon 500 has plastic tanks so unlikely to be hole. Moor likely to be coming in via a vent line or via a faulty fresh/salt water changeover valve. Think wash down pump or galley sink. Or as suggested plumbing connection mix up.
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Old 11-03-2016, 21:06   #8
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Re: Salty freshwater tanks mystery

Wow thank you so much for these great suggestions!

The scariest one is hole in the mini keel. As pointed out the freshwater tanks are big blue plastic tanks, not built in to the hull so I don't think that is it.

I like the suggestion about the freshwater wash down pump valve somehow playing a role in the contamination. I will check that. I will also double check that there is no contamination from the brine line out of the watermaker to the fresh water tank, but since we have not changed the plumbing hoses related to the watermaker and it was working properly before I am doubtful of that.

It is a 2011 lagoon 500. Any other ideas?

Thanks again you guys are great!
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Old 12-03-2016, 03:57   #9
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Re: Salty freshwater tanks mystery

I have a changeover tap that allows me to use a foot pump to feed salt water into a couple of sinks. This is to save fresh water on long journeys. Maybe you've got something like that that Po installed
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Old 12-03-2016, 22:41   #10
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Re: Salty freshwater tanks mystery

where is the tank air vent located. I've experienced cases of water being driven in thru the vent out in the ocean, beating to windward.
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Old 30-09-2016, 13:27   #11
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Re: Salty freshwater tanks mystery

Hey guys,

Well, we found out what the problem was. I figured I would share it on the board in case someone else comes up with a similar issue! It turns out that the "Y" valve we have to select whether head water flush is fresh or salty was corroded and due to back pressure in the system, salt water was getting sucked into the head flush system, and going into the freshwater tank.

All we did was replace that Y valve and now we are good to go again! Mystery solved!
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Old 30-09-2016, 14:01   #12
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Re: Salty freshwater tanks mystery

Hmm, are you sure you really want the heads connected in anyway to a drinking water tank?

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Old 30-09-2016, 14:08   #13
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Re: Salty freshwater tanks mystery

LOL that's a great point! This set up is the manufacturer's, not mine, but another good reason we bottle water directly from our watermaker and don't drink out of the holding tanks! We still shower in it though... better not let the Admiral know about this potential for contamination
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Old 30-09-2016, 15:42   #14
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Re: Salty freshwater tanks mystery

1 x Non return valve between tank and Y vale would solve the concerns of saltwater by-pass.
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Old 30-09-2016, 17:03   #15
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Re: Salty freshwater tanks mystery

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Originally Posted by cbrooks07 View Post
Hey guys,

Well, we found out what the problem was. I figured I would share it on the board in case someone else comes up with a similar issue! It turns out that the "Y" valve we have to select whether head water flush is fresh or salty was corroded and due to back pressure in the system, salt water was getting sucked into the head flush system, and going into the freshwater tank.

All we did was replace that Y valve and now we are good to go again! Mystery solved!
You need to change that horrible system. A Y-valve that allows seawater to back into your fresh water tanks? Ugh!
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