Gravity holding tank plumbing

[Boat441]

We fitted a gravity tank. It has a y piece to to either pump into the tank or overboard. The tank has both a gravity discharge; with help of a pump and a deck pump out facility. Our vent is a loop to deck level then down to just above the boot topping.

If you regularly empty it you shouldn't have any smell problems.

Trek tank have some useful example system diagrams.

Cheers

 

[matthewriches]

I fitted a gravity drain tank to Jimi's boat of this parish. Very simply comes from toilet, into tank, to seacock (with vent and wash out/pump out fitting).

You choose whether you want to discharge to sea or to fill tank. When you get to sea, pull seacock and dump the contents or have it pumped out via the deck drain.

Assume all was ok, he had the tank made by a local plastic fabricator.

 

[Sea-Fever]

I have decided to fit a gravity holding tank.

I intend to use it as little as possible to avoid smells etc. But there are occasions when it is too anti-social not to! Should I therefore connect a y-valve so that I can bypass the tank most of the time? Or are y-valves more trouble than they are worth?

Where is best to lead the tank vent? Is there a lot of persistent smell from the tank vent - i.e. is it worth leading it to the rear of the boat so it is downwind when at anchor?

I have read that it is best to fit a valve just below the tank to prevent solids sedimenting in the exit pipe - how much of a problem is this if I empty the tank fairly quickly after use?

I also plan to fit a holding tank this year but have planned to fit a dip tube instead of gravity pipe. I will use a Henderson Mk V to suck up waste from the base of the tank after which point it will effectively siphon with very little pumping effort down to the outlet seacock. Entry to the tank from the head will also be via the top of the tank as will the vent thus very little chance of leakage from the tank itself. Won't bother with a Y connection. I may later fit a deck pump out.

 

[LittleSister]

A good gravity-tank install should have one continuous length of smooth-wall pipe from toilet outlet to tank - that's fewer joints than a non-tank installation because you don't need the anti-siphon, and should be almost impossible to block.

I'm struggling to see why you wouldn't need an anti-syphon valve. If the pipe run is such that water could back flow to the toilet, what difference is made by having part of that run through a tank?

 

[prv]

I'm struggling to see why you wouldn't need an anti-syphon valve. If the pipe run is such that water could back flow to the toilet, what difference is made by having part of that run through a tank?

This is assuming a fairly tall gravity tank with the toilet hose entering at or near the top, above the boat's waterline. Essentially the run of pipe is like the upward side of the traditional loop, the body of the tank is like the downward side, and the vented airspace at the top of the tank is like the anti-siphon valve.

To get back to the toilet bowl, the contents of the tank would have to somehow jump upwards to the pipe entry at the top.

If you completely overfilled the tank you might manage to get a small amount of drain-back (along with shit squirting out of the vent...) if the hose enters at the top of a side-wall instead of through the top face, but you still wouldn't get a siphon because any negative pressure would just suck air in through the tank vent until equalised instead of lifting any weight of water upwards.

Pete

 

[LittleSister]

This is assuming a fairly tall gravity tank with the toilet hose entering at or near the top, above the boat's waterline.

I see. Thanks.

 

[elenya]

We have a simple gravity tank plumbed to always gobthrough the tank. We also fitted a valve in the 1 inch air vent. When the tank outlet is open we close the air vent. This reduces and possible smells on deck. The tank cannot pressurise as the discharge seacock is open. Works for us as liveaboardd.

 

[RupertW]

For both our heads we use the simplest system which works well - outlet straight up to top of gravity tank with a vent and with a pump-out outlet directly above so we could always rod out any mess (never used for either purpose in 8 years). The only control is the outlet seacock below the tank. The only smells we have ever encountered are once when a holding tank overflowed through the vent (alas into the dinghy which was parked alongside the hull below it - ugh), and more commonly after 3 years or so when the pipe from toilet to holding tank started "perspiring" in an unpleasant way and needed changing - not a hard or long job.

I actually would expect the pipe to be the weak link as it has pretty much undiluted sewage sitting in it for about 2m. Obviously flushes are kept to a minimum to clean the bowl only as you don't want to fill the holding tank with lots of water.

But the reason why we have never had any blockages and never expect any is that we follow the Greek practice of nothing you haven't eaten going down the loo and have a bagged little pedal bin by each loo.

 

[vyv_cox]

There is an account of our holding tank experience on the website under Cruising.. In our case there is insufficient space to fit a diverter valve. The current arrangement has worked well for many years now. We had one blockage, caused by a flake of scale that fell off the tank wall. Fixed by rodding through the inspection hatch. We have had some problems with scale in the discharge pipe, mainly because the Jabsco Lite throughput is kept low to avoid filling the holding tank.

