Tank vent basics
Dear all,
maybe someone can help with a thought -- I am trying to understand the consequences of the non-original tank vent layout on our 48.
The original four tanks are all under the floorboards. The tank vents are all on the top side of the tanks, going up with some horizontal stretches to the vent outlets in the sink; there is no bend where a water pocket can build up.
When we bought the boat, the former owner had installed an additional freshwater tank under the port settee in the saloon. The tank is connected to one of the stainless steel tanks, using the original vent tube to fill: water comes in through the filler tube, fills the lower original tank, then overflows through the former vent tube which is connected to the bottom of the additional tank (sitting in the settee above the floor boards, the bottom of the additional tank is above the top of the originals). So far, no problem.
However, the former owner led the vent tube of the additional tank from its top side connection downwards, then along the side of the boat to the sink and up again to the outlet. With that layout, there is sure to be a water pocket in the vent tube. The owner advised me to always open the inspection plug when filling the tank so air could get out. Which I always did. Without thought.
As I am thinking about changing the layout, I started thinking about the consequences of the current one. While it is true that opening the inspection plug lets all air out of the tank while filling, the vent tube still probably has a water pocket: as soon as the boat heels, the tube will certainly fill as the tank water overflows into the vent and collects in the low part.
So, assuming that the pump is strong enough to empty the tank (and so far it was), would it be possible to leave the current layout, accepting that there is a water pocket, and just stopping to open the inspection plug? In that case, while filling the compressed air should push out the water pocket until the vent tube is completely full. And while using water from this tank, the pump will slowly build up a vacuum in the additional tank ... until the low pressure in the tank is strong enough to suck the water from the vent pocket back. After that, things should be like they are now. Or am I missing something?
Nice way to spend a saturday ...
Best, Martin (Age of Swan, 48/039)

Dear Martin
Odd solution for the vent. If you could re-route the vent tube from the additional tank to the sink so it is without pockets the vent tube would work as intended, and overflow into the sink when the additional tank is full.
An additional comment for this arrangement - the water head in the additional tank will cause the content to attempt to drain into a lower tank if you keep any of the other tank valves open. The lower tank's vent tube will then fill to the level of the additional tank, but as this is lower than the sink it will not overflow when the yacht is upright. But heeled with a full tank on the high side there will be overflow into the sink, and it is advisable to keep only one tank valve at a time open.
Wishing you a pleasant Saturday evening
Kind regards
Lars

Dear Lars,
thanks as always!
As we have no tank gauge installed, we have ever only one of the tanks open, and the other valves are closed -- that way I keep an eye on consumption. Of course you are right to point out the heeling effect with interconnected tanks ... I had not thought about that, but then, didn't need to.
I will try to figure out if re-routing is possible. The top of the additional tank is almost settee level, that is, higher than the bottom of the fridge. So to reach the sink without drop in the tube, I will have to route around the fridge, pass into the aft head, and then lead back into the pantry area. Hope that no other project claims priority ...
Thanks again,
Martin

Dear Martin
If the re-routing to the sink causes trouble there are alternatives.
You could simply terminate the vent tube high up in a cupboard near the tank, and keep an eye on it when filling, with a transparent canister for collecting the overflow. If the tube end is high enough heeling will not cause overflow.
Or would the toilet wash basin be easier to reach? It is not necessary to install a fixed drain on the basin edge if a flexible vent hose is kept in the toilet locker, and pulled out into the basin when filling water. One more thing to remember though.
Adding a small drain at the lowest point of the present vent tube would allow it to be emptied before and after filling, but you indicate that heeling also fills it, assume this happens as long as the tank is nearly full.
You could also use the vent arrangement used for fuel tanks with a hose across the main bulkhead, this will lead the overflow to the outside, and is easier to see from the water filling station on deck.
Kind regards
Lars

Lars -- you are a genius!
Thanks, Martin