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Old 12-02-2020, 10:53 AM   #1
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Is combining two water tanks a good idea, even possible?

We recently purchased a 2000 Grand Banks 46. Shes in splendid shape, well cared for by a doting previous owner. Slowly, were getting to know her better following a great trip down from her home in Maine. One situation that has me confounded has to do with our two water tanks, fore and aft. The GB manual schematic show the tanks are connected - and filling the fore tank slowly does ultimately fill the aft. Alas, the fill appears to be their only connection. As the fore tank empties theres no resupply from the aft. Id hoped thered be one since the fore tank, which supplies the forward cabin head and the galley, drains far quicker than our aft. My question is 1) can these tanks be linked in a way that allows simultaneous access to both tanks and 2) if so, is this a good idea? Wisdom wanted.
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Old 12-02-2020, 11:00 AM   #2
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why do you have two water systems is a better questions . just get the water from the aft tank .I have two tanks but the are both aft it runs thru one to the second all water is pulled from the second.
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Old 12-02-2020, 11:09 AM   #3
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1) can these tanks be linked in a way that allows simultaneous access to both tanks and

2) if so, is this a good idea? Wisdom wanted.
Both my fuel tanks and my water tanks have passive crossovers. Each tank has a fitting at the bottom of the tank, connected via line to the fitting on the bottom of the other tank. As you draw from one, equilibrium balances the volume. In my case both the fuel tanks and water tanks are stored on opposite sides of the boat, so balance is key to avoid listing.
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Old 12-02-2020, 11:12 AM   #4
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I have a GB42 and both water tanks are in the lazerette next to each. Both tanks are connected at the bottoms with a valved-crossover pipe. Your schematic should show a valve somewhere between the fore and aft tanks. It seems your fore and aft tanks need to be at the same static level for them to be able to communicate, otherwise you’d have to pump water from the lower to the higher tank. Just my thoughts...
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Old 12-02-2020, 11:26 AM   #5
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You can put a Y valve on the intake to your pump, and switch when one tank is empty. This is actually quite common on offshore sailboats, partly the thinking is if one tank leaks or becomes contaminated the other is still good. If you want them connected otherwise, you need to T their output at a point lower than the bottom of the lowest one.
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Old 12-02-2020, 11:30 AM   #6
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I had a 2009 Grand Banks Europa for a number of years with a similar setup, and struggled with the same issues. However the details may be different, so don't apply this literally.


I wanted to operate with the two tanks connected and acting as one, simply for the convenience of having a larger working supply of water, and not having to play around with tank monitoring and switching from one to the other.


My memory may be a bit fuzzy on this, but here's what I recall, in no particular order:


- The tanks were at different elevations, both their tops and bottoms. In my case the forward tank was lower than the aft tank.


- The plumbing allowed the pump to draw off either tank, or to have both tanks and draw off both. The later is how I operated.


- I would fill the aft tank, with the expectation that water would flow to the forward tank, filling it first because it was lower. Then when the pump would draw off the water, I expected the higher aft tank to empty first, then the lower forward tank. But this didn't work as expected.


- The problem turned out to be a low spot in the forward tank's vent line. The low spot would flood, and then the standing water would block the vent. The blocked vent would prevent water from flowing from the aft tank to the forward tank, both when filling the tanks, and when drawing water out.


- To work around this, I would crack open the deck fill for the forward tank when filling. You could clearly hear the air release as the tank vented, and water then flowed freely. Then over the course of using the water, I would periodically open the deck fill again to allow the water to equalize.


- The solution would have been to reroute the forward tank vent to ensure there were no low spots, but I sold the boat before I got around to doing it. The vent line of course ran through a difficult to access area, so I kept putting the project off.


Hope this helps. It was all a good lesson in the importance of ensuring that hoses are self-draining to prevent air locks.
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Old 12-02-2020, 11:45 AM   #7
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Many many thanks for the insights. In our case the forward tank appears higher than the aft. We’re hauling her in a month or so and we’ll definitely take the advice to heart then. I’m particularly interested in the possibility that re-fitting the lines so the lower aft tank becomes the primary supply and gravity wins the day. Again, thanks to all of u.
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Old 12-02-2020, 12:11 PM   #8
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I have a fill that goes into aft tank, then it is connected at the bottom to the forward tank. They both fill, then they both empty maintaining equilibrium at all times. It's a centerline setup so no balance issues, other than adding 2000 pounds aft of center and digging in the stern trying to get on plane.......
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Old 12-02-2020, 02:45 PM   #9
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You know I have a very similar problem -- the Mainship manual and the schematic for the plumbing shows the two tanks are connected so they equalize, but they're not -- or if they are, it doesn't work, they don't equalize. It's kind of annoying in our case because we have one very large tank under the master berth (80 gal) port side, and then another much smaller tank under the salon couch against the hull, starboard side. If the big tank is full and the small one is empty, that's a (8.34 x 80 gals = 667 lbs) imbalance and we list to port. I hadn't thought of a kink or low or high spot in the vent line or connection hose, I'll have to check that.
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Old 12-02-2020, 03:00 PM   #10
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Many many thanks for the insights. In our case the forward tank appears higher than the aft. Were hauling her in a month or so and well definitely take the advice to heart then. Im particularly interested in the possibility that re-fitting the lines so the lower aft tank becomes the primary supply and gravity wins the day. Again, thanks to all of u.
You should be able tee the supply lines from the fore and aft tanks, but only if the lower aft tank vent line is above the top of the other tank.

I would still ensure there are isolation valves on the outlet of both tanks. Also d ensure the lower vent line is in good shape and not leaking, as the water level would be part way up the vent line when both tanks are full.
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Old 12-02-2020, 05:50 PM   #11
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I just solved a similar issue. Our boat has main 250g water tank under aft queen bed. Forward 100g tank is under floor of forward cabin. In the two years we have used the boat we have never been able to access the forward tank. PO said they never used it also. Finally dawned on me that with the forward tank being much lower, when the higher aft tank emptied the FW pump would just suck air from that tank. I opened the forward inspection hatch and saw an extremely clear/clean tank full of water(monel steel) - yep, any time there was at least 1/2 full aft tank, it would flow down (1/2' pex lines) to the lower tank. The lines came to a tee, then to the FW pump. I simply installed a valve in the line from the aft tank. When turned off, the pump pulls perfectly from the forward tank. Since that tank had not been used, possibly in the 30 yr life of the boat, I commissioned it with an appropriate amt of bleach. 350 gal should help on our planned Bahamas trip - when covid is over.
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Old 12-03-2020, 06:11 AM   #12
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Personally I would contemplate two tanks not interconnected so I could draw off either tank to operate the boat.

Cruising we found some areas have awful water ,, worse than drinking from a public swimming pool .

The ability to turn a valve and use the crap for washing and then turn a valve for cooking and drinking water should not be difficult.
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Old 12-03-2020, 04:19 PM   #13
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We have 5 tanks onboard for around 5500 litres (1500 gallons.) total

The two aft are cross linked with ball valves to close off each and with separate fillers each side and are the ones we use.
Same with the midship 2, but not connected to the main tanks
And there is one fwd in the forepeak

The fwd and midship tankage as they are not plumbed in are currently filled as ballast but I really would love to get some hose run through to the back and bring them all online .
All would gravity feed aft if done.
Pretty much at the bottom of my priority list at the moment.
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