 

[Daydream believer]

My vent outlet is a chrome fitting that sits 20mm proud on the side. Normally, if not using the holding tank for retention, I tape the vent up with insulation.
This stops air coming out. The theory behind this is that when one pumps fluid into the tank if there is a tendency for anything to start a blockage it compresses the air in the tank.
The slightly "compressed" air tends to push the blockage through the pipes. one can get quite a pressure build up & it does make my stainless steel tank bulge if there is a blockage. The other advantage is that we get an early warning of an impending blockage as it becomes harder to pump long before the tank is full. That means there is less mess when unblocking.
I have a clear flexible pipe between the tank & final seacock as I can see what is in it & I can "work" the pipe to aid free movement of blocking material. Most times it is paper at the seacock.
Being flexible I can disconnect from the seacock & position it over a pan (makes the veg taste better!!) to collect detritus
Because it is flexible I can clamp the pipe between 2 bits of wood with a small G clamp.
If the blockage is at the seacock I can easily take it off the seacock, open the cock & shower myself with the blockage as sea pressure blows it back & clears it
So i recommend a clear flexible pipe at this point rather than a stiff wired one, regardless of what the " safety" experts say about having decent pipes here.

 

[RupertW]

My vent outlet is a chrome fitting that sits 20mm proud on the side. Normally, if not using the holding tank for retention, I tape the vent up with insulation.
This stops air coming out. The theory behind this is that when one pumps fluid into the tank if there is a tendency for anything to start a blockage it compresses the air in the tank.
The slightly "compressed" air tends to push the blockage through the pipes. one can get quite a pressure build up & it does make my stainless steel tank bulge if there is a blockage. The other advantage is that we get an early warning of an impending blockage as it becomes harder to pump long before the tank is full. That means there is less mess when unblocking.
I have a clear flexible pipe between the tank & final seacock as I can see what is in it & I can "work" the pipe to aid free movement of blocking material. Most times it is paper at the seacock.
Being flexible I can disconnect from the seacock & position it over a pan (makes the veg taste better!!) to collect detritus
Because it is flexible I can clamp the pipe between 2 bits of wood with a small G clamp.
If the blockage is at the seacock I can easily take it off the seacock, open the cock & shower myself with the blockage as sea pressure blows it back & clears it
So i recommend a clear flexible pipe at this point rather than a stiff wired one, regardless of what the " safety" experts say about having decent pipes here.

Why does your tank block so often? Those sound like really good suggestions for dealing with regular blockages but a lot more to set up if blockages are a once a year or a decade event?

 

[Daydream believer]

Why does your tank block so often? Those sound like really good suggestions for dealing with regular blockages but a lot more to set up if blockages are a once a year or a decade event?

Does not block "often". It has been down to visitors, & limescale. That is 3 times in 15 years. But if one is not prepared it makes one hell of a mess & if one does not catch the detritus it can go into inaccessible parts of the bilges.
On one occasion I sailed from Ostend to Bradwell with turds floating in the shower tray when the limescale blocked the heads pump & the crew( suffering from diarrhoea) left the boat to come home on the ferry, just as I was about to sail for home.
It was their parting gift because the RNSYC was shut.:encouragement:
Not funny clearing it up afterwards.

Now all my pipes are flexible so that any limescale can be crushed. However every winter sees a full flushing with brick cleaner to ensure pipes are clear& the tank is left with some in it for a while as well.

Once bitten, twice shy.

 

[Daydream believer]

It's not very often that I read something completely new on these forums but that is it. :encouragement:

The concept of being in a bay and diverting the No. 1's directly into the sea along with the toilet paper (you do have women aboard sometimes presumably?) whilst people are swimming around is not going to go down too well.

I really like the concept though.

Richard

Yup a really good one:encouragement:

 

[RupertW]

Yup a really good one:encouragement:

As said before, we don't put paper down the loos but as we have two heads we do occasionally (if we are stuck in a bay for longer than normal), use one with seacock open for no 1s and use the holding tank on the other. But that's probably only been 2 or 3 times in 8 years.

 

[Birdseye]

I put a system in my cat and it worked brilliantly. The essential points were:

1/ all piping was solid abs solvent welded - smelly flexible piping comes about because the process to make it flexible makes it micro porous.
2/ The tank had a pipe to empty it going in through the top and down to about 1 inch from the bottom. The pipe then led through a diaphragm pump to an ordinary seacock. Emptying the tank was done by using the pump to create a syphon and then leaving the tank to empty itself
3/ I was able to route the vent pipe in between the two hulls - no smell.
4/ All the toilet output was routed through the tank to avoid a diverter valve. Any valve or connection is a site for a blockage
5/ On a Greek flotilla I was introduced to the practice of putting used toilet paper into a bin rather than down the loo. We inisisted on that with the holding tank and we never had any sort of blockage.
6/ I included a decent top access hole just in case.
7/ with holding tanks bigger is better - I was surpsied how quickly we filled ours.