Holding tank vent lines

[Larry Bliss]

Peggy, I've been reading your advice with great interest, but am not clear on a couple of points about your vent line recommendations. 1. I envision two 3/4" to 1" thru hulls on opposite sides of the bow. What keeps sea water from filling up the holding tank?2. Are check or ball valves necessary?"We prefer to put holding tanks in the bows of sailboats--under the v-berth--because the hull just behind the point of the bow is the only place on the hull except the transom that will never be under water even when the boat is at maximum heel; it’s the perfect place to install vent-line through-hulls, because the though-hull is always into the wind, forcing air into the vent line, when the boat is underway or on an anchor or mooring. The vent through-hull should not be the same type as a fuel vent through-hull (a cap with a slit in it), but should be a straight open "bulkhead" through-hull."

 

[Peggie Hall/HeadMistress]

Splashing doesn't put much if any water...

down a vent line. Even decks awash (unless they continuously stay that way for hours) won't put much water down a vent line. If you've read my repeated advice to keep all seacocks closed except when the device at the other end of the hose is actually in use, you know the effect that "ram water" can have--overflowed toilets and sinks--if they're left open when a boat's underway. That same effect can very easily happen if the vent through-hull is in a location on the hull that can stay submerged when the boat's heeled...but if the vent through hull is far enough forward, it stays out of the water and is only subjected to splashing. There's enough backpressure in the tank to keep all but a few drops from going down the vent when water breaks over the bow. The last thing you'd want to put in any tank vent line is one-way valve...you'd defeat any exchange of air in a holding tank vent line, and even in a water or fuel tank vent, air has to travel in both directions at some time--out when the air in the tank is displaced as the tank is filled, in as the water or fuel is drawn down. However, if you're a passagemaker and can find yourself riding out a 3 day blow at sea with decks continually awash, by all means put a shut-off valve in the holding tank vent line. You won't be using the tank at sea, so you don't have to worry about pressurizing it... It will get "ripe,"...but when the storm is over, you dump, rinse and that takes care of that problem.