Plastic holding tanks- Are they legit?

[stiffwind]

My orginal gravity operated holding tank which was built into the hull with fiberglass matting was recently replaced because after 25 years it appeared to be deteriorating. Instead of doing a patch job, the yard recommended a plastic tank. So after about $1,800 to build a shelf, buy the plastic tank in the shape to maximize size, custom configuration of tank outlets, installation, etc., I lost about 10 gal of capacity...and of course now have a tank that is leaking from the bottom hose fitting to the tank. After finding the leak, I puzzled about what would cause the leak in what appeared to be a secure and caulked connection. Then I noticed when flushing the toilet that the sides of the tank were moving with the pressure created by pumping and sinking inward when the tank was emptied. I know the first reaction will be to check to make sure vent is not clogged. But if a clogged vent...which happens....will render your plastic holding tank vulnerable to malfunction, why take the chance of installing such a vulnerable system instead of installing a more rigid tank?
Has anyone had similar experience with the plastic holding tanks? And is there any way too insure that pressure differences won't occur? For instance I would think that when a pumping station hose sucked the plastic tank empty it would also tend to cause the sides of the tank to draw in. Any repeated movement of the tank wall would seem to eventually cause even the best hose connections to leak.

[treilley]

If you have space, always install a tank with top fittings. Or minimal bottom fittings.

No need to better engineer the tank because it requires a vent. This is just part of regular maintenance. Keep the vent clean. Not filling your tank to the top helps. It is also a good idea to locate the vent line centerline on the tank.

[PCP]

Never saw an holding tank that it was not made of plastic but sure, there are many different types of plastic. The ones I have seen are pretty rigid and strong.

Regards

Paulo

[stiffwind]

Tim, you would use a top fitting in a gravity operated tank for emptying path, instead of bottom fitting directly to the seacock fitting??

[SlowButSteady]

Originally Posted by stiffwind

Tim, you would use a top fitting in a gravity operated tank for emptying path, instead of bottom fitting directly to the seacock fitting??

That's how my boat's tank is configured; input, output, and vent fittings are all on the tip of the tank. You just need to fit "dip-tube" from the output fitting on the top of the tank (the input fitting doesn't need one). The dip-tube shouldn't quite touch the bottom of the tank and its end should be cut at a bit of an angle on one side.

It doesn't take much of a leak in a holding tank to stink up a boat. Not having any thru-hull fittings, except on the top of the tank, eliminates most of the potential for such leaks.

[SkywalkerII]

I've built two waste systems using custom made holding tanks from Ocean Link. They make great products, have a long list of stock sizes, and can make what you want. They put all fittings on the top and use PVC tubes inside the tank they call "dip tubes" so waste never sits in hoses. They recommend dedicated lines for overboard and pump out fittings, so no Y valve. Great system.

[boatpoker]

I have rarely seen a plastic (mostly polyethylene) tank that did not leak an odor if not actual liquid. The tank fittings are a more rigid plastic and expand/contract at different rates than the tank itself therefore the leaks.

Aluminum tanks can have a very short life if installed improperly and a shorter life than other materials because aluminum corrodes in the preence of uric acid.

Stainless steel works quite well but is expensive.

FRP with an epoxy interior coating is my favourite because it does not deteriorate (in my lifetime), the expansion/contraction differential is minimal and thay can be easily built to fit in any space or even built in place.

As to fittings, I'm all for top fittings only. It just makes sense to put all your holes in the top surface to minimize leaks.

[mitiempo]

Unless it is a gravity draining tank as the original poster has.

[jackdale]

One advantage to plastic tanks is that you can see the level of effluent. The only one I have ever had leak was from a top seam. When we were heeled the seam would open up due the weight of the liquid.

This has resulted in my advising folks to look at, then smell any "water" on the cabin sole before tasting.

[boatpoker]

Originally Posted by mitiempo

Unless it is a gravity draining tank as the original poster has.

Only if it's open all the time and we know thats not kosher in lakes, harbours, rivers or wihtin 3 miles of an ocean shore.