Gravity Holding Tank Install... need advice

[thruska]

Originally Posted by atoll

a gravity tank needs no pump when emptying into the sea and is less likely to get blocked as the toilet flushes through it when not being used as a holding tank,closing the seacock,turns the tank back into a holding tank.
no nasty 3way valves,macerator pumps or extra seacocks needed.

Or compost head....

[J.Kitchens]

I've been hating the twenty one gallon turd tank under the v-berth. I was thinking about ditching my new LaVac and using a port-potty or a bucket just so I could get rid of the giant turd tank.

Maybe I should rip out the cabinet I built behind the head earlier this year and build a gravity tank in its place. I'd get more storage volume under the v-berth than behind the head. However with the port potty I would get storage behind the head and under the V-berth.

I like to keep my boat simple and a gravity holding tank seems simple enough not as simple as a porta-potty though. I have roughly twenty feet of sanitation line, in the current system, that I wouldn't mind getting rid of.

For me, right now, composting isn't even an option. I may get there someday.

Gravity tank or porta-potty?

[Pizzazz]

There are three things you can do to dramatically improve the marine head experience: PVC tubing, fresh water flush and gravity drain. On my Hunter 31, I just used PVC tubing to create a downhill path for the waste from the tank position below the v-berth to the through hull. The waterline is about 1” above the bottom of the waste tank. Love the system, never had a clog, the easiest way to fix it if it ever develops is to create overpressure in the tank via da vent line (using a dinghy pump). It works very well.

Check out the Hallberg-Rassy manuals for a nice drawing of a gravity drain waste tank design.

[atoll]

i think you also need to consider local laws and how you intend emptying your holding tank,this in turn will dictate what capacity of tank you will need,regardless of how the tank functions.

currently in europe, turkey(+ croatia ?) is the only country that legally requires and enforces the use of holding tanks on small seagoing vessels and requires that they are emptied offshore,or at a pump out station,unfortunatly pump out facilities are few and far between,making dumping at sea the most common option.

with this in mind, for the charter market,and the potential for other eu countries to adopt similar regulations,yacht builders started incorporating these small holding tanks on vessels,mainly to satisfy regulations.

in reality though plumbed in, the tanks are rarely used in europe,except turkey.
the main problem being capacity ,as a 5 gallon tank with 4 people on board would require emptying every 2-3 days,not a problem if changing anchorages every day,or with access to shore side facilities.

however i would suggest a 5 gallon tank would be impractical in places like the usa for liveaboard use where a larger tank would provide more options for extended periods between pumpouts.

it is entirely understandable the popularity of composting heads and porta potties etc in areas where no discharge is the rule rather than the exception.
worth thinking about if going to the expense of retrofitting a tank onboard.

[Steven UK]

Atoll good points, I was looking at the five gallon tank so that I could keep it high under the corner of the ‘V’ birth just forward of the head. This was to try and keep the bottom of the tank above the waterline. I have seen many tanks behind the head against the hull and this looks like an ideal solution if you don’t mind gutting the cabinetry that’s already there. If I did go this route then 10 gal would be achievable. I will buy ‘the book’ on the subject and see how it goes.

[johnjuliano]

To clear the outlet on our gravity tank when it clogs, which it does, we have added a shut off valve to the vent line.

We close the vent valve and add pressure to the tank by pumping additional water using the toilet pump.

It has never failed and is a hands-off method.

[Steven UK]

Originally Posted by johnjuliano

To clear the outlet on our gravity tank when it clogs, which it does, we have added a shut off valve to the vent line.

We close the vent valve and add pressure to the tank by pumping additional water using the toilet pump.

It has never failed and is a hands-off method.

Genius ...!

[atoll]

Originally Posted by johnjuliano

To clear the outlet on our gravity tank when it clogs, which it does, we have added a shut off valve to the vent line.

We close the vent valve and add pressure to the tank by pumping additional water using the toilet pump.

It has never failed and is a hands-off method.

this is how submarines would empty tanks when submerged using compressed air,just be careful that the tank can withstand the pressure.

having had full 25 gallon tank burst into the bilges under pressure caused by barnacles blocking the outlet is not an experince i would wish on anybody

[Jdeuel]

Piggybacking on an old thread. I started the install on my electric head today. Part of the job was removing old hoses (yuk).

Head is just above waterline. Holding tank discharge is about 8 inches above waterline. The current setup has the head discharge going directly into the holding tank. The discharge of the holding tank has a Y-valve that diverts to either the seacock or the Pumpout line.

My question is, do I actually need a y valve there, or can I just use a ‘T’ fitting? The pumpout plate is a good 3-1/2 feet above the T on deck.

Seems like eliminating the y valve would eliminate something to forget, but educate me!
Jeff

[Pizzazz]

You do not need the valve, a simple T will do. This is how I operate my discharge system, just with a T and gravity drain.

One of the benefits of the a valve is that if you get a clog and your tank is full, you can work on one portion of the hoses at a time. With a T only, you have no way to isolate/work on the system unless you empty it completely.

[Rough Magic]

Yes, you need the valve, or someway to shut off the ocean dump valve. The dockside pump works on suction, you won't have it with a tee, and won't be able to pump out.

[Sailmonkey]

Originally Posted by Rough Magic

Yes, you need the valve, or someway to shut off the ocean dump valve. The dockside pump works on suction, you won't have it with a tee, and won't be able to pump out.

Not true.

Ours is plumbed as the OP describes. The outlet of our holding tank goes to a T. One side to the deck fitting, the other to the discharge seacock.

Our drain line is about 2’ long, so to prevent clogs I put a big Banjo ball valve immediately on the tank discharge. This prevents waste from sitting in the discharge hose while the seacock is closed.

[Rough Magic]

Yes, and that allows the pump to create vaccum to remove the waste. If the peacock is always closed, then yes, no additional valve is necessary.

[Jdeuel]

Awesome. Yes-there’s a seacock on the dump side of where the tee will go.

Hoping the electric head with the macerate will keep clogs to a minimum.

Thanks!

[Sailmonkey]

Originally Posted by Rough Magic

Yes, and that allows the pump to create vaccum to remove the waste. If the peacock is always closed, then yes, no additional valve is necessary.

That’s how a gravity tank works. If the sea valve isn’t closed you dump the waste overboard.