Can't work this out !

[oakleyb]

I have a water tank in each hull and a single water pump supplying heads and galley. Now obviously I want these to be drawn from equally but they don't? ?? The pump is situated close to port tank and therefore has a shorter pipe length, would that cause an unequal draw ?

 

[FirstAway]

Assuming that the tanks are connected together with no closed valves, the pump will draw preferentially from the closest tank, but as soon as the pump stops the two tanks will level out with gravity flow.

 

[Sticky Fingers]

...Now obviously I want these to be drawn from equally but they don't?...

Why? Every boat I've been on with two tanks has had a system of changeover valves. Draw from the 'open' one, close it when empty and switch to the other. Means you know when you're half way through your supply.

 

[Big Andy]

Assuming that the tanks are connected together with no closed valves, the pump will draw preferentially from the closest tank, but as soon as the pump stops the two tanks will level out with gravity flow.

That might not happen if air can enter the line and stop it syphoning. Assuming the conecting line runs higher than the tank tops when crossing between the hulls.

 

[oakleyb]

I have a cat so would prefer where possible to keep weight in each hull fairly equal

 

[Sticky Fingers]

I have a cat so would prefer where possible to keep weight in each hull fairly equal

Are there two fillers? Or one?

200 litres of water (or whatever) won't make much difference but I take the point, what does the manufacturer suggest? There cannot be a balancing pipe, it would have to rise above the top of each tank to cross the centre of the cat, so no free-flow possible, so it's either a manual changeover; or pumped levelling (unlikely); or common draw but you'll get uneven take-up as you suggest.

 

[QBhoy]

Pressure of head
Trim of boat
direction of impeller and piping flow.
Think you need isolation valves or levelling x pipe.
Who knows...just a thought !

 

[FirstAway]

Scala is right what I said is only true if there is a balance pipe running between the two tanks. What happens at the moment when you run the tanks down, does it empty one tank first?
If you only have one filler the tanks probably balance, if you have two and isolation valves they may not.

 

[Lon nan Gruagach]

I have a water tank in each hull and a single water pump supplying heads and galley. Now obviously I want these to be drawn from equally but they don't? ?? The pump is situated close to port tank and therefore has a shorter pipe length, would that cause an unequal draw ?

If the pump is attached to a tee that has a leg to each tank then your starboard tank wont get much use. The pump will preferentially draw from port. Any air in the pipe from the starboard tank will effectively isolate it.

2 options:
fit a valve on each leg of the tee
fit another identical(ish) pump at the starboard tank wired to run at the same time as the existing pump.

The second option wont be precise, but pretty close.

 

[AndrewL]

You could try restricting the flow in the shorter pipe run very slightly to balance them. Maybe an inline ball valve partly closed.

 

[JerryHawkins]

Now I may be having a complete brain-fade here, but it seems to me that there must be a balance pipe between the two tanks which would (as its a cat) have to go above the level of the tank tops . If the balance pipe is full of water and the ends are below the water surface in each tank then they should self-level by siphon action. If the tanks have been emptied below the balance pipe ends and air has been drawn in then the self leveling won't work. You may just need to 'purge' the air from the balance pipe to make it work again.

 

[Graham_Wright]

Assuming that the tanks are connected together with no closed valves, the pump will draw preferentially from the closest tank, but as soon as the pump stops the two tanks will level out with gravity flow.

Gravity shows scant respect to boats!

 

[rogerthebodger]

I have a water tank in each hull and a single water pump supplying heads and galley. Now obviously I want these to be drawn from equally but they don't? ?? The pump is situated close to port tank and therefore has a shorter pipe length, would that cause an unequal draw ?

This is an age old problem with hydraulics and can only be solved with a flow splitter.

The reason for a flow splitter is that the 2 inflow legs will have different resistances to flow and thus the one leg with the least flow resistance will take precedence.

To do this automatically you need to measure the flow in the 2 legs then adjust a distribution valve to adjust the resistance.

This can be done manually but if the resistance changes in either leg the flow will change anyway.

I have a similar issue and I simply have a change over valve to select which tank I wish to use, monitoring my water level gauges as I use water.

Having a mono hull I could simply fit a balance pipe but I do wish to keep my 2 tanks separate in case of contamination.

You will not ever get it right all the fine and will need to keep adjusting the manual distribution valve.

Using one of these is the best you can hope for.

 

[oakleyb]

Thanks for all the replies, i think a y valve and two gauges is the way ahead

 

[ghostlymoron]

I can't see how it will work as it is. I'd think that if you start with both tanks full and connected to the pump, you'll draw from the nearer tank until it's empty then start sucking air. You definitely need valves on the pump auctions so that you can select which one to draw from. This must be common on cats so perhaps a cat owner will enlighten us.