Integral water tanks in fiberglass sailboats

Discussion in 'Boat Design' started by MJT, Jun 10, 2014.

  1. MJT
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    MJT Junior Member

    I hope this post will begin a general discussion about integral freshwater and wastewater tanks in fiberglass boats. I've found very little information on the internet and practically no project pictures.

    My project is the restoration of a 1970 Cheoy Lee Offshore 40. The ballast is internal, with concrete and lead forming the keel. The first image shows a view of the keel area looking down from above. The area has been gutted and is being rebuilt. The original wastewater tank was small (20 gal), and located under the port settee. The original water tanks were in the bilge areas, with a total capacity of 100 gal. Integral tanks in the bilge area would provide a larger wastewater capacity (perhaps as much as 70 gal) and the freshwater capacity would remain about the same (100 gal). The red and green lines suggest rough outlines for the wastewater and freshwater tanks.


    The next photo shows a later stage in the rebuild.


    There are two common arguments against integral tanks:

    (1) Hulls flex, which may cause failure of an integral tank. Many Beneteau yachts have integral water tanks, and several incidents of failure can be found on the internet. Beneteau yachts, however, are thin-skinned racer/cruisers. Thalassa, on the other hand, is heavily built, and there is little concern about flexing in the bilge area, especially with three or more athwartship baffles serving as additional bulkhead-like strengthening.

    (2) The contents of an integral tank may, through osmosis, penetrate the laminates that form the hull. Osmotic blistering is the result of the same concern, but originating from outside the hull. Hulls, however, are laminated with polyester resin. Epoxy resin is much less permeable by liquids. Additional layers of glass along the hull and keel, and a buildup of layers of epoxy would largely alleviate this concern.

    There are three common arguments for integral tanks:

    (1) Integral tanks utilize space most effectively. In Thalassa, a 50-60 gal holding tank and a 90-100 gal freshwater tank would be constructed. More capacity is likely possible, but the additional weight would be a concern.

    (2) The tank and internal baffles form structure that makes the boat stronger and stiffer.

    (3) In the unfortunate event that the hull is holed in the location of an integral tank, no seawater would enter the boat.
  2. Ike
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    Ike Senior Member

    As far as I know there are no regulations or standards preventing you from using integral freshwater or waste water tanks. There are specific regulations prohibiting integral gasoline tanks, but that is not the case here. You have addressed most of the issues. Of course if you holed a wastewater tank in RI you may run into a problem with RIs no discharge regulations.
  3. Rurudyne
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    Rurudyne Senior Member

    I don't know if this is something you would consider, but some time ago I read some very favorable things about lining water tanks with a suitable cement plaster (not concrete, of course): in terms of durability, ability to keep clean, and not leak.
  4. JSL
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    JSL Senior Member

    All kinds of stuff can accumulate in the bilge (in this case a sump). Do you want this 'stuff' sitting on the top of your potable water?
    Perhaps you have cut them out but there does not seem to be much in the way of floors/framing, especially as this is in way of the keel/ballast.
  5. MJT
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    MJT Junior Member

    The floors and framing are gone, and I am rebuilding from a bare hull. The bilge, in fact, will be aft of the two tanks, and lower than the bottom of either tank.

    Thanks everyone for the responses so far.
  6. GTO
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    GTO Senior Member

    Use food grade epoxy

    I did see the exact type modification in a magazine. Had to have been Good Old Boat or Small Craft Advisor.
    The author pointed out the need to seal the water tank interior with a food grade epoxy.
  7. MJT
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    MJT Junior Member

    It was decided that integral tanks will be built. You can see some of the work here:
  8. rwatson
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    rwatson Senior Member

    This is one instance when I might have opted for high density foam instead of plywood, due to the constant exposure to water vapour. Theoretically though, it should do the job.
  9. MJT
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    MJT Junior Member

    The constant water exposure is a concern, yes, but I'll be extra diligent here in sealing everything.
  10. viking north
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    viking north VINLAND

    Mmmm-- If I were doing this job I would have collapsible tanks custom made to fit between the installed bulkhead spaces to double counter any danger of leakage /contamination and the remote possibility of internal osmosis. A section of the top could be dedicated for an inspection/replacement hatch.
    Last edited: Jun 23, 2014
  11. LP
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    LP Flying Boatman

    VN - Any ideas for getting collapsible tanks built? I was considering tanks along the same line as the OP, but the idea of a custom bladder type tank is intriguing. Tankage is such an inefficient use of space unless the area is designed around the tank.
  12. MJT
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    MJT Junior Member

    I also like the idea of flexible tanks, but I don't think they'd work well for large capacities. I don't like the idea of a 90-gallon water balloon below my floors.

    On the other hand, connecting multiple smaller bladders seems like a workable system for water.
  13. LP
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    LP Flying Boatman

    I am looking into food grade epoxy as the bladder idea no longer appeals to me. I am curious if anyone in the industry is using integrated tanks or if aftermarket containers are the norm. An alternative to food grade epoxy would some other form of coating. Does anyone have any suggestions regarding a roll on food grade sealer to apply to the epoxy substrate?

    What about a non-food grade paint like a topsides paint that is fully dried and outgassed. Do you think there would be any kind of leaching from this type of a seal coat?
  14. redreuben
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    redreuben redreuben

    I have done integral fuel and water tanks in vinyl ester, in both cases tissue or veil in US parlance was used plus a waxed resin coat. The key though is a thorough (heated) post cure. Epoxy would be even better.
    Topside paint no way, I like my coffee isocyanate free !

  15. SukiSolo
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    SukiSolo Senior Member

    There are food grade epoxy paints available, at least in Europe. They are used in food prep areas such as on one of my local farmers, who makes excellent ice cream from the milk, from his Jersey herd. About 2010, the guy had to strip out and repaint the preparation area for converting the milk and the ice cream blending and flavouring areas. Other than stainless steel surfaces,tables etc, just about everything else was painted in epoxy to meet current hygene standards.

    Some of the resins are medically approved for use touching internal body parts. You can research this. Even sticking a mirror on a stick with epoxy and putting it in the mouth requires an approved adhesive.
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