Making new gravity dump holding tank setup with electric head.

[DocSjulle]

Hi.

When we bought our boat last year, it had a not working holding tank setup with an old electric head.
The tank was mounted in the bottom of the boat and was a stainless 100 liter tank.

I have removed everything and want to replace it with a new smaller custom made fiberglass tank above the waterline to eliminate the pump out pump.
A fellow boat owner has made that setup in a similar boat and he is very pleased with it.

He used a manual toilet, but I want an electric toilet and have already bought the Jabsco Lite Flush because both my wife and I thought it looked much cleaner and easier to keep clean.

But now I wonder if the Lite Flush will be able to push the waste uphill to the tank.

Do you think the Lite Flush will work in this setup ?

Thx in advance.

 

[Peggie Hall HeadMistress]

According to the information in the data sheet, which you can download from here Jabsco Lite Flush datasheet. (click on "downloads" to get it), the Lite Flush can lift up to 5'.

Before having a custom fiberglass tank made, check what's available from Ronco Plastics Ronco Plastics marine Tanks They make TOP quality thick-walled water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank. So if you want a gravity dump tank, just specify the discharge fitting IN the bottom of it.

Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein

 

[jviss]

I have never heard of a gravity dump tank in a boat. Why would you want that, and how, and into what are you dumping the waste?

 

[Peggie Hall HeadMistress]

Gravity dump tanks are actually fairly common on blue water cruising sailboats. They make it possible to dump a tank without the need for y-valves or an overboard discharge pump and also to flush the toilet directly overboard without the need for a y-valve in the toilet discharge line.
The tank is a holding tank, same as any other holding tank, but it's installed entirely above the waterline with a vent line, same as any other holding tank, and a discharge fitting IN the bottom of the tank...the discharge line goes directly to a thru-hull. The toilet always flushes into the tank. When the thru-hull is open, the flush just keeps going down the discharge line and out of the boat. When the thru-hull is closed, the waste is held in the tank, same as it is in any holding tank. The tank can also be pumped out, either using a y-valve in the discharge line with the line going to the deck pumpout fitting, or a separate dedicated line from the tank to the deck pumpout.

Gravity dump tanks have definite advantages, but also some disadvantages. Since they have to be installed entirely above the waterline, it almost always means that the toilet discharge line is an uphill run...sometimes a STEEP uphill if the tank is very close to the toilet. Inadequate flushing in the dry mode can leave flush water or even bowl contents in the line to run back down to the bowl. If the tank is used for very long in the holding mode without rinsing it out, sludge can build up in the discharge line and pack it. So the distance from the toilet matters and they system does require a bit of maintenance. So gravity dump tanks are simple, but they're not for everyone.

Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein

 

[jviss]

Wow, @Peggie Hall HeadMistress , thanks for the detailed description. I never knew. I'm still trying to wrap my mind around this. All of this to avoid having a macerator pump or waste pump to empty the holding tank? Seems one is trading the occasional pump overboard issue for a daily pump uphill issue. I certainly wouldn't modify an existing design or installation to avoid adding a pump. I don't understand the OP's intended operating mode. If you can pump overboard, why not just flush overboard?

 

[DocSjulle]

Hi Peggie.

Thank you very much for your reply

I checked the datasheet last night and couldn´t find the info about how high it can lift the waste, but today looking closer I found the 1,5m / 5' specification.

I´ll make a test setup this weekend with a piece of hose to see how much is remaining in it after flushing, and I´ll check out the Ronco Plastics site for a suitable tank.

 

[Peggie Hall HeadMistress]

If you can pump overboard, why not just flush overboard?

A gravity dump tank allows you to do that by simply leaving the thru-hull open.
The whole point of having a gravity dump tank is simplified plumbing--one line with no y-valve from the toilet to the tank, a second line from the tank to the thru-hull...the seacock provides a choice of holding or flushing overboard--close it to hold, leave it open to flush overboard. A second discharge line--or a y-valve in the line from the tank to the thru-hull--can be added where it's necessary to also have the ability to pump out the tank (all US waters)...but in coastal waters with easy access to open sea beyond the "3 mile limit," it would rarely be used.

You'll rarely find a gravity dump tank on a "weekend warrior's" boat...they're mostly found on boats that spend most of their time away from the US. The simpler the system, the less maintenance required and the fewer spare parts needed...something that's very important to cruisers that may stay away from the US for 2-3 years or even longer, 'cuz having parts shipped all over the world or buying them in foreign ports can be VERY expensive!

--Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein

 

[DocSjulle]

The idea is to get rid of the tank in the bottom of the boat, because the space would be great to store food and other stuff.
It will also greatly simplify the setup, with a lot less pipes and less possibility for leaks and odors.

We will leave the seacock open all the time, and only close it during stays in the harbor, or when using the head close to the coast.

The old setup had about 7-8m of waste tubes and 3m of vent tubes. Combined with an old diaphragm pump it took up a lot of space and smelled really bad.

Wow, @Peggie Hall HeadMistress , thanks for the detailed description. I never knew. I'm still trying to wrap my mind around this. All of this to avoid having a macerator pump or waste pump to empty the holding tank? Seems one is trading the occasional pump overboard issue for a daily pump uphill issue. I certainly wouldn't modify an existing design or installation to avoid adding a pump. I don't understand the OP's intended operating mode. If you can pump overboard, why not just flush overboard?