Holding Tank: Dip pipes

[laika]

I've been planning a new holding tank for 8 years. Everything I've read advises using dip pipes rather than an exit pipe at the bottom of the tank, reducing the danger of leaks and avoiding having to stick a valve on the outlet in case your immediately-downstream macerator/pump burns out whilst the tank is full.

I've never actually got round to installing it, mostly through lack of ability in designing a model to allow construction of the tank itself rather than plumbing ability.

With solid work for the next 6 months and a desire to escape the country as soon after quitting as possible I decided to outsource the job to professionals. However they're vehemently arguing against dip pipes and in favour of outlets at the bottom of the tank saying "you'll never completely clear a tank with dip pipes: it'll always smell.

I'm not convinced. What does the forum think?

 

[MoodySabre]

Tek tanks are professionals too! Their tank with dip pipes and a proper vent system doesn't smell after 8 years. Clearly it never completely clears but most of what goes is water from flushing. A large vent with a carbon filter takes care of it.

 

[matthewriches]

My boat is fitted with 2x fuel tanks, water tank and black water tank. All have dip tubes for everything. There are no holes below the "lid" so to speak. Whether that was standard or because it was coded from new, I'm not sure. I think this is the best idea for absolute foolproof "no leak" solution but that obviously doesn't stop connected pipes from leaking.

We fit a lot of Tek tanks. I am aware some gravity drain so therefore, there must be a hole in the bottom for it to work.

Personally I wouldn't mind a water tank leaking but diesel and waste water...... yuk!

 

[RupertW]

I prefer the simplicity of an inlet at the top and an outlet at the bottom straight to a seacock (plus an extra hole at the top to the deck so I can theoretically pump out or rod down to the sea cock.

In 8 years with two heads I’ve never had a leak and have changed all pipes at least a couple of times and resealed fine.

With a dip outlet do you have to actively pump to empty the tank rather than just open the seacock?

 

[Yngmar]

Ours is a Bavaria standard gravity drained tank (behind the shower wall). Poo goes in at the top and out the bottom (with a seacock). There's also a vent pipe at the top and the usual decorative deck pumpout fitting. The only pump in this is the Jabsco Twist+Lock on the toilet that pumps the poo up into the tank. Oh, and there's a diverter valve, so you can pump straight to sea or into tank. Tank had quite a lot of calcium when I got the boat, but once cleared hasn't caused any trouble in the years since and nothing stinks. Simple & good system if you ask me.

The Twist+Lock has failed once though (overeager attempts of clearing a blocked straight-to-sea pipe) and that was much yuck.

 

[laika]

With a dip outlet do you have to actively pump to empty the tank rather than just open the seacock?

Yes in this case by virtue of the placement of the tank: it won't be gravity to the seacock either way, i.e. up or down, there would be a macerating pump involved. My thought from research echoed what MoodySabre suggested but I was canvassing on whether that thought might be flawed.

 

[Norman_E]

My holding tanks are both top fill - bottom empty types with the usual and useless deck fittings. In Turkey you do have to pump out at an official station so as to get your Blue Card used and my solution is a tee piece in the outlet pipe leading to a deck fitting. That way the pump out station suction can completely empty the tank. The original deck fittings lead to the tops of the tanks so no amount of suction on them could ever empty anything and their only use is that they are directly above the outlets so that it is possible to rod through and clear a blockage.

 

[Tranona]

Yes in this case by virtue of the placement of the tank: it won't be gravity to the seacock either way, i.e. up or down, there would be a macerating pump involved. My thought from research echoed what MoodySabre suggested but I was canvassing on whether that thought might be flawed.

If you have a pumped system then presumably it will be low down in a locker. If that is the case then a bottom outlet will seriously compromise the size of your tank, although you could have a side outlet at the bottom of a side panel. However, a dip tube would be the obvious thing to use as presumably you will have all the other fittings on the top.

There are schematics of different layouts and hints on things to consider when designing tanks on the Tek Tanks website. I expect also they have already made a tank for your boat and will have worked out the best way of doing it. Again details on the website.

 

[langstonelayabout]

I’m one of the lucky ones that has had to fit a new macerator pump to a ¾ full, low down holding tank. I will never tell the story, ever…

Although I’m currently boatless, if I have to fit a holding tank to my next boat it will be a ‘top fill, gravity empty’ type of the sort on the Beneteau 323’s we’ve chartered in Greece. Low maintenance, no electric motor to go wrong, etc. One of the people that has already commented on this thread has suggested the use of a T-Piece taken from below the tank to the deck fitting. That will be added to the installation too!

Whatever you do with dip pipes or not, avoid electricity, water and sewage: you know it will only end in tears and a very character building DIY session…

 

[laika]

I’m one of the lucky ones that has had to fit a new macerator pump to a ¾ full, low down holding tank. I will never tell the story, ever…

When I bought my boat I discovered the holding tank was full and the only exit was via pipe at the bottom connected to a burned out macerator. So you don't have to tell me your story... Suffice to say that I wouldn't have any bottom outlet pipe which didn't have an on-off valve close to the tank.

Thanks for useful advice: I'm aware of the tek tanks site and I believe one of the designs on there was designed by fellow oceanlord owner and forumite chris_robb who has also given useful advice on this.

Gravity fed would be my preference but I'm not sure the space I have available in the heads would allow for a sufficiently large tank there.

Over the weekend (thanks partly to the forum's confirmation) I've decided to stick with my original concept with dip pipes at the top. I can see this reverting to its original status as a DIY project.

 

[Tranona]

Space is always a problem retrofitting a gravity tank and the ready made ones are invariably the wrongs shape - usually too fat. I have fitted two, both custom made and managed to get 40l+ which is OK for one or two persons for a couple of days. One had a ball valve on the bottom outlet but the assembly is very bulky and long so the tank needs to be mounted high. The second one (fitted in a locker behind the washbasin in a Bavaria) did not have room and only a short run to the seacock. Did, however, by design have the inspection hatch immediately above the outlet so the one time it did block was easy to rod.

 

[Steve_N]

Over the weekend (thanks partly to the forum's confirmation) I've decided to stick with my original concept with dip pipes at the top. I can see this reverting to its original status as a DIY project.

I made and fitted my tank about 15 years ago - it's low down beneath the forepeak berths and is emptied using a Gusher-type pump and dip pipe. I've not had to touch it since it was installed and the dip pipe method seems to work just fine for me. Yes, there is always an inch or so of 'fluid' left in the bottom of the tank but I always flush it through with clean seawater when leaving the boat for any length of time and thus far it's been odour-free.