Waste tank fittings

[mait]

I have a question about waste tank fittings.
I have a company near me that can produce a welded HDPE holding tank according to my drawings at reasonable price.
The tank will be somewhat complicated upright box shape, slightly conical and following the hull shape. 60L of volume, gravity emptying. On top there will be inspection hatch.

Connections on the top of the tank-
straight for vent
90-degree elbow for inlet
straight for pump out
Connection in the bottom - straight for gravity emptying.

Unfortunately the company has no weldable fittings available nor won't they bother threading the tank for fittings.

Would it be OK to use brass or plastic thru-hull type fittings? I can easily assemble these through the inspection hatch. If so, which sealant to use with HDPE?

Any other ideas are welcome

[ranger58sb]

Can't speak to your real questions, but...

FWIW, I'd prefer a top-mounted dip tube and a pump for emptying overboard at sea. Because fittings -- including bottom mounted fittings -- can fail whereas gravity always works.

Search for threads here where PegHall (the Head Mistress) names the gizmo she recommends for inserting tubes in waste tanks. (I just don't remember, off hand...)

-Chris

[Bellinghamster]

I believe fittings in rotomolded tanks usually spin-welded in (friction heated). It seems odd that someone who makes tanks doesn't have that capability. What do they normally make?

Since it's a custom tank, they should be able to set up the mold to make thickened areas, i.e. "bosses", where the fitting go (~5/8" thick, 2x the diameter of the fitting feels about right). Then you, or any machine shop or plumbing shop can drill and tap the bosses for the appropriate female pipe thread.

I just put in a very similar system w/ gravity drain from a bottom fitting because I didn't want to have to use a pump to empty. I've had both setups on previous boats, gravity drain was super simple and never had a problem. . The bottom fitting is on the side, short nipple to a PVC tee. A PVC "riser" from the top of the tee to the top of the tank then short hose connection to the deck pumpout. A hose nipple and short hose out the bottom of the tee to a PVC ball valve (just above static waterline), then short hose to the seacock/through hull. This minimizes the standing sewage in hoses, only potential plug point is the tee, which can be poked or flushed from the deck fill.
I added 2x 3/4" vents, one on each end of the tank w/ short hoses to forward and aft facing vent cowls through the cabin sides. All seemed good in theory but have a leak somewhere in the tee fittings, probably due to the crappy Shields 148 super-stiff hose... (I had less than a week to put it all together and that was all that was available locally).
Also, crew mentioned catching a whiff in the cockpit when the head was flushed. I debated long and hard whether to put the vents in the cabin side or hull side. Went with cabin because it was a very short (10") hose run, and to avoid filling the tank with seawater in rough weather. In hindsight, hull side vents would have been better, and aft vent farther aft, even though it'd be a longer run. Sailing anywhere that seas are rough enough to get in the vent likely doesn't require saving the blackwater so it would have just drained out.

[mait]

Originally Posted by Bellinghamster

...What do they normally make? ...

They make all kind of industrial stuff, like welding pipes together, large tanks for chemicals etc.

By the way, it is not roto-molded tank. Think of Tek-Tanks - welded together from flat HDPE sheets

[peghall]

Ask them if they can make the tank using polypropylene instead of HDPE. PP is a far superior welded tank material 'cuz it's easier to cut in odd shapes and--unlike PE--can be "glued" (PE must be heat welded...there is no sealant that can permanently bond to to PE), making it easy to install plastic female threaded tank fittings. You can even do that yourself if you have a hole saw (available as a bit for an electric drill).

NEVER use BRASS anywhere on a boat, with the possible exception of the ship's bell!


You'll use plastic hose fittings in the tank...bronze or Marelon for the vent and overboard thru-hulls.



-Peggie

[mait]

Thank you all for constructive replies! At least I know now what to ask, PP it is.

[peghall]

You're welcome to give me a shout via PM or email if you have any questions I should be able to answer.


--Peggie

[Miguelso]

I am also contemplating a new holding tank and was wondering if anyone has experience with these Uniseal types of fittings. I am a little hesitant specially in a moving environment, but some people seems to be using them.

https://www.usplastic.com/catalog/it...x?itemid=89951

[peghall]

The Uniseals UNISEAL do not leak when installed according to directions, so they're great for inlet and vent fittings and discharge fittings installed in the top of a tank using a diptube. They are NOT recommended for use in or at the bottom of a tank. Short pieces of PVC that have an OD that's a match for nominal size of the hose, which is always its ID) can be used as hose fittings (if used with a diptube, just make sure the pipe sticks out of the top of the tank far enough to double clamp a hose onto it). Be careful that no hose has any sideways pull...that can create a leak.


More and more companies are using Uniseals instead of traditional fittings replacement...for example, Dometic uses 'em in their Diptube retrofit kit.


--Peggie

[mait]

Here's an update - I decided to order the waste tank from a reputable company specialized in HDPE tanks and they will produce the tank with all the fittings etc.
Now I have started another thread about water tanks https://www.cruisersforum.com/forums...ml#post3679514