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Holding Tank Vent

7K views 21 replies 8 participants last post by  chall03 
#1 · (Edited)
How do you vent your holding tank ?

Suggestions for how to cap the vent above deck would be appreciated. I'm concerned about water washing over the deck simply filling the tank through the vent pipe.

Also, how important/advisable is a macerator ?

The Womboat has a Lavac head. The Henderson pump on the Lavac performs as near as damnit as a macerator anyway, so I'm wondering whether I need the actual macerator itself.
 
#2 ·
td- our vent is on the outside of the hull, near the top and a bit ahead of midship. It isn't submerged when well heeled over although I'm sure that waves wash over it regularly in heavy weather. It has one of those fittings that has the cover over it and is open toward the stern. Hope that makes sense, I don't have a picture of it.
 
#3 ·
Hey Fuzzy-

I would recommend using a 1" hose for the vent line, per Peggie Hall's recommendations. Terminate the fitting as high on the hull as you can as close to the holding tank as you can using a standard 1" through-hull and cover with a stainless steel clamshell vent cover.



As for a macerator...not really needed. I would recommend plumbing the head directly into the holding tank, then adding a DIVERTER VALVE to the holding tank pumpout hose, and leading one side of the diverter valve to the deck pumpout fitting and the other to a Whale/Henderson Mk V diaphragm pump and then to the seacock and through-hull. This will allow you to dump the tank when out past the three-mile limit...
 
#4 ·
My vent is in the hull just under the deck edge, about amidships. It is mounted so the opening is down and aft. It's like the one below currently but I may change it to a larger one for the new holding tank which will be gravity draining. It will be mounted at the back of a hanging locker just under the deck and everything will go into it so no "Y" valves will be needed. I also have a Lavac and am sorry I didn't change to one sooner.
 

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#6 ·
ah..OK then....vent in topsides not in the deck. That's good to know. Dog's vent idea, which I guess is what you are talking about JRD, looks good.

mtiempo.....how far above water line is your tank ? When you say gravity drain where is the through hull ?
 
#7 ·
The through hull will be right below the tank. As everything will go into the tank first by leaving the through hull open it is in effect a straight pumpout. The tank will be mounted a few inches under the deck and its bottom will be above the waterline by 6" or so. The deck pumpout will be directly above the through hull so if there is ever a clog a rod can be pushed through to clear. I will finish when I haul out and install the through hull.
I have come across many installations like this from a Vega 27 to a Nicholson 35 as well as others and nobody has had a problem with it. And it couldn't be simpler.
 
#9 ·
Sounds good to me. When you say "under deck" where is the thing actually located ? Forward head, outboard of that ?

I'm planning on ours going into our cockpit locker. The VDS 34 has an aft head not forward btw. I would prefer to avoid another through hull though.
 
#8 ·
The idea behind a large diameter vent hose is to allow simultaneous bidirectional circulation (in and out) via one vent. Smaller diameter hoses tend only allow unidirectional flow (in or out) at any one time. The other option is to have two small vents (preferably placed on opposite sides of the boat) so that air will follow any pressure gradient and flow through the tank, keeping it well oxygenated. I prefer the two small vent option, using vent thru-hulls like the one in Brian's post (post #4) above. The vents on my boat are on the side of the cabin (both on the same side actually, just angled a bit differently), rather than the hull. This avoids any chance of water siphoning into the tank (or worse) when the rail is buried. If you do put the vents on the hull, I would place them either close to the bow or on the transom as those are two areas unlikely to be submerged for any length of time.
 
#10 ·
Yhe head is forward on the port side and the tank is going in the starboard hanging locker which is pretty deep. I will still have a good sizes hanging locker. Unfortunately I can't put the tank on the same side as the commode due to the structure already in place.
 

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#11 ·
Wombat,

Sounds like we have a similar configuration on our boat to your planned arrangement. Aft head, holding tank in cockpit locker, etc. Our holding tank vent exits at the transom. We have the same fitting as JRD, shown in mitiempo's illustration.

There's a bit more to our plumbing than the standard configuration. We can divert (Y-valve) to direct discharge without first going through the holding tank. For simplicity sake, I prefer this approach (although in actual fact we can't use it except when more than 3 miles off-shore).

We also have an electric macerator pump for pumping the tank out when off-shore. I have mixed feelings about the electric pump -- seems to me a manual Whale would work every bit as well and more reliably. However, the electric PAR (or is it Jabsco?) pump is very compact. The Whale takes a lot of space and needs a location where the handle can swing through the full arc. Our electric macerator pump is tucked away on an overhead and takes up no space -- helpful on a compact boat.

In my opinion, the single most important element of head plumbing is the size of the holding tank. Bigger is better.;)
 
#12 ·
Hey TD,

Had a nasty enocunter with the good folk from NSW Maritime of late have we ??? :)

We belong to a yacht club, and like all good bored well-intentioned, over regulated bureaucracies in addition to insisting that now that all members boats meet Cat 7 despite whether they are raced or not, last year they also decided to make sure that we were all compliant with discharge regs despite the continuing limited pumpout facilities in Pittwater.

Correspondingly we have fitted a holding tank, using a turtle-pac flexible tank, vented above waterline forward(where our head is) and using a Whitworths purchased fitting identical to what SD detailed.

I do agree with JRD that the Y valve is good for when you are offshore.... Alternatively however you can set up a valve post tank so that the contents of the holding tank can also be simply 'direct discharged' when offshore....

