Fresh water tank vent plumbing

[mitchisword]

Hello CF,

I have my boat apart for a wiring project and now have great access to the fresh water vent plumbing. A lot of if is cracked and needs to be replaced.

Here is the scenario: we have 4 tanks, for this exercise we'll say we have two on port and two on starboard. The plumbing (3/8") originates at the forward tanks of each side, then it's routed (trough the unaccessable depths of the boats construction) to under the side deck, where it is more accessible. Then the aft tanks tee in and the hose continues to the vent fitting on the cockpit gunnel, port and starboard, respectively.

Here is the question: Can all four tanks be tied in together under the sole with one line running to a vent pipe (gooseneck like fixture) over the galley sink? My life would be a lot easier if this is the case, as all the original vent plumbing needs to be replaced, and I cannot get to some of it!

Perceived pros and cons:
-pro, ease of upgrade
-pro, venting overflow happens in the sink, where we can see it when we're filling tanks.
-pro, the runs would be much much shorter
-pro, I already have hole in the countertop for the new style vent!
-con, would there be too much resistance bringing four tanks into one 3/8" line? (which is technically twice as many as came to each vent before, but the lengths are shorter (by A LOT)).

Also, we're never filling more than one tank at a time, so the pressure from the one fill would overflow into the next tank in the line, or so it would seem. Drawbacks to this?

Am I missing anything?

Thanks for your help.

[slug]

A tank would typically have two vents

One big vent for taking on water

This vent goes over the side and is closed when sailing

The second vent is small , open full time and can be inside the boat anywhere that makes sense

A sink sounds ok

[thinwater]

From an engineering perspective, the vent need to be able to handle the maximum filling airflow (not liquid flow) without excessive pressure. You will only fill one tank at a time. The worst that will happen is that filling might be restricted.


Into the sink is a good plan. Fewer bugs, no risk of seawater in the tank.

[wrwakefield]

You described how our two 110 gallon water tanks are plumbed: two independent fills with combined vents that empty into the galley sink. [Our combined vent line has a high loop so water can't pour into sink when heeled...]

To avoid too much back pressure when filling from a pressurized supply, be sure to keep the vent into the sink the same diameter as the vent line... [e.g., don't restrict the size by using one of the small diameter water faucets commonly used for foot pumps...]

We have experienced no downsides with this set-up, and recommend it often. It is nice for knowing when the tank[s] are full— especially when running the watermaker... [And never having to worry about raw water or insects contaminating the potable tanks from an external vent...]

Cheers! Bill

[Toccata]

if the vent line is full of water, as I reckon it will be when the tank(s) is/are full;
Will the tank vent through that line while filling? or vent back up the filler so slowing down the fill rate?
Will the water pump create sufficient suction to empty the vent line so that it works properly (as a vent)?
A water works engineer or physics student can probably answer this. Possibly only a theoretical problem and it will work anyway....??
I am considering a vent hose rolled up and hung up high in lockers adjacent to the high end (in my case forward) of the water tanks, then while filling the tanks, unrolled and draped and secured into a nearby sink.
No external connection/potential source of contamination.
Simple

[slug]

Originally Posted by thinwater

From an engineering perspective, the vent need to be able to handle the maximum filling airflow (not liquid flow) without excessive pressure. You will only fill one tank at a time. The worst that will happen is that filling might be restricted.


Into the sink is a good plan. Fewer bugs, no risk of seawater in the tank.

Big vent. Many times you must take water from a ship , water tanker or 1000 litet cube

The volume per minute is huge

You need plenty of vent

[Hermia II]

In my boat, each tank has a vent coming out in a small tube in the kitchen sink. One for each tank. This is a Nautor Swan, and originally Sparkman & Stephens, design. It is perfect since you can see each tank filling. In your case, connecting all four tanks to the same tube is as good if you are filling each tank separately.

[GordMay]

Originally Posted by Toccata

if the vent line is full of water, as I reckon it will be when the tank(s) is/are full;
Will the tank vent through that line while filling? or vent back up the filler so slowing down the fill rate?

If the tanks are full, you won't likely be filling.

[Toccata]

To be more precise,
On an occasion when the tanks are completely filled, spilling into a vent pipe that has a "low spot" the water will stay in the vent pipe, blocking it.

Will this stop the vent system working on a subsequent fill OR in fact prevent the vent working to allow air back into the tank as water is drawn out??
Height/Weight of water column will probably be the issue.

[DefinitelyMe]

We have a similar system. One tank on each side plumbed into a single vent into the sink. It works but there are issues.

The plumbing for one of the tanks runs down through the bilge and then up again to the top of the tank. I can't see away around it.

Problem is that water collects at the low point, blocking the vent. We do two things to combat the problem:

1) installed a small valve in the vent line of the problem tank, which can be shut off when appropriate to prevent spill over via the vent pipe.

2) often have to suck or blow down the pipe to clear blockages.

Not a big deal, but not ideal as it requires some management.

[Hermia II]

Originally Posted by DefinitelyMe

We have a similar system. One tank on each side plumbed into a single vent into the sink. It works but there are issues.

The plumbing for one of the tanks runs down through the bilge and then up again to the top of the tank. I can't see away around it.

Problem is that water collects at the low point, blocking the vent. We do two things to combat the problem:

1) installed a small valve in the vent line of the problem tank, which can be shut off when appropriate to prevent spill over via the vent pipe.

2) often have to suck or blow down the pipe to clear blockages.

Not a big deal, but not ideal as it requires some management.

If possible to lead the vent pipe high, under deck, then problem solved (but I agree could be a lot of work).

[Toccata]

Original installation, My vents were run up to just under the deck, turned down and ran down to the bilge
Fill point is obviously at deck level.
Filling the tanks until the water runs out of the deck fill probably means water is running out of the vent pipe (inside the boat)
So I'm interested in solutions.
So far, putting the pipes into a nearby sink for the tank fill up, then lift them up to drain retained water back into the tank and coiling up the hose, hang it on a hook above tank level in adjacent locker, seems to address some/most of the issues????

[mitchisword]

Thanks for all of these replies. I've looked at routing options and can get all four vent hoses to the sink. I'd like to have each tank on their own vent but making something like that out of stainless sounds pretty custom!

For now I'll probably tee them all together but when galley remodel happens, I'll make them all individual vents.