Friday, November 18, 2011

Polytheylene Water Tanks?
By Michael

The chain plate for the port aft lower stay. Again,
evidence of water penetrating the sealant at the
deck and running down behind the chain plate to
corrode the bolts and back face. I won't know
whether we'll have to replace this until I remove
it and polish it, looking for cracks and
extreme pitting.
Today the bedding compound around the chain plates for the upper stays is set. This means I can reattach these stays and detach the lower stays so I can remove and inspect those chain plates. I began removing wood trim to access the four chain plates for the lower stays. There is evidence of water leaking down these, just as there was on the chain plates for the upper stays. Accordingly, I removed additional trim to see if I could find additional leaks.
Yesterday, I began the water tank replacement project. I cleared out the storage spaces under the settees (both reorganizing a lot of stuff that needed to be reorganized, and finding a lot of things we will go ahead and try to sell at the marine swap meet tomorrow). I also removed all of the bungs in the floor that have to come out to remove the water tanks and I figured out how to disassemble the dinette. I am working at a pace to remove the tanks late Monday so that I can get them to the shop Tuesday morning (Monday is a big Mexican holiday, Dia de la Revolucion, commemorating the start of the Mexican revolution in 1910).
A couple folks commented and others emailed about our likely choice of stainless steel for replacing our problematic stainless steel water tanks. I failed to mention we did consider polyethylene tanks and we haven’t ruled them out. In fact, polyethylene was my first choice and before we realized the leaking was getting progressively worse, I figured we would live with the leaks until we got to San Diego, find a place that made custom polyethylene tanks, and replace our two tanks with these (I don’t think we can have these fabricated in Mexico and I don’t want to wait for large, custom tanks to be shipped to us here).
Bolts securing a deck fitting are shown here,
just forward of the chain plate pictured above.
The mold in this picture and the one above,
is likely caused by water slowly leaking
around these bolts and along the intersection
between the hull sides and deck underside.
More rebedding is in order. 
But about the time I realized we have to remedy this problem before we leave the dock, I learned more about polyethylene tanks. They can’t be built with traditional baffles. My understanding is that they are roto-molded and any baffles have to be part of the shape, which creates a lot of negative space that decreases holding capacity. The approach then is to use multiple small tanks, which is a poor compromise for a few reasons, including cost, potential leaks from additional fittings in difficult-to-access places, and additional mounting considerations (for example, whereby a single large tank may span two supports, multiple tanks may require additional supports). Another drawback is that polyethylene tanks cannot be glassed in, they must be secured using straps.
Our metal tanks are wedged shaped to take advantage of the interior hull space. One is starboard and one is port, mounted symmetrically. It is difficult to see how well they use the space (there is bilge area beneath them) until I remove them. It may be that we can use the existing space in a different and effective way with multiple, rectangular-shaped polyethylene tanks, but I doubt it.
Don’t get me wrong, were I building a boat from scratch, I would configure it to use off-the-shelf polyethylene tanks. They are non-toxic and will last forever. But given where we are and the configuration we are working with, I suspect we’ll see that is straightforward to repair our existing tanks (maybe cutting off and replacing the bottom, for example) and that they make good use of the available space. I think we will then reach a conclusion about the reason these failed and move forward with stainless steel. After all, the previous tanks were stainless and were not replaced until 30 years passed (and our neighbor in the marina, Cactus Tree, is a Mariner 31 with 40-year-old stainless steel water tanks). I’m convinced it is all about installing them properly, ensuring the exterior stays dry, and preventing anything caustic (including chorine) from corroding the tank. I hope I’m right.

Eleanor proudly displaying the doll she made for me for my
43rd birthday. She enjoys sewing and proudly keeps
her own sewing kit in a tupperware container.


  1. Good follow up on the reasoning for SS tanks! When we installed our new fuel tanks (aluminum) the yard and the vendor both recommended we paint them with an epoxy barrier paint (which we did). Not sure if the same applies to the SS tanks but you might want to check into it as a means of keeping them dry. I'd be interested in seeing photos of the tanks and how they failed when you pull them.

  2. Bob Perry told us that if water tanks on boats were easy to get to they'd call it the Water Tank 42 (or whatever size).

  3. Aww... happy belated! It's been a while since I checked your site. Hope all of you are well.

  4. It's amazing that you're building a boat from scratch. My uncle has had a stainless steel water tank in his boat for as long as I remember. Yeah, this type of tank is built to last. It's so sweet of your Eleanor to make you a cute doll for your birthday. =)

  5. Yes, it was a good thing to see like this. This very important to look forward on any project.

    water tank australia


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