Bit of theory about flexible water tank.

SiteSurfer

SiteSurfer

Well-Known Member
Joined
18 Jun 2015
Messages
1,262
Visit site
Heres an interesting bit of conjecture after a discussion in the pub tonight. I have a triangular Plastimo water tank in the bows, this is the type that has no internal suction tube but relies on the bag collapsing under pressure.

I have noticed that im not getting full recovery of the water in the tank and having to fill it very much more often than should be nessecary (its not empty). After discussion in said pub, the concensus of opinion is that my filler tube (which is quite a rigid thick walled affair) is actually responsible for this, holding the bag up and not allowing it to collapse.

Chaps have suggested that more flexible or lay flat tube would provide the option for the bag to collapse and thereby provide the ability for the pump to get a lot more water out.

So my thoughts are this: is that a valid point - would a more flexible (longer when needed) filler tube help or would I be better of adding an internal feed tube to the bag to assist the draw of fluid from the base of the bag.

Its this bag:
http://www.piratescave.co.uk/plastimo-triangular-watertank-120l.ir
 
SiteSurfer said:
Heres an interesting bit of conjecture after a discussion in the pub tonight. I have a triangular Plastimo water tank in the bows, this is the type that has no internal suction tube but relies on the bag collapsing under pressure.

I have noticed that im not getting full recovery of the water in the tank and having to fill it very much more often than should be nessecary (its not empty). After discussion in said pub, the concensus of opinion is that my filler tube (which is quite a rigid thick walled affair) is actually responsible for this, holding the bag up and not allowing it to collapse.

Chaps have suggested that more flexible or lay flat tube would provide the option for the bag to collapse and thereby provide the ability for the pump to get a lot more water out.

So my thoughts are this: is that a valid point - would a more flexible (longer when needed) filler tube help or would I be better of adding an internal feed tube to the bag to assist the draw of fluid from the base of the bag.

Its this bag:
http://www.piratescave.co.uk/plastimo-triangular-watertank-120l.ir
Click to expand...

My triangular pillow tank has the filler near the apex of the triangle and the outlet (IIRC) near the centre of the base. The filler pipe has very little scope for movement but it does not prevent the tank collapsing and allowing me to use just about all the water in the tank. It does lie on a fairly flat platform ( bit of ply) rather than in the V of the hull

Similar to this only smaller

16658.jpg

Are you sure your filler cap is air tight?
 
Last edited:
yes, i get a hiss of air when i undo it.

when empty does your filler tube stay above the bag or is it 'lower down' - i think the input on mine might be a bit high.
Interesting supposition though. Unless I am using 120 litres of water in a couple days.
 
SiteSurfer said:
yes, i get a hiss of air when i undo it.

when empty does your filler tube stay above the bag or is it 'lower down' - i think the input on mine might be a bit high.
Interesting supposition though. Unless I am using 120 litres of water in a couple days.
Click to expand...

The filler tube runs down through the chain locker right in the V of the bow, through a cut out in the ( loose) bottom of the chain locker and through a hole in the very bottom of the bulkhead that forms the chain locker. The connection on the tank is not then many inches away. Possible movement of the filler pipe is minimal but its not raised

Interesting, puzzling , that you end up with a slight vacuum in the tank. Mine just collapses completely flat.

Something else to add to the list of things to photograph!
 
Last edited:
Just something related to be aware of.
My tank arrangement is similar.
I tidied up the filler pipe on it's run to the tank and prevented the bag collapsing as water is used.

This eventually pulled the filler connection to the bag apart and started a leak.

So now I put up with excess pipe in the run.
 
That's interesting, when I replaced the bag I decided to have the shortest possible route to the bag, the hose is quite inflexible and also I suspect - as yours - too short.

Armed with both of the posts above I will have a better understanding of this now.
 
When I fitted the flexible tank the first time I used it, I only got about half the water out. I then watched it as the pump tried to get water out and saw air getting in via the filler cap. Once I replaced the seal on the cap, the tank collapsed fully, allowing all the water to be used. Anything that stops the tank collapsing will mean you can't use all the water, so I'd think that too short a hose run would cause a problem.
 
Something else to bear in mind - when you fill the tank, are you drawing in air at the same time? We have an oblong flexi tank lying flat on the hull in a locker next to the main water tank, filled with an extension to the pipework for the main tank, with a valve to select which tank is filled. The pipework is clear plastic and we can see air as well as water going into the tanks.

