What Holding Tank Fittings

[Desert Island]

Hi there. Its my first post so please forgive me if I am doing something wrong!

I am looking to install a 23 gallon polyethylene holding tank under the side deck of my sailing boat in the next 6 weeks and would value thoughts, comments and any helpful suggestions on my plan and help with some questions.

The bottom of the tank will be about 600mm (2ft) above the waterline and I propose to have the drain outlet at the bottom of the tank and gravity discharge it through a 50mm (2”) TRU Design ball valve attached to the bottom of the tank, then through 500mm of sanitation hose to a 50mm (2”) TRU Design ball valve and skin fitting.

The heads discharge pipe will go straight to the top of the holding tank without an anti-syphon loop, my logic is that the holding tank vent is in effect the anti-syphon.
The tank will vent from the top of the tank to the side hull using 38mm (1.5”) pipe taking a horizontal path and a length of approx. 500mm (2ft).
O and not to forget the pump out from the top of the tank directly above the discharge at the bottom of the tank. I got that gem from Peggie Hall’s book.

Questions:
1 – Should I have one 38mm or two 25mm vents (the top of my tank measures 530mm fore and aft by 240mm?
2- What should I use for the outlet at the bottom of the tank? I am thinking of using a skin fitting which I can fix the ball valve to.
3 - What should I use for the outlets at the Top of the tank? Again I am thinking of using skin fittings although it has been suggested by the company I am going to buy the tank from that I use ‘Uniseals’ instead. Does anyone have experiance of these?

Mark

[HopCar]

Sounds like a good plan. You've learned from the Head Mistress about the need for big vents. I'd probably go with one 25 mm but two won't hurt. Just use regular thru-hull (skin) fittings with out covers. That makes it real easy to clean them if they get plugged.

Peggie has mentioned Uniseals and likes them. They would be a very good way to install the pickup tube for your pump out.


I buy my tanks from Ronco and they weld threaded fittings onto the tanks where ever I want them to. I think I'd prefer that for the drain, vents and inlet.
Ronco Plastics - Marine Water Tanks, RV Water Tank, Auto Detail Tanks, Water Tanks

A nice big cleanout on top of the tank would be nice as well.

[Desert Island]

Thanks for the reply HopCar. I will probably go with the Uniseals for the fittings on top of the tank then.

Regarding the vent, I am unsure whether I will get a better flow of air or not using two slightly smaller vents over one bigger if they are just 300mm apart?

[Boracay]

I used two 18mm OD hoses from the top of my tank as vents. Other than a rather spectacular result if the tank becomes overfull and more effluent is forced in I have not observed any adverse results.

From diagonally opposite corners of the top of the tank one was run to the cabin top and one to the cabin side

I did not use any special seals on the outlet at the bottom of the tank. The fittings supplied by the manufacturer have worked just fine.

The only problem that I have experienced is the long run of hose to the top of the tank causes effluent to flow back through the pump to the bowl.

I installed a tank of about 150 litres (about 40 US gallons) and this is barely sufficient for two. 23 US gallons may not be big enough for convenience.

[Desert Island]

Thanks for the info Boracay. I am keen to have large vents in order to try to get the air flowing and minimise bad odour issues as I have a desire to keep the boss keen on sailing! Just not too sure if two vents will be more effective so close together or not.

The discharge at the bottom of the tank could have a boss fitted (welded) to the bottom of the tank however the supplier wants £50 to supply that which is clearly way more than a skin fitting, hence my query on that point.

That is a big holding tank you have, unfortunately I would have difficulty finding the room for a tank your size. I am from the other side of the pond to yourself and 23 gallons UK style is 105 litres which about 28 gallons US style. I have read so many varying reports on how much volume is required in the tank per person per day (understandably there are quite a few human variables!) that I have just got the biggest I can in the available space.

[Boracay]

I set the vent outlets on the cabin top and side as there would almost always be a (small!) pressure differential so there is some slight airflow.

So far we have not had a major problem with odor.

[Desert Island]

I like the idea of that Boracay. I will have a look at my boat to see if I can do something similar. Thanks.