Plumbing/watertank question...

[canucksailorguy]

Hello all - I have bladders under my cockpit benches and I've decided I'm not happy with them. I was planning on removing the bags, glassing the interior of the space and sealing it off, with piping as needed to fill and use.
Then I came across a suggestion that food grade plastic is 'weldable', and that one can build tanks from it - a far less messy job than using fibreglass. I'm researching the possibilities and wonder if anyone here has experience with this technique? My thinking is to place the plastic sheets in the space now occupied by the bladder, which is wood on three sides, the inner hull on the fourth and bottom, and then place a sheet overtop with inspection plates, and the appropriate fittings.
Looking for your thoughts.

Wally
s/v Gypsy Wind
about to head south on the ICW
for Paradise yet again...

[Leftbrainstuff]

High density polyethylene is the, usually black, plastic you see being used for irrigation use and some quite large pipelines.

It's very durable, safe to drink or eat from and relatively easy to fabricate. The pipelines are joined using a special hot plate 'welder' but tanks are fabricated by relatively low temp solid state methods. You can use a hot air gun for thin sections but for a water tank you typically need to use a commercial joining method.

The material is weaker than steel or aluminium so you need to support the tank to seperate the mass loads from the sealing functionality.

[overbored]

There are many companies that make marine water tanks in thousands of shapes and sizes. Ronco is one company. they will cost less then you can buy the materials and tools to make them yourself. they are natural color HDPE material and drinking water safe. fiber glass and polyester resin is not safe and will effect the taste of the water. epoxy resin some say is OK for drinking water but the resin alone will cost you the price of a pre-made tank.

[canucksailorguy]

The reason for building a tank in place is so as to not lose the capacity of the available space. Using a built in tank would probably cost me 30% of my tankage. I sail in the Bahamas, Cuba and other areas that are not always easy to secure water in, so maximizing capacity in a 34 foot boat is important, price is a secondary issue (who am I kidding here?). Showers are nice....
Overbored, you should check out the prices of tanks someday - they are not nearly as cheap as you think.
Leftbrain, the material would be surrounded by the wood of the bench on two sides, plus the inner hull on one and on the bottom. It sounds like you're saying that I should reinforce on the long side of the bench, am I right?
I had been hoping to find a PVC type material that can be 'welded' using a PVC type 'glue' that I've used successfully in my house in the past. Any thoughts on that?