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Old 30-01-2016, 04:24   #1
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Osmosis inside F/W tanks

Just taken possession of new to me boat, and sailing her home. During first bit of wave action, water from the tanks became tainted with some sort of brown deposit, so once in harbor, I decided to open up the freshwater tanks (2 * 2000 liters) to see what was going on.

Turns out there are osmotic blisters on the inside of the tanks and I suspect the brown deposits in the water could be a byproduct of this. It's quite interesting because the outside of the hull came out completely osmosis free during the pre-purchase survey.

The question is how do I deal with this. Ideal solution would obviously be to get in there, repair the blisters and then barrier coat, but this will require extensive surgery since the inspection ports are once around 30 cm diameter. I really do not need 4000 liters of water since I will be installing a watermaker soon. Maybe I empty the tanks, let them dry out and use them to hold a 500 liter bladder in each one? Any experience with this?

Any other suggestions?
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Old 30-01-2016, 04:44   #2
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Re: Osmosis inside F/W tanks

I have no experience with blisters inside a water tank or repair of this problem, but I have placed a bladder tank inside a large failed tank, leaving the old tank as a structural frame for the bladder. I've found this works well with the bladder holding about 60% of the volume of the former tank when the bladder is not secured. If you're placing a relatively smaller bladder in the huge tank, I think you will need to attach the bladder securely in place. 'sounds like a suitable idea to me.
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Old 30-01-2016, 04:47   #3
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Re: Osmosis inside F/W tanks

Another option could be cut the tank at the top , get inside, fix the blisters , coating the tank, and reglass the top ... a nice project ....
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Old 30-01-2016, 11:13   #4
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Re: Osmosis inside F/W tanks

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Originally Posted by pwillems View Post
...During first bit of wave action, water from the tanks became tainted with some sort of brown deposit...
I would think something like this would encourage you to do a proper repair--not look for more shortcuts.

Either properly repair or replace the tanks.
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Old 30-01-2016, 11:32   #5
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Re: Osmosis inside F/W tanks

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Another option could be cut the tank at the top , get inside, fix the blisters , coating the tank, and reglass the top ... a nice project ....
Yes this, or better yet, just buy a plastic tank that will fit in there. Never had a fiberglass tank that I would use for anything but cleaning. YUCK.
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Old 30-01-2016, 15:19   #6
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Re: Osmosis inside F/W tanks

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Yes this, or better yet, just buy a plastic tank that will fit in there. Never had a fiberglass tank that I would use for anything but cleaning. YUCK.

The thing is, these are integral tanks, one side is part of the hull, the top side supports the floorboards in the saloon. If I am to cut out the top to get in there and do the proper fix, this is going to be a VERY big project.

Is there something inherently wrong with bladders? It seems to me the old tanks will make ideal spots to put said bladders and hold them safely in place... And I will never have to worry about the osmosis problem again. Just replace the bladder every X years etc...
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Old 30-01-2016, 15:44   #7
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Re: Osmosis inside F/W tanks

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Originally Posted by pwillems View Post
The thing is, these are integral tanks, one side is part of the hull, the top side supports the floorboards in the saloon. If I am to cut out the top to get in there and do the proper fix, this is going to be a VERY big project.

Is there something inherently wrong with bladders? It seems to me the old tanks will make ideal spots to put said bladders and hold them safely in place... And I will never have to worry about the osmosis problem again. Just replace the bladder every X years etc...
If these are integral tanks, that means your hull is blistering from the inside. If detectable amounts of brown liquid came out of the tanks, it is almost certainly a severe issue. Much bigger deal than just tank problems. This is why I never recommend integral tankage.
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Old 30-01-2016, 17:48   #8
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Re: Osmosis inside F/W tanks

OK, so I've gone in and had a much better look. While the inside of the tanks are blistered, the blisters are small and have not "popped". The brown stuff in the water is little more than a healthy coating of "slime" on the inside of the tanks which I am assuming afflict pretty much all boats/RVs.

I am in the middle of doing a "shock" bleach treatment which should at least make sure that whatever makes up the slime is dead. I am currently pumping out the bleach water and am going to scrub everything I can reach before flushing a few times. Issue is that it seems I can only reach a small section of the tanks. They are baffled and there is no way I am going to be able to scrub > 50% of the surface without using an angle grinder to "open it up".

I am going to install a carbon filter in the line just after the pump and also another more serious RO type filter at the galley faucet specifically for drinking/cooking water.

This should give a whole lot more time to determine how to deal with the minor blistering in a suitable way.

