Plumbing Short-cut / error. Is this common?

[papawads]

According to the owners manual, both the main 220l water tank and the 150l starboard water tank have independent breathing pipes (to allow air out when filling) which exit into the anchor locker.  On my boat, someone has merged the two with a T-joint (see pic) behind the access panel in the saloon.

This means that when one tank fills, rather than overflowing to the anchor locker, water comes up the breathe pipe and down the second breathe pipe, filling the second tank.  That screws up any calculations of water-purifier amounts for the second tank.

It’s likely an easy fix, but it’s worth others knowing in case this is a common short-cut.  Left as is, I’d be hugely over-treating (or under treating) the overflowed tank

[sailingabe41ds]

hi..
the only concern i have is that when the second tank is filling with water from the breather overflow and there is no breatger hose to ven and relieve the pressure it can expand causing damage or leaks.  a rigid tank needs a breather hose itself .  i did mine that way and the tank inspection port leaked from too much pressure. 
imho
abe

[rene460]

Hi Papawads, As I see it, the main disadvantage of combining the vents in that manner is the potential some contamination of the the tank not being filled, which may or may not be important to you, depending on your water sources and filling routine.  

If you are in any doubt, it is not a big job as you say, to separate the two vents and keep them independent as the manual seems to recommend.

In practice, I don’t think there will be much water flow to the wrong tank.  None until the tank being filled is nearly full, then some slugs of water lifted up the vent coffee percolator style, which, when they reach the tee, will go part to the second tank and part up the vent to the anchor locker.  How much each way will depend on your fill rate.  Very difficult to tell how much water goes into the second tank, and the importance will depend on whether you are filling from a dubious source and want to keep them separate.

In any case the maximum pressure will always be down in the tank being filled.  The maximum pressure at the tee will never be much above the height of water from the tee to the anchor locker vent.  The flow down to the second tank will be limited by the fact that air will have to come up past the slugs of water, while the water flows down, and the pressure in the second tank will never be more than the pressure in the one being filled.  I hope that is clear, I am sure you have seen air bubbles rise in a clear tube if you try and pour water down the tube.  Sort of in agreement with Abe, just a variation in the explanation.

Please let us all know what you decide to do.

rene460

[papawads]

Thanks for the thoughts guys.

Possibly because I tend to fill slowly, I find that once tank 1 is full and the water level reaches up the breather to the T, the whole flow then travels down the second tank’s breather pipe, until (as said) the air pressure in tank 2 is sufficient to block furher water.  When I then open the second tank, there a rush of air escaping etc.  

incidentally I also noticed that the fill pipes for these two tanks is also opposite to the owners manual, so clearly someone has been messing with this in the past.  Grrr... 

Come March, when I’m back on the boat (just home today) I’ll run a second vent pipe AND switch the filler pipes around.  Also, I don’t like the way that my filler pipes leave a small water residual in them, so that’ll be fixed too.

thanks again

:-)

[sunrise4]

One possible solution could be to extend the breather hoses from each tank with a loop up higher then the T and the hose to the exit into your anchor locker.  That way any overfill will drain out to the anchor locker before making it up over the loop into the second tank.  Unless there is a restriction into the anchor locker... or the hose into the anchor locker is too small to drain the overflow without backing up over the other loop... 

[rxc]

My boat is setup this way, as well. I have not had any problems with it, except for one time when we "made like a submarine" coming out of Strangford Lough. We buried the bow in one of the rollers at the bar and had green water from bow to stern, over the top of the bimini. One cat got a few drops of water on her because she was sitting under a zipper. The rest of us were bone dry.

The problem was seawater ingress via the vents up forward. All three tanks were contaminated and we had to drain and flush all of them, twice, I believe, to get rid of the salt.

The cat was not happy.

[andreshs1]

Hi there

mine has the same setup, as the 2nd breather is used by the forward tank.

Now when I fill the tanks I just open both tank caps on the deck, so issue resolved

cheers

[saltymetals]

Well you guys are a bit ahead of me. I realised there was a conflict somewhere in the breather tubes for the central tank and the starboard tank. In fact if the central tank is full it is deceptive when filling the starboard tank since it appears to be full very rapidly. So, while i have not traced the setup of the breathers it would appear they share the same one. Solution? I have numbered the filler caps so the outer one (stbd tank) is always filled first. This system seems to work ok.
Andrew