Holding Tank Vent

tdw · #1 (**permalink**) 08-31-2010

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.

jrd22 · #2 (**permalink**) 08-31-2010

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.

sailingdog · #3 (**permalink**) 08-31-2010

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...

mitiempo · #4 (**permalink**) 08-31-2010

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.

mitiempo · #5 (**permalink**) 08-31-2010

Dog's suggestion is a good one - that is probably what I will do.

tdw · #6 (**permalink**) 08-31-2010

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 ?

mitiempo · #7 (**permalink**) 08-31-2010

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.

SlowButSteady · #8 (**permalink**) 08-31-2010

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.

tdw · #9 (**permalink**) 08-31-2010

Quote:

Originally Posted by mitiempo

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.

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.

mitiempo · #10 (**permalink**) 09-01-2010

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.