Holding tank venting

[thinwater]

Locating one tank higher than the other will also increase the static pressure on the lid of the lower tank by about 1 psi for every 2 feet of waste. I don't know how the tank was designed, but the lid may not like this.


Just somethin' to consider. If the lid is glassed-tight or well boated, probably a non-issue. If it gasketed, this would be a good time to consider if the gaskets need renewed.

[GordMay]

Originally Posted by nickbrown

... So in your diagram 1 illustration, if the vent was lower than the water level then the contents would empty to the height of the vent.

In my set up, my first tank, the funnel one, would be above the cylindrical one. If the vent to the lower cylindrical one is below the funnel tank then the funnel tank would not hold any liquid. Do my physics make sense to you?

Exactly.

[peghall]

Nick, you keep using the term "vent thru-hole" which I've been interpreting to mean "vent thru-HULL" (the hole in the hull of the boat through which the vent line exits the boat)...but it's just occurred to me that what you're calling a "vent thru-hole" may actually be the vent fitting in the tank wall. If so, it changes the entire discussion.


--Peggie

[nickbrown]

Hi Peggie, yes, apologies, it is a vent thru hull not vent thru hole, well it is a hole too in the hull side but sorry for the confusion there.

[ramblinrod]

But a bigger question, why the need for a larger holding tank?

Is that the best use of premium space?

If one marina hops, the holding tank needs to be sufficient for one day.

If one anchors out over the weekend, the holding tank needs to be enough for two days.

If one lives aboard, the holding tank needs to be enough for a week.

For ocean passage making, direct discharge.

I don't recommend storing more than a week's worth aboard.

30 gallons of sanitation tank capacity should last a family of 4 about two weeks.

So this means pulling into a marina once per week in NDZs (easy enough) or heading outside the 3 mile limit once per week (also easy enough) with a week of reserve capacity should bad weather prohibit moving to empty the tank.

[peghall]

I'm even more confused now. No argument with Gord's drawing...but there shouldn't BE any liquid in either vent line TO equalize...no way that either vent line should be able to transfer tank contents from one tank to another. Whether either vent line exits the HULL above, below or parallel to either tank shouldn't have anything to do with it.

I've also had time to wonder why you're doing it this way instead of just replacing your existing tank with a larger one. Ronco Plastics (no relation to the VegOmatic Ronco) Ronco Plastics marine Tanks makes TOP quality thick-walled water and waste tanks (they made tanks for SeaLand...I'm assuming they still do for Dometic) 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 they should have one the right size and shape for your space.

--Peggie

[nickbrown]

Hi Peggie, I would love to just have one tank, it would make it much simpler. But unfortunately the existing tank is glassed into the keel and not accessible and we need to use that plus another one to make up the capacity.

No plans to do any contents transfer through the vent lines

Essentially I think the easiest way to think of my set up is that it's one tank, but with a very skinny middle bit. The skinny middle bit being the hose connecting the top tank to the bottom tank.

[Red Sky]

Nick, If I’m picturing it correctly, I’m thinking an hourglass type of set-up? If so, I’d think the lower tanks vent would have to be above the full level of the upper tank to prevent waste from exiting as the upper tank fills. If you vent only the top tank, I doubt the lower tank, prior to being full, would get the fresh air exchange need.

How about installing a y-valve to direct the waste to the tank of your choice? Of course you would have to monitor tank #1 and change the flow to tank #2 accordingly. You could then connect the tanks with a valve to use for pump out or install a deck fitting for the second tank. ???

[nickbrown]

Thanks, some good points there and your idea on the lower tank vent position is what I was thinking about doing. A pump out for each tank could be a good idea too.

[ranger58sb]

Originally Posted by ramblinrod

If one lives aboard, the holding tank needs to be enough for a week.

I don't recommend storing more than a week's worth aboard.

Why would you say that?

-Chris

[nickbrown]

Ramblinrod, re the tank size, we stay on the boat sometimes for more than a week and the existing tank fills up quite quickly so we wanted extra capacity to go a full week between pump out's if needed.

[ramblinrod]

Originally Posted by ramblinrod
If one lives aboard, the holding tank needs to be enough for a week.

RR: I don't recommend storing more than a week's worth aboard.

Originally Posted by ranger42c

Why would you say that?

-Chris

If one installs sanitation tankage for greater than one week (leaving 100% reserve) then they are needlessly consuming very valuable space for waste.

Most need to pull off the water at least weekly for provisioning, water, a meal out, or just to get some land time off the boat.

So when topping up water and fuel is a great time to get a pump-out (in NDZs).

Else they will likely be on the move at least once per week and can discharge beyond the 3 mile limit.

Weekend warriors don't even need that much if they pump out every Sunday before returning to slip.

For those who swing on the hook indefinitely, they need all of the storage space they can get for provisions and stuff; increasing sanitation tankage just takes up too much room that can be better used for other things.

I normally recommend 1 week of capacity for the normal crew compliment with another week of reserve capacity, should weather prohibit emptying the tank after the first week for a few days, and still have a margin of safety.

Also the more stored aboard, the greater the risk and mess from a tank, hose, or fitting failure.

[ranger58sb]

FWIW, I think your "most" focuses only on a single segment of the market.

A week's capacity would cost us $20/week to pump-out (at our own marina), and we'd have to move the boat each week to do that (to get to the pump)... even when we're simply spending time aboard in our slip. (Pumping out 4x the amount would cost the same.)

We can carry enough supplies for about 2 months aboard, without really taxing our storage spaces. We usually top off fuel once every 3 months or so.

I think I'll carry on with our additional holding tank capacity.

-Chris