Sound like a plan?
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Old 30-01-2016, 17:51   #9
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Re: Osmosis inside F/W tanks

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Old 30-01-2016, 18:06   #10
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Re: Osmosis inside F/W tanks

Quote:
Originally Posted by pwillems View Post
OK, so I've gone in and had a much better look. While the inside of the tanks are blistered, the blisters are small and have not "popped". The brown stuff in the water is little more than a healthy coating of "slime" on the inside of the tanks which I am assuming afflict pretty much all boats/RVs.

I am in the middle of doing a "shock" bleach treatment which should at least make sure that whatever makes up the slime is dead. I am currently pumping out the bleach water and am going to scrub everything I can reach before flushing a few times. Issue is that it seems I can only reach a small section of the tanks. They are baffled and there is no way I am going to be able to scrub > 50% of the surface without using an angle grinder to "open it up".

I am going to install a carbon filter in the line just after the pump and also another more serious RO type filter at the galley faucet specifically for drinking/cooking water.

This should give a whole lot more time to determine how to deal with the minor blistering in a suitable way.

Sound like a plan?


I'd consider getting some good moisture meter readings on the hull at the next haul out, with the tanks empty, obviously.
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Old 31-01-2016, 03:55   #11
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Re: Osmosis inside F/W tanks

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If these are integral tanks, that means your hull is blistering from the inside. If detectable amounts of brown liquid came out of the tanks, it is almost certainly a severe issue. Much bigger deal than just tank problems. This is why I never recommend integral tankage.
My boat have 2 FW integral tanks , the builder dont build the tanks using the inner hull side as part of the tank, kind of dropping a FG premade tank in place and glass the sides , top and bottom to the hull, then is imposible the hull itself get any amount of moisture or water unless the tank is ruptured.. to me is a good idea since if the boat is holed there you have something like a watertight compartment .... but i dont know how they made the tank in this isue, i know CSY made the tanks in a mold outside and they glass thick, inside is finish with smooth gelcoat , the top have a wide and easy inspection cleaning plate , and they are well glassed inside....
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Old 31-01-2016, 09:50   #12
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Re: Osmosis inside F/W tanks

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Originally Posted by neilpride View Post
My boat have 2 FW integral tanks , the builder dont build the tanks using the inner hull side as part of the tank, kind of dropping a FG premade tank in place and glass the sides , top and bottom to the hull, then is imposible the hull itself get any amount of moisture or water unless the tank is ruptured.. to me is a good idea since if the boat is holed there you have something like a watertight compartment .... but i dont know how they made the tank in this isue, i know CSY made the tanks in a mold outside and they glass thick, inside is finish with smooth gelcoat , the top have a wide and easy inspection cleaning plate , and they are well glassed inside....


I would not call that an integral tank, that's a glass tank glassed in. Integral is with one tank wall being the hull of the boat, IMHO.


Have seen fuel tanks done this way seep diesel right through the hull laminate-imposssible to repair. Same for water tanks and hydrolysis.
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Old 31-01-2016, 10:26   #13
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Re: Osmosis inside F/W tanks

Well they are listed by the builder as integral tanks, since the only diference is the double fg shell ,I don't know if any builder use the inner raw fg part of a hull to make a tank but sounds a lame idea to me, in any case the function is the same or similar and sometimes practical , I mean my boat have a FW capacity around 400 gallons, its a lot and sometimes I be tempted to make the tanks smaller or even cut the tanks to gain space and found another location for a smaller tank but then I think again and those 2 places are ideal for the tanks and hull contour and location call for FG tanks glassed to the hull since 200 gallons weight a lot , I don't see a better way to stick a tank to the hull with that weight... for the Op I will be sure those tanks are made like you mention , or the other way, in the 1 case could be a problem for the hull itself and the other is something not to worry for the hull , but anyway that brown stuff fluid make the water a no no, ,, so in any case a inner blader tank could be a fix , I don't like those tanks, they are noisy and not 100% free from leaks , a plastic tank droped inside of the other is another lame idea to me, since you need to cut the top to slide a tank then is more simple and logic to fix the OEM tank and get rid off of plastic .... Cheers.
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Old 31-01-2016, 11:15   #14
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Re: Osmosis inside F/W tanks

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Sound like a plan?
No.

The bleach will kill the bugs but won't remove any existing toxins.

You are either going to have to cut lots of holes and fit inspection hatches like these or fit new bladders inside them.

Access Panels|tank|hatch|boat|water|TCL4

Good water quality is essential.
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