The flexi tank needs to be 'burped' during the filling procedure by pressing it down, otherwise it will end up half full of air which prevents more water going into it as there is no breather. Once the selector valve is closed no more air can get into the tank if the filler cap leaks, and suction from the water pump flattens it.
 
Returning to this thread.
At the boat today and ran out of water after a day (from full) and decided to have a look. The bag is almost full still but simply doesn't collapse. I pushed it down and got the outlet lower and it started pumping again.

I'm wondering if there's something in the seal idea so will replace the filler in the morning and see if that helps. The filler tube appears to be fine and not holding the bag up, and I'm estimating that I have probably got a scope of about 10 litres from the 120 litres it should provide.
 
SiteSurfer said:
Returning to this thread.
At the boat today and ran out of water after a day (from full) and decided to have a look. The bag is almost full still but simply doesn't collapse. I pushed it down and got the outlet lower and it started pumping again.

I'm wondering if there's something in the seal idea so will replace the filler in the morning and see if that helps. The filler tube appears to be fine and not holding the bag up, and I'm estimating that I have probably got a scope of about 10 litres from the 120 litres it should provide.
Click to expand...

Like some others here my flexible tank collapses under atmospheric pressure because the filler is air tight. When it's empty the lever on the hand pump can't be moved.
 
Mine is a vee 130 lt - in the vee berth locker. My filler is sealed as far as I know.
I just have a whale hand operated push down tap,

Strangely - both the filler and the outlet are on the bottom ( sort of odd - but I suppose if the outlet is not going to leak neither is the filler pipe ? )

Never had a problem - except it can tend to crease up when nearly empty.
However - I do have the spare wash boards on top of it ( only place to stash them ) and on top of that a load of little used soft gear stowed.

Mine is in a canvas liner that straps down - so the bag inside is loose - apart from attachment hoses

So the answer might be to pile a load of stuff on top of it ! Squashes it flat !
 
nemodreams said:
Mine is a vee 130 lt - in the vee berth locker. My filler is sealed as far as I know.
I just have a whale hand operated push down tap,

Strangely - both the filler and the outlet are on the bottom ( sort of odd - but I suppose if the outlet is not going to leak neither is the filler pipe ? )

Never had a problem - except it can tend to crease up when nearly empty.
However - I do have the spare wash boards on top of it ( only place to stash them ) and on top of that a load of little used soft gear stowed.

Mine is in a canvas liner that straps down - so the bag inside is loose - apart from attachment hoses

So the answer might be to pile a load of stuff on top of it ! Squashes it flat !
Click to expand...

The problem will be that if any air gets in, by being carried down with the water while you are filling it, it'll be trapped, effectively reducing the useful capacity of the tank.

If its just a plain pillow tank I would have assumed it had simply been installed upside down although I appreciate the pipework may have been run to suit and that it might not be possible to simply turn it over and reconnect so that the inlet and outlet are both on the top
 
Sandro said:
Is the outlet in the lowest point of the tank?
If so, the tank should empty regardless to the filler cap being sealed or open.
Sandro
Click to expand...
On the particular bag in question both fillers are on top and on the same level. My understanding is that the bag collapses under pressure.

Am about to test with filler pipe blocked up to see if it is my filler seal.
 
Treble whammy!

Filler seal dodgy, bag outlet high, small tears at creases...

Bag gone back to plastimo as only 8 months old. Does rather explain the puddle under the bag too.
 
SiteSurfer said:
Treble whammy!

Filler seal dodgy, bag outlet high, small tears at creases...

Bag gone back to plastimo as only 8 months old. Does rather explain the puddle under the bag too.
Click to expand...

The idea with a pillow tank is that it just collapses flat with no creases!


I had a shaped flexible tank in the bow originally but it had no vent so it creased as it collapsed and it always creased in the same places until it failed at these points. it lasted about 3 seasons.

The replacement is a double skinned pillow tank That's still OK after more than 30 years but it holds a lot less water than the original.
 
I'll see how the new one gets on and go from there, was an intersting half day with my head stuck in the bows sorting it all out.
 
VicS said:
The problem will be that if any air gets in, by being carried down with the water while you are filling it, it'll be trapped, effectively reducing the useful capacity of the tank.

If its just a plain pillow tank I would have assumed it had simply been installed upside down although I appreciate the pipework may have been run to suit and that it might not be possible to simply turn it over and reconnect so that the inlet and outlet are both on the top
Click to expand...

Maybe it is upside down ? - but it works fine and gives me no problems - and no leaks.
I put it in new ! - I figured it best that way !

If I ever capsize - at least my water will be the right way up :)
 
You must log in or register to reply here.

Share this page

Top