TWO WATER TANKS

[Endeavour]

I have the space to install two large water tanks.

 

If a tank is completely full or completely empty there is no 'free surface effect' of the liquid (try googling if you are unfamiliar with the term). so for reasons of stability (the boats rather than mine!) it would be preferable to completely empty one tank, before starting to drain the other.

 

I am looking for as simple a solution as possible to switch between the tanks, preferably a solution that will switch over automatically.

 

Any ideas or thoughts would be much appreciated.

[Yoda]

I have the space to install two large water tanks.

 

If a tank is completely full or completely empty there is no 'free surface effect' of the liquid (try googling if you are unfamiliar with the term). so for reasons of stability (the boats rather than mine!) it would be preferable to completely empty one tank, before starting to drain the other.

 

I am looking for as simple a solution as possible to switch between the tanks, preferably a solution that will switch over automatically.

 

Any ideas or thoughts would be much appreciated.

 

Simple solution is to have a common feed from both tanks and no switches except all off. The only other way is to independantly switch each tank, manually or motorised valve.

[Big Steve]

Won't any half decent tank have baffles in it to reduce the free surface area problem?

[DaveR]

How big are these tanks to have FSE as a problem?

[pagan witch]

If you are at the desaign and build stage rather than trying to retro-fit then a simple electronic water level guage in each could be connected to a selenoid change over valve via a suitable interface.

 

As a retro-fit things get a bit trickier as your choice of guages becomes less.

 

For a narrowboat though I'd not bother adding complications to the boat.

[Endeavour]

Each tank is approx 7' long x 6' (full width of interior of narrowboat) x 1' deep, divided by baffles into sections approx 1 foot wide.

 

It may be that the free surface effect will not be that significant, however if someone can come up with a simple automatic way of draining one before the other (without resorting to sensors and motorised valves as that is getting too complicated for long term reliability), then I would be interested to hear from them.

 

I guess I am really asking does anyone know of a clever change over valve that when air rather than water is drawn through one port, it it can switch to a second port?

 

Many thanks

[Bojangles]

..............

 

I guess I am really asking does anyone know of a clever change over valve that when air rather than water is drawn through one port, it it can switch to a second port?

 

Many thanks

 

Yeh, it's simple - you !!

 

When air starts to come thru with your water - manually switch to your other tank. You'll then become the 'clever change over'.

[bottle]

and it will remind you to refill or the next time you get air the clever change over valve will be redundant.

[DaveR]

The FSE for the tank set up will be absolutely minimum. For each baffle in the tank (5) this is the root value of the whole FSE.

 

So what you have is the 5th root of the full tank, really not worth thinking about.

 

Manual valve work is your best option.

[GUMPY]

I run two tanks, one in the bow under the bed so its almost the full size of the double bed approx 900L the other a 500L tank on the Starbord side opposite the diesel tank in the stern. They are connected with no auto changover just gate valves at the exit of each tank.

When the front tank empties the back tank still has about 125L left I simply turn off the front tank and carry on. I do find that to get both tanks full I have to fill the back one first then swap the hose to the front as the pipe between the tanks is not large enough.

 

remember Keep It Simple

 

Julian

[PeterF]

Rather than setting up the tanks in parallel set them up in series. Tank number 1 is connected to the pump as normal. There is no vent pipe on tank number 1 and it has to withstand a small vacuum. The top of tank number 1 is connected to the bottom of tank number 2 (assuming both tanks are on the same level). Tank number 2 has a vent on it. When you pump out of tank number 1 a small vacuum is created and this sucks the water over from tank number 2. When tank number 2 is empty air vents through it into tank number 1. The trick is when you come to refill them, you always have to fill tank number 1, you cant have either or. You may be able to fit the relative heights of the interconnecting pipe to mean that you will tank number 1 and when full this overflows into tank number 2 so you do not have to use 2 filling connections.

 

I know this is strictly not what you asked for but it does solve the problem of completely emptying one tank before the other.

[Endeavour]

Thanks everyone for the replies:

 

The FSE for the tank set up will be absolutely minimum. For each baffle in the tank (5) this is the root value of the whole FSE.

 

So what you have is the 5th root of the full tank, really not worth thinking about.

 

Manual valve work is your best option.

 

DaveR, in that case it sounds like I'm worrying about a problem that isn't a problem at all

 

 

Rather than setting up the tanks in parallel set them up in series. Tank number 1 is connected to the pump as normal. There is no vent pipe on tank number 1 and it has to withstand a small vacuum. The top of tank number 1 is connected to the bottom of tank number 2 (assuming both tanks are on the same level). Tank number 2 has a vent on it. When you pump out of tank number 1 a small vacuum is created and this sucks the water over from tank number 2. When tank number 2 is empty air vents through it into tank number 1. The trick is when you come to refill them, you always have to fill tank number 1, you cant have either or. You may be able to fit the relative heights of the interconnecting pipe to mean that you will tank number 1 and when full this overflows into tank number 2 so you do not have to use 2 filling connections.

