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Old 18-07-2007, 14:54   #1
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Question Dual fuel tanks

Question: On a small boat with dual (port and starboard) saddle tanks, is it possible to have a siphon tube from one to the other to keep their levels even? Otherwise, how do you keep the levels near each other? I know on larger installations with a fuel manifold you x-fer the fuel by pump or gravity. I don't want to mess with a lot of valves and it'd be nice if the system would just adjust itself or am I way off base here?
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Old 18-07-2007, 15:34   #2
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But you are going to have a fuel polishing system aren't you? With a fuel polishing system it's easy to keep your tanks fairly level. But of course that requires a couple of manifolds. On a small boat you could just go with just a small electric fuel pump to transfer between tanks, hopefully through a filter, with pickup and return tubes near the bottom of the tanks. Oh, oh, that's almost a fuel polishing system...<g>

With the system you describe, I don't think that you would get leveling of the tanks unless the tube was plumbed in to the bottom of the tanks, which isn't a "good" thing in my books. Not sure if it's against CG regulations or not but I do believe that it is against ABYC guidelines.
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Old 18-07-2007, 15:35   #3
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If you routed both sets of fuel lines to 2 Y valves you can just switch the supply and return lines between tanks. It gets more complicated to rely on gravity to even up the tanks. Sort of makes the fuel gage readings confusing.

The siphon tube would have to pull off the bottom of the tank and connect to the bottom of the other tank to work by gravity and that assumes the elevation of both is the same. That means you are pulling the bottom sludge and it is possible the siphon could become plugged with sludge. So then what happens to the other tank? Switching tanks manually lets you treat each tank separate and allows one tank that has no problems to work while the other can be say cleaned.

The current boat has it all by gravity for both tanks. It works well because the aft tank is trashed with growth inside and I'm only using one tank right now until I can get it cleaned out.
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Old 18-07-2007, 16:26   #4
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Thanks fellas. The reason I'm asking is I'm studying some photos of a build of the same boat in Oz. From the photo attached you can see (blue arrows) what appears to be a fuel line from the bottom corner of one fuel tank to the other. This would automatically level both tanks. The same appears to be true for the water tank">fresh water tank (red arrow). Now I'm not advocating drilling holes in the bottoms of tanks but more of a siphon that picks up about 3/4 of the way down into a tank, out the top across to the other tank and back down 3/4 of the way. If a constant syphon was maintained it would level both tanks (eventually depending on size of hose).

Frz, I don't know of too many boats in this bitterly cold part of the world that need fuel polishing systems. Maybe it's different out there in Winterpeg!

I don't really care about CG regulations cause I work there!

I'm really thinking of ways I can make this as simple and hands free as possible. We're only talking about 40 gals in each tank. I'm more concerned about having the fuel return going to one tank and forgetting to switch over and possibly overfilling a tank. Dunno but that sounds like it could happen.
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Old 18-07-2007, 18:48   #5
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Because diesel engines don't use all the fuel that goes in, some of it is returned to the tank(s). Typically, with twin tanks, the fuel system will have an inlet line from both tanks with a "y" valve (i.e. a valve with 2 inlets and 1 outlet) and a outlet line to both tanks with a "y" valve. Using the y-valves you can determine from which tank you draw fuel and to which tank you return the fuel. So you can draw fuel from one tank and return it to the other, which will allow you to redistribute the fuel as you want (although the process is pretty slow). For long passages, where you are on the same tack for hours (or even days) at a time, you can use this to move your fuel fom the low-side tank to the high-side for iproved windward performance under sail!
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Old 18-07-2007, 20:48   #6
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Rick, I thought you might get some warm weather down there. <g> But asphaltene(sp) is a hazard as well as things that grow in your tank. Especially if you leave fuel in the tank over winter. There is also the chance of getting dirty fuel. Of course if you only plan on cruising near home it's not that important. One spare filter should get you home, even with bad fuel.

Although in the case of a plugged fuel line, which I experienced off the coast of Cuba, a bunch of filters wouldn't of helped.
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Old 19-07-2007, 02:42   #7
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The fuel supply & return should always be plumbed to the same tank.
Don't use "crossed" returns to effect fuel transfer.
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Old 19-07-2007, 10:15   #8
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Another issue with a free-flowing cross-connect is just that: free-flow. Picture a list made worse as the fuel from one side seeks its level on the low side of the boat. If you're already fighting a minor flooding - or even an uneven loading issue, this unwanted liquid ballast transfer would just make the situation worse. (Ooooh - look at the dolphin right under the boat.... Aunt Bertha runs to that side to see and takes her fat b*** with her... You can imagine the rest..)

If you're concerned about unequal tank levels and perhaps running one side dry, maybe go for a small day tank mounted on centerline. Or just put wifey on watch with a sounding tape...
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Old 19-07-2007, 16:24   #9
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Quote:
Originally Posted by knottybuoyz
Now I'm not advocating drilling holes in the bottoms of tanks but more of a siphon that picks up about 3/4 of the way down into a tank, out the top across to the other tank and back down 3/4 of the way. If a constant syphon was maintained it would level both tanks (eventually depending on size of hose).

.
If you ever ran the tanks down to less than 1/4 you would get an airlock in the syphon line and it wouldn't work anymore.
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Old 19-07-2007, 18:42   #10
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Quote:
Originally Posted by 44'cruisingcat
If you ever ran the tanks down to less than 1/4 you would get an airlock in the syphon line and it wouldn't work anymore.
Understood. I fingered that out all by myself! *lol* I'm slowly learning there's no way around using a manifold and a buncha valves.
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Old 19-07-2007, 19:12   #11
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2 Y valves isn't that bad. It's a good thing. It helps if you have an electric fuel pump for bleeding. The last boat had that and it was a breeze. Just be sure you mark both Y valves the same for port and starboard and put them in the same place. Forgetting the return line when you switch tanks would be bad.
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Old 20-07-2007, 05:17   #12
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Ok, I located a little thingy that'll return fuel to two tanks equally. Normally installed in big rigs with dual saddle tanks.

Return Flow Splitters on Velvac Inc.



I'm thinking this would work