Onboard water supply puzzle

[F104G]

Hello all
my first post, so I hope it's in the right place.
I have a 150 litre flexible water tank on board. It works fine (supply to shower, wash basin, water heater & galley) when it's full but not when it gets past the half-empty point. The supply at all taps is very spluttery and I hear loud gurgling noises throughout the boat. This always happens - at first I thought the tank was getting empty but that is not the case. It always starts to happen once I have used about 70 litres.
So, is this "a thing" with flexible tanks or has mine been installed incorrectly in some way?

 

[Graham376]

Sounds like the outlet is badly positioned and you're sucking air.

 

[macd]

Sounds like the outlet is badly positioned and you're sucking air.

That could be the case. Flexi tanks can also take in air when filling, especially filling quickly: the tank seems full when prodded, but not everything that's in it is water... Some are fitted with an air bleed for just this reason. If you have one, it may be automatic (-ish), or even a small manual plastic valve.

P.S. Welcome to the forums, F104G. You'll get a lot more hits on a straightforward practical issue such as this on the Practical Boat Owner's forum.

 

[F104G]

Thanks Mac & Graham.
I will have a look for that bleed valve.
I have no idea where the outlet is - need to get myself a decent torch and have a rummage

 

[duncan99210]

I've found that if you don't screw the filler cap on so that it seals, then the tank doesn't collapse as you draw water from it. Rather, you pull air in leading to the gurgling spluttering tap syndrome. Check the o ring sealing the filler cap and make sure it's on tight.

 

[VicS]

I've found that if you don't screw the filler cap on so that it seals, then the tank doesn't collapse as you draw water from it. Rather, you pull air in leading to the gurgling spluttering tap syndrome. Check the o ring sealing the filler cap and make sure it's on tight.

I think you can divide flexible tanks into two groups.

Pillow tanks, as the name suggest pillow shaped, which will completely flatten when MT They do not need a vent and the filler cap should be air tight. Both filler connection and outlet may well be on the top.

Shaped tanks. Ie with top, bottom and sides. These are best used like ordinary tanks with a vent and the outlet on or near the bottom. The sides , esp of large ones, may be supported so that they do not collapse.

 

[F104G]

Thanks duncan99210 and VicS for the input.
I need to have a proper look but I think (from memory) that it's a pillow-shaped jobbie.
I'll be on the boat next weekend so I now have a list of things to look at.

 

[F104G]

Well, it was blown up like a space hopper :-D
Unscrewed the filler hose at the tank end and a huge rush of air came out. Once I had flattened the tank I reattached the filler hole and everything seemed OK. No more spluttering. I will be back on the boat again next weekend so I will take a look to see if it has expanded or not.
I'll keep you posted.

Update: the tank is behaving itself now. No gurgling or spluttering and I'm getting twice as much water out of it
Thanks for the help everyone.