Venting a waste tank

[RIWATER]

I recently purchased a boat and the former owner had installed a waste tank without a vent. He had capped the vent outlet. The tank was empty but it still smelled bad. Out of the water the smell has gone.

I have been looking into vents and have not found a nice looking one. I am thinking about a plastic through hull with a clamshell covering. I cannot vent through a stanchion because my stanchions are removable.

I had one suggestion to add an in line carbon filter (Expensive)and let it vent into the boat. The filters are supposed to remove all odor. Any ideas would be helpful.

[Willin']

Venting inboard is always a bad idea, even with GAC filters. Eventually the charcoal loses it's adsorptive capacity and you have methane and hydrogen sulfide accumulating below. Even if you change the GAC regularly, it's still a bad risk.

I'm in the same situation with my boat and have been looking around for an outboard vent that won't look obtrusive or wind up underwater when heeling. So far, I'm stuck. Guys?

[mmd]

Regardless of what filter you install in-line, DO NOT vent a holding tank into the interior of the vessel. The vent has to be outside of the living spaces of the boat. Locate the vent on the side of the hull or superstructure in a place away from opening ports, hatches, and the cockpit so that the odor doesn't enter the living spaces. If the vent opening is located in a place where water might splash in while underway, put a check valve in-line or run the vent hose to the through-hull fitting in an inverted 'J' to prevent seawater from filling the tank.

[ssor]

I vent inboard with a respirator cartridge for a filter directly below a solar powered cowl vent. My holding tank is only twelve gallons so I have a surge tank in the vent line with a sight tube along side of it. In six years with this arrangement I haven't had any odors in the boat from the vent. Sometimes if the head wasn't flushed completely that will raise a stink. I believe that keeping the boat well ventilated is the most important boat keeping effort that you can make.

Ross in Bel Air

[FSS172]

If you want the vent itself to be completely hidden in something shippy-looking that will fit in with other on-deck elements, how about constructing a dorade-style vent box that doesn't open to the interior and terminate the vent line inverted inside the box? Pack the line with crumpled mesh or screen to keep bugs from blocking it and so air will only flow from the tank when it's being displaced, and odors outside should be minimal especially if it's located carefully as mmd suggests. Jim

[seo]

Venting a sewage tank inboard seems like a really bad idea.
There's no reason not to vent out through the hull, even on a boat that heels deeply. So what if the vent goes underwater? As long as the vent line has a riser loop that rises above the heeled waterline, water won't enter into the tank. The trick is to have the riser loop located toward the center of the hull. If you have a boat that regularly puts its lee rail underwater, then the loop might have to go up into the cabin trunk. If she sails with the cabin trunk underwater, you might consider a different boat.

Another workable scheme would be to have two vents, one on each side of the hull just below the sheer. A cross tube connects the two vents, with some rise (say at least two inches) in the cross tube. The vent tube from the tank is tee'd into the cross tube at the high point of the cross tube, near the center of the boat.
This is similar to an exhaust pipe arrangement used on some big sailboat, usually english. When the wind is on one side of the hull, the exhaust automatically discharges out of the downwind side.

There's no need for a big vent line or some gommy chrome fitting. A flared copper tube can be fitted into a countersunk hole in the sheer strake, and painted with the hull. A tiny (1/8") line would be adequate as a discharge vent. Depending on how the tank is set up to pump out, it may be necessary to have the vent big enough so that the discharge (macerator) pump won't be able collapse the tank.

Having the vent outboard on the hull (instead of on deck) makes it less likely the venting gas will blow on board and be noticeable, just because of diffusion. Probably venting aft is better than forward.

A cigarette is a useful tool when figuring out venting and air flow. The smoke make airflow visible, and the smell makes distribution of airflow sensible. An incense stick is just as good for people with a more highly elevated consciousness.

Still another approach to venting is to figure that methane is lighter than air (unlike propane, which is heavier). So you could run the vent line up through a mast or flagstaff or radio mast or something, and vent it up aloft. A plastic tube running up to the lower spreaders?
(Joke) Maybe the ideal arrangement would be to discharge into the mast , with the idea of filling the interior of the mast with lighter-than-air methane, which would contribute to stability! A pulse-jet could be fitted at the masthead which would add thrust at critical moments.

I once worked as engineer on a sailing research vessel (Westward), which ran its diesel exhaust inside the steel mainmast, up maybe 85' above the deck. Fumes and soot were not a problem! In certain conditions of engine speed and load, the exhaust would sometimes develop a very peculiar cyclical flatulant noise. Oh well. It was a german engine. Anfahrt!
seo

[Mike Vogdes]

Your vent line wants to be as short and strait as possible to outboard the hull and the largest diameter possible. A loop is fine if your concern is heeling. 1/8" is definetly NOT a good idea. 5/8" is the norm. Google Peggy Hall and you will learn all you want to know about marine sanitation.

[Presuming Ed]

Um, she spells it Peggie

http://boatbuilding.com/article.php/MarineSanitationFactvsFolklore

http://www.amazon.com/gp/product/1892399156/002-7435851-7476023?v=glance&n=283155&n=507846&s=books&v=glance

[Presuming Ed]

Um, she spells it Peggie

http://boatbuilding.com/article.php/MarineSanitationFactvsFolklore

http://www.amazon.com/gp/product/1892399156/002-7435851-7476023?v=glance&n=283155&n=507846&s=books&v=glance

[seo]

I don't know what's meant by "5/8" vent line is the norm..." Who established that "norm?" A line that size on a big (say, 200 gallon) tank with a 2hp discharge pump would be a pretty sure prescription for a collapsed tank.
The same vent line on a little tank with a manual discharge pump would be overkill.
Some sewage tanks are rigged to discharge overboard by gravity, and can make do with a very small vent.
Others (usually on freshwater lakes) are designed to only discharge to a shore pump-out. Some of those require a vent that's as big as the discharge line.
That, for example, is the "norm" on fuel tanks that are hard-connected to the fuel dock. If you're using a 2" delivery hose, you need at least a 2" vent. Anything less can cause the tank to rupture (surprisingly, hull tanks on ABS ships are tested to less than 3 psi. Obviously, when you have a 60 psi fuel hose connected to a 3 psi tank, you want to watch pressure. Also, when you're discharging a tank through a 10" line running at a thousand gallons a minute you want to make sure that the vents are wide open.
Anyway. The idea of going straight and short to discharge is a good one. A useful addition is to make sure the hose slopes up continually from the tank to a high-point, then down continually to the hull discharge. Make sure there are no low dips, because these can get filled with water and freeze in the winter, blocking off the vent line. On a fuel tank this can cause the engine to starve out. I once spent hours looking for the reason why a boat's fuel injector pump was air-locked. Plenty of fuel, no obvious hose leaks. Once bled out, the engine ran fine. In a moment of inspiration I took the vent line off at the tank, and poured a couple of ounces of water out.
It had warmed up, and the ice melted. When the vent was blocked off the transferpump pulled enough suction to suck air in through a hose connection, causing the air lock.
Live and learn.
seo

[Mike Vogdes]

Sorry, Peggie it is... She has a very good book on the subject.

[Shortman]

Peggie is the authority. She has a forum @ sailboatowners.com. Do not block vent w/charcoal & do vent outside hull. She preaches the gospel of airflow. I have two vents for one tank. Both "clamshell". If you are heeled that far over, you are moving forward at a pretty good clip. Opening facing aft, I doubt clamshell vent will pull in any significant water, if any at all.

[RIWATER]

Thanks for all the helpful info! It looks like I am going to have to put in a through hull and cover with a clam shell. I dot have any other place to install it without it being completely visible from the inside.