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Old 06-06-2020, 11:33   #1
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Drawing down dual water tanks

Hello friends,
I have two water tanks on board, 25 gals under the V and another 25 +/- under a settee. Each has its own shut off.

My question is should I draw down one before the other (and if so, is there a preferred order for weight distribution)? Or should both valves be left open so that they draw down together?
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Old 06-06-2020, 11:51   #2
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Re: Drawing down dual water tanks

How does the weight of each affect the balance of your boat? The answer lies in that.
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Old 06-06-2020, 12:03   #3
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Re: Drawing down dual water tanks

I would definitely draw from one tank at a time so you have a feeling of how much water you have left. When switching to the second tank it is time to think about filling up. I do it that way.

You don't say if you are out cruising, on a mooring or in a marina. That would be a big factor in how closely you need to watch water consumption.

As for which tank to draw down first, OloteleMtn has your answer. I'd probably get the weight out of the bow first.
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Old 06-06-2020, 12:16   #4
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Re: Drawing down dual water tanks

If 1 tank is higher than the other, the higher tank will drai into the lower one. If they are both full, the lower tank will overflow. Bob
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Old 06-06-2020, 12:25   #5
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Re: Drawing down dual water tanks

roberttigar,
I see what you mean, if both valves are open.

stormalong,
Drawing them down separately to keep track of usage makes good sense. Seems bow tank first is the way to go.
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Old 06-06-2020, 12:49   #6
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Re: Drawing down dual water tanks

Suppose you have a fitting come loose in your pressurized fresh water system and suppose you are either away from the boat (hopefully on the beach having a cocktail) or perhaps you are motoring and don't hear the fresh water pump running. If you draw from both tanks all your fresh water will end up in the bilge. If you are drawing from only one, and the second isn't already empty, then you will have some still in the tanks when you discover what happened.
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Old 06-06-2020, 15:18   #7
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Re: Drawing down dual water tanks

Or... for more control (and complexity) you can always draw from one and just transfer from the other as needed with a small transfer pump.

Also, don’t assume emptying the forward tank first will result in better trim and/or sailing performance. Most boats I’ve owned were far more upset about squatting and digging their tail in than sailing with their bows down.
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Old 07-06-2020, 09:18   #8
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Re: Drawing down dual water tanks

If you draw from both then whichever empties first may then result in your pump just sucking air so that even though you still have water it will not be available.
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Old 07-06-2020, 14:29   #9
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Re: Drawing down dual water tanks

I have 3 tanks and draw one at a time.
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Old 07-06-2020, 14:36   #10
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Re: Drawing down dual water tanks

Quote:
Originally Posted by bobgarrett View Post
If you draw from both then whichever empties first may then result in your pump just sucking air so that even though you still have water it will not be available.
?????
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Old 07-06-2020, 14:50   #11
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Re: Drawing down dual water tanks

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?????


I think the missing word was “simultaneously”.

Which would be a pretty odd setup, I’m sure.
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Old 07-06-2020, 15:04   #12
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Re: Drawing down dual water tanks

Not that odd. I have tanks Pt and Stb which come together and have a take off from there. I recently added a valve to Pt tank and made a new header so I have Tail; Valve; Take off with valve; Take off with valve; Valve; Tail. So it joins the tanks and gives me the manifold with Pressure pump take off and a spare for possible foot pump take off.
Next task is to restore Pt tank vent that PO removed for some insane reason.
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Old 07-06-2020, 15:20   #13
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Drawing down dual water tanks

Quote:
Originally Posted by Djarraluda View Post
Not that odd. I have tanks Pt and Stb which come together and have a take off from there. I recently added a valve to Pt tank and made a new header so I have Tail; Valve; Take off with valve; Take off with valve; Valve; Tail. So it joins the tanks and gives me the manifold with Pressure pump take off and a spare for possible foot pump take off.

Next task is to restore Pt tank vent that PO removed for some insane reason.

Roger


By saying simultaneously, I am suggesting there is no isolation of the tanks.

Which would be pretty silly as all the water would drain to the lower tank when under way. (At least on a mono).
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Old 07-06-2020, 17:37   #14
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Re: Drawing down dual water tanks

^^^
Everything being equal - If both tank valves are open and one tank is empty you will definately draw some air.

I have to use one tank at a time at least when one of them is approaching empty.
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Old 08-06-2020, 05:50   #15
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Re: Drawing down dual water tanks

If the settee tank doesn't cause a notable list then I would draw down from the bow first to take the weight out of the bow. JMHO
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