It isn't pleasant subject matter to ponder, but if the womboat is halfway up the QLD coast, the ability to empty a holding tank offshore in accordance with discharge regulations would be preferable than a 150nm mile passage to civilization merely to 'pumpout'.
 
#13 ·
Chall,

Ha bloody ha......:p:p. No mate we haven't had a run in with maritime as yet and if we did we have the chemical on board. Perfect compliance unless caught in the act. :eek::eek: Nonetheless its a damn nuisance to lug around and simply takes up otherwise useful stowage space, whats more you don't want to know about emptying the damn thing. As for pumpout facilities, you know and I know just how pathetic a situation that is. You only need to follow a couple of stinkers out of Smiths or down here in the Lane Cove early in the morning to understand how the locals cope with the problem.

We'd definitely install a Y valve and it will be a solid tank. I'm afraid the bladder type scares the sh.....well you get my drift. I've also never been on board a boat with an oldish (more than a year) bladder that didn't smell.

Maritime is pretty slack in Sydney Harbour but end of year we will either be heading up to Port Stephens or down to Jervis Bay and possibly Eden. Maybe even Tassie. We need to get this sorted properly.

John,
How big is your tank ? I'm getting somewhat concerned about having no space left in my cockpit locker. :eek: I reckon I'll have two y-valves as well. One for direct discharge/to tank the other tank discharge at sea/discharge at pump out station. Lava uses Henderson which is almost a macerator in itself so probably another on of those. Vent to or near transom. Vent pipe will be two metres or so if I do that. I thought shortest possible was best but if vent comes out alongside cockpit what about odour ?

I cannot see at this stage that an instalation that would allow gravity through hull discharge is workable for us.

btw...has anyone seen latest Good Old Boat ? The tube affair has definite possibilities.
 
#14 · (Edited)
Andrew,

We have a 16 gallon (U.S.) tank. I think that translates to a little better than 60 liters. Using it carefully, our family of five can get by for 4-5 days. I wouldn't mind one that was double that size. But we are only 31 feet...

Our vent is aft of the cockpit, on the transom, so at anchor it is downwind. We have never had odor issues (at least not from the vent -- my kids might say differently when I'm using the head).

A single Y-valve should do it for you. This is placed immediately downstream of the toilet, to allow selection of direct discharge or holding tank.

You do not need another Y-valve downstream of the tank, all you need is a through-hull through which the macerator can eject the contents when legal to do so. The macerator pump is of course "downstream" of the tank. Open the thru-hull, turn on the macerator pump and let it do its work.

The same line out of the tank (downstream) that feeds the macerator, also has a "T" upstream of the macerator pump. The other part of the "T" feeds the deck pump-out fitting. With the deck pump-out capped airtight, the macerator pump only sucks from the tank. Similarly, the line downstream of the macerator is air-tight, because the through-hull where it terminates is closed. So when pumping the tank at the deck fitting, the macerator line is not an issue either. I.e., the deck-pump out can only "draw" from the tank, which is vented.

I hope that makes sense?

One other important component is an inspection port on the holding tank. I would not be without one -- at least 4-5 inches diameter.
 
#15 ·
I agree with John.

Our vents are both Midships. We do not have a Y valve (yet) between the tank and the head. We use two Jabsco Quietflush Heads which both have macerators. I might caution that after some time you may get some sludge buildup in the tank so for that reason, I think the installation of another macerator for tank discharge is helpful. The cost is minimal anyways and now is the time to do it.

I would also highly reccomend putting in a tank monitor of some type before the first use! Nothing worse than the tank getting overfilled and the overflow finding its way out the vent tube!!! And remember that the tank has to be very tight because methane seems to be heavier than air and even the smallest leak can fumigate a boat. Spend the extra money on very good hoses and be thoughtful of running the head-to-holding tank line in a way that it is removeable and serviceable (if possible). After some use you may get calcium buildups in it that can cause clogs.

Anyways, there are my thoughts. Luckily my crap doesn't stink so much of this is uncessary for me. Like John, though, I do have kids...

B
 
#16 ·
The reason I recommend adding a diverter valve to the pumpout line on the head is to protect the macerator or diaphragm pump in the case that you've left the seacock open. The vacuum on a decent pumpout station can be fairly significant and the water trying to force its way back through the pump under the pumpout hose's vacuum can damage the pump.
 
#17 ·
I guess folks would have to consider whether they'd be more likely to leave the thru-hull open, or the second Y-valve in the wrong position. I would reason that adding the Y-valve adds another variable and therefore increases the possibility of a mistake. Your mileage may vary.;)
 
#22 ·
I had to check in on this thread while eating my lunch didn't I???

For what it's worth TD my flexitank has remained odour free so far(been in there about a year), it is the proper, designated holding tank model however, if one was to use the standard freshwater flexitank as a holding tank I can see how it might be an issue. Mind you ours has had light use as we are often utilizing the direct discharge option*...............
*NSW maritime this is of course because we spend most of our time the required 3 nautical miles offshore......despite there still being no pumpout facilities whatsoever in the area......
......and of course being offshore yes we do also carry your knee-jerk reaction....poorly thought out, newly mandated EPIRB .........oh and we thoroughly enjoy the 200% increase on mooring fees, the lack complete lack of facilities and lets not say nothing of your ridiculous no live-aboard mandate....
 
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