Water tank connections

[LankyStreak]

My narrowboat has its water tank beneath the well deck floor in the bow, just behind the gas locker.  I was told when I bought the boat that it's a stainless steel tank but there's no way to verify this.  However whenever we fill the tank there's always one or two loud 'boom!' sounds as presumably the tank walls bulge out under the weight of the water.  Then later, after we've used the first few dozen litres of water there's another boom, presumably as the tank sides revert to their usual profile.  I think this was described in another topic on this forum but I can't find it now.

 

Anyway lately the water tank has been going boom! repeatedly for no apparent reason.  When having a shower it might boom 5 or 6 times, then afterwards when the water pump isn't even running the tank will randomly boom again several times.  Today, after three days of cruising which probably resulted in around six showers and several sinkfuls of water for shaving and washing up, plus many pots of tea, we stopped at a fairly slow (i.e. low pressure) water point.  I put the hose into the filler on the gunwale, opened the tap, and almost immediately the water was overflowing out of the filler and into the canal.  The tank was definitely not full so I pushed the hose deeper into the tank.  This resulted in a mini-Vesuvius where an air-lock (I assume) caused water to gush up out of the filler all over the side of the boat, almost to the roof.  However after doing this the tank was again overflowing out of the filler.

 

The tank has two gunwale-mounted fillers, one on each side of the boat, as I think is fairly normal.  In addition on the right side of the hull is a vent hole a few inches lower than the filler (see photo).  When filling the tank this vent usually gushes with water a few seconds before the filler overflows.  I assume it also admits air into the tank as water is consumed.  I have no idea how these tank connections interact but in an attempt to increase my understanding of the problem with my overflowing but empty tank I inserted a screwdriver into that vent.  I didn't encounter any obvious obstruction or resistance but there was an immediate hissing of air out of the vent so clearly I had broken some kind of seal.  After the hissing stopped the overflowing from the filler also stopped and the water tank filled normally.  However when the tank was full the flow of water from that vent was only a trickle compared with normal.

 

I suspect there's some kind of blockage still in there but I can't think how to free it.  I did try pushing the hose from the water tap into that vent to let it push any blockage back into the tank but although there was a flow of water into the tank it didn't seem to increase the flow out of the vent when I reinserted the hose into the filler.

 

Could anyone please explain to me where the two filler pipes, and the vent pipe, go in a typical water tank?  Do they all just feed into the top of the tank?  Without access to these pipes, which must be all behind the cabin lining, how can I clear any blockage in the vent?

 

Thanks.

 

[Guest]

Not sure how two fillers will connect as mine has only one. Our deck filler joins the tank with a simple flexible hose, as does the vent. I use a trombone (snake) cleaning brush for clearing pipes such as that vent. Just a small brush on the end of a long flexible wire.  Plenty on ebay.

Edited July 15, 2019 by Guest

[Tony Brooks]

All three pipes should come from the top of the tank. 2 larger ones for your fillers and one smaller one for the breather. However it is possible that the pipes from the two fillers meet in a T beside or above the tank so you only need one filler pipe in the tanks. All three pipes should have virtually no "stand down" in the tank. The booming is not uncommon as the tank fills and empties but normally only as yours used to be.

Edited July 16, 2019 by Tony Brooks

[nicknorman]

As you say, the problem is a blocked vent. On our boat there is a detachable panel behind the steps that gives access to the pipework. I would imagine yours is the same. It would be odd to totally seal in the plumbing. 

[Yellowback]

Name builder and age, then maybe someone with a similar build may assist you.

[Boater Sam]

Classic blocked vent syndrome, its getting a vacuum in the tank as you use water. And pressurising as you fill.

Leave one of the fillers loose, if the problem stops then that proves it.

I would guess that the vent is connected to the tank top by a hose which has kinked or collapsed.

Try inserting a length of say 10mm nylon /plastic pipe down the vent  for a couple of feet, if you can get it through it will act as a vent inside the existing hose.

[Guest]

4 hours ago, Boater Sam said:

Classic blocked vent syndrome, its getting a vacuum in the tank as you use water. And pressurising as you fill.

Leave one of the fillers loose, if the problem stops then that proves it.

I would guess that the vent is connected to the tank top by a hose which has kinked or collapsed.

Try inserting a length of say 10mm nylon /plastic pipe down the vent  for a couple of feet, if you can get it through it will act as a vent inside the existing hose.

I had forgotten until I read that, but same happened to ours many moons ago.