 

I know this is strictly not what you asked for but it does solve the problem of completely emptying one tank before the other.

 

Peter F - A beautifully ingenious solution

[howardang]

I don't think you need to worry about free surface effect if your tanks are subdivided as you describe - i.e. 6 compartments 1 foot wide.

 

The FSE in an unbaffled tank would be small in any event, but when subdivided any free surface effect is divided by the square of the number of compartments - in your case 36 - so it would be very very small so as you needn't worry about it.

 

I like the tanks in series idea, but one thing might be of concern. That is any change of trim (full versus empty fuel tank, full versus empty gas bottles) might cause air locks. I also would be slightly concerned about the need for tank 1 to work under a vacuum which would suggest that the tank top should be strengthened to prevent flexing.

 

I think the best solution to this is to have a manual change over valve. You would then be always aware of the amount of water you have in the tank(s0 and there are no mechanical/electronics to go wrong!

 

Howard Anguish

[John Orentas]

If ever there was a case of complication for the sake of it, this is it, people have been building large unbaffled water tanks into boats and ships for hundreds of years.

 

So your boat rocks around a bit, so what, it's a bloody boat.. Sounds like another of the Chelsea apartment brigade.

[howardang]

If ever there was a case of complication for the sake of it, this is it, people have been building large unbaffled water tanks into boats and ships for hundreds of years.

 

So your boat rocks around a bit, so what, it's a bloody boat.. Sounds like another of the Chelsea apartment brigade.

John, you really can be a grumpy old f**t at times!

 

The man asked a question because thought he might get some help and maybe he isn't a world authority on all subjects like you are. How about live and let live

Howard Anguish

[BarnBrian]

Thanks everyone for the replies:

DaveR, in that case it sounds like I'm worrying about a problem that isn't a problem at all

Peter F - A beautifully ingenious solution

Surely the idea of running two tanks is so that you always have a reserve. By using the in series method there will be no indication when the first tank is empty. As others have said, keep it simple.

 

Brian

[John Orentas]

John, you really can be a grumpy old f**t at times!

 

The man asked a question because thought he might get some help and maybe he isn't a world authority on all subjects like you are. How about live and let live

Howard Anguish

 

 

Hi Howard.

 

The chap is getting help but there are times when we all bark up the wrong tree, we do not do anyone any favours by responding in a complacent, patronising, oh yes thats a clever idea sort of way.. If you say Peter is building a boat for the first time well you obviously know more about that than I do but when you embark on projects such as he is doing the first task is to take a look at the general conventions and if you can improve on them then do so.. If he is a novice boater how did he hear about the FSA effect.

 

This free surface area effect is a factor but I would doubt that more than 10% of boaters would even be aware of it, certainly I had a boat for a couple of years before I even noticed the phenomenon, I used to think it was my imagination that sometimes the boat rocked more at some times than at others.

 

The phrase 'Live and let live' is probably probably the the most ill used one there is, if we all wanted to 'Live and let live' we would not join any kind of forum in the first place.

Edited November 24, 2007 by John Orentas

[ChrisPy]

If ever there was a case of complication for the sake of it, this is it, people have been building large unbaffled water tanks into boats and ships for hundreds of years.

 

So your boat rocks around a bit, so what, it's a bloody boat.. Sounds like another of the Chelsea apartment brigade.

I totally agree with you John.

 

But then I'm just another grumpy old f*rt .................

[pagan witch]

Smug alert . . . .

 

Some short while ago I asked for comments about a water level gauge and got a mixed bag of 'What for' - 'mark the boat like a plimsol line' and the less than helpfull 'fill up whenever you go past a water point'

 

As the idea was to know how much I had left so I could stay put in one place for a few days if I felt like it the latter view was a no brainer.

 

So . . . . I built an 8 LED level gauge out of a plan I found on the net, about £9 of parts from Maplin and a length of bell wire I had laying around. I now know how much water I have at the press of a button - very re-assuring and no need to worry about a 'spare' or second tank which weren't practical to me as the boat was already built.

 

Can't remember what I called the thread now but dare say if you search for 'water gauges' and 'pagan witch' you'll find it. If not I'll PM you details if it is of any use.

 

Alternatively there are a couple of ready made gauges that measure head of water and are on the market at around £70 or so - normally at most boat shows and some swindlers.

Edited November 25, 2007 by pagan witch