Flexible water tanks recommendations and advice please

[Tarka1]

I've just taken out two very horrible grp water tanks out of a Moody 333 and due to the osmotic blisters and fungal growth want to replace them with flexible tanks.
Questions
1. Should I buy slightly oversized tanks and let them fill the space available?
2. The original tanks have one fill point and are then linked together with a balance pipe with the water outlet taken off this pipe - could I use the same arrangement with flexible tanks?

Any advice on the best products or installation tips appreciated

 

[milfordman]

On your original setup with the GRP tanks. I would assume that the balance pipe linked at the bottom of both tanks to allow the full capacity to be used.

The tank without the fill point would have had to have a breather/ overflow pipe inorder for the air to vent when filling, and return when water is drawn off.

I think the arrangement will work ok with flexible tanks. As long as you either arrange for the new flexible tank without the fill pipe to be vented as before or ensure that it is fully deflated of air before you fill the tanks, as any air in the tank at the start will always be trapped in there.

The tank with the fill pipe should be able to fill and vent through the same pipe as long as you ensure that it slopes up along its length. If there is an S bend in it you could setup a water trap that would restrict the filling.

 

[VicS]

Flexible tanks must be double skinned IMHO. Maybe they all are now anyway. I had a single skin one. 'twas Ok for 2 or maybe 3 years. The double skinned replacement is still in perfect nick after more than 25 years .


Be aware that there are two sorts of flexible tanks.
Pillow tanks which are, well, pillow shaped and are intended to go flat as they empty. They are not vented and the outlet is usually on the top.
Shaped tanks are shaped three dimensionally. They are intended to retain their shape as they empty and must therefore be vented. the outlet must therefore be on the bottom. All but the smallest need support.

I imagine you will be looking at fully shaped tanks.

 

[Gordonmc]

The original set-up on my boat had one double-skinned tank under a port side berth, soon proving to be inadequate as filling opportunities are limited. I got an identical tank and installed it under the opposite berth, simply taking a tee off the original's filler pipe.
Venting isn't a problem.

 

[Tranona]

The set up I have is a rigid tank that has the filler and vent connected with a pipe and ball valve to a Vetus double skinned pillow tank - not vented. There are take offs on both tanks to a changeover valve. To fill I open the ball valve which fills the flexible tank and then the rigid one. I then close the valve. Can't think of any reason why this should not work with two flexible tanks.
The beauty of this system is that I end up with two independent supplies selected by the change over valve. I think I would avoid a single take off point in the connecting pipe - and as others have said pillow tanks have take offs from the top.

 

[macd]

'Fraid my experience is contrary. I installed a Plastimo tank (which is double-skinned) as a second tank two years ago. It split along a seam after 4 months (which admittedly is a better way than some to discover why your bilge pump's so busy). This certainly doesn't damn double-skinned tanks per se. You get what you pay for, I suppose, but it doesn't necessarily make them any good, either.

The Plastimo was replaced by a very much dearer custom-made flexible tank from Hovercraft Consultants, made from similar material to hovercraft skirts. It's single skin but seems bullet-proof.

The boat's original tank, in the keel, is single skin and of similar material to the HC tank and still going strong after 30 years. The key, I suspect, is that it should be well restrained/contained to avoid chafe. Over-filling is bad new, too.

 

[macd]

For switch-over valves, consider cheap garden hose fittings rather than expensive boaty ones. B&Q and the like stock a Y-piece with two integral valves for half inch hose for just a few quid.

 

[davidpbo]

We have plastimo flexible bags on our boat and they have been fine. They fit in another bag which is supported. With reference to B&Q fittings, I would suggest using equipment intended for potable water they may well be a different plastic.

 

[Graham_Wright]

[ QUOTE ]
We have plastimo flexible bags on our boat and they have been fine. They fit in another bag which is supported. With reference to B&Q fittings, I would suggest using equipment intended for potable water they may well be a different plastic.

[/ QUOTE ]

Or try your local aquatic centre;- fish are even more sensitive it seems than humans. EVE make suitable ones (but if you google EVE you receive all sorts of interesting offers based on unsullied ladies.)

 

[captainslarty]

also worth pointing out that space the tank occupies should be as smooth and protution free as possible.
We intend to fit two 50 gall bag tanks in the bottom of the huge cockpit lockers to use at anchor. We have sourced some great neoprene rubber to line the grp and wood walls with making a smooth surface. I am sure there are many other ways of achieving this though. This neoprene is solid, around 6 mm thick and flexible. perhaps wetsuit neoprene 10mm or similar, especially the fabric coated stuff, would also be excellent ?
Joe