Can I build a fiberglass tank inside leaking metal tank? - SailNet Community
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post #1 of 3 Old 11-11-2007 Thread Starter
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Can I build a fiberglass tank inside leaking metal tank?

I appreciate the knowledge base of the forum members, so I am throwing out this question.
Boat has leaking metal water tanks. I can't remember if they are aluminum or galvanized. The leaks are probably not very big, because the tanks seem to hold water to some degree when the bladders (PO installed bladders) broke last year. I am ready to bite the bullet and cut open the teak & holly to do it right if necessary. Bladders just don't provide the capacity and reliability I want. Since the boat is located far from my usual haunts, I am thinking of the second best way to do it. Obviously the best way would be to get exact measurements and have new tanks professionally fabricated, but this would be very expensive in terms of shipping not to mention the customs duty this caribbean island imposes. And no matter how exact the new tanks are made, there would be some fitting and adusting to make them snug.
The tanks are pretty big, and conform to the bottom of the hull (with a layer of insulating foam), so they are the shape of 1/4 of a cylinder.
What I am thinking about is cutting off the top of the metal tank, leaving about 1" all around on top to fit a lid. Then I was thinking of laying down, inside the tank, two layers of heavy roving with West Epoxy (available locally) and building in baffles as well. Then I could fit a lid of fiberglass on the top. I suppose I would chemically (acetone) clean the metal and rough it up so there was good adhesion, although this is probably not critical. The existing metal tank would provide strength so that the fiberglass would be primarily to seal.
In fact if I do it this way I might not have to cut up the floor. I might be able to access it all through the existing access doors. This would save me a week of work.
Does anyone have thoughts about this? Will this work or will I just create a mess? Is epoxy okay for potable water tanks or do have to put on a coat of another epoxy?

Last edited by dohenyboy; 11-11-2007 at 07:56 PM. Reason: grammar
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post #2 of 3 Old 11-11-2007
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My boat is made with fiberglass tanks for potable water. It is coated with CeRAM Cote 54, which is an epoxy-based coating specifically designed for potable water tanks.

How large are these tanks... if they're more than 10-20 gallons, you really need to have vertical baffling inside them to help reduce free surface effects and sloshing. I would cut the metal top off completely, rather than try and clean the metal to get a good bond. One real reason you don't want to deal with trying to bond the metal to the fiberglass is the difference in their expansion and contraction characteristics.

That may also be a problem for the tank itself. One possible solution is to place a layer of foam down inside the existing tank, and then laying up the fiberglass tank over the foam. The foam would help prevent the metal tank contracting from damaging the tank and help support the fiberglass tank when the metal tank expands.

Trying to install a tank into an existing tank, that probably has internal baffling, will probably take more time than cutting off the lid, gutting the tanks and then laying up the new tanks.

Finally, unless your boat is homeported on the island in question, you may not have to pay customs duties on imported tanks, since they are going to a yatch in transit. Generally materials to yatchs in transit are not considered imported...since the yatch doesn't stay in the country in question.


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post #3 of 3 Old 11-11-2007
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Dohen...there is a good article on building tanks on the West system website (search there for "potable") but it does not look like a good idea to do at home for potable water systems:

Considerations for potable water

We have adopted the broad policy of not
recommending epoxy for drinking water
tanks because of regulatory and safety issues.
The potential problems outweigh
the benefits. To date, none of Gougeon
Brothers’ epoxies meet FDA regulations
or any other drinking water certified approval.
The major long-term concern
with any plastic water tank is extractives
leaching out in the water. Off-ratio,
poorly cured epoxy can release extractives,
as noted above. In the fabrication of
water tanks and food handling equipment,
the successful use of epoxy requires
thorough mixing and adequate
elevated temperature post-cure to assure
themaximum cross-linking and cure of
the polymer. These process controls are
not always possible with the home-built
tank. Unfortunately, neither Gougeon
Brothers, Inc. nor any certification
agency can verify the level of quality
control exercised in the fabrication of
the tank.

There is an excellent thread on your issues over here:
which indicates it CAN be done, but note all the cautions in the thread as you make your own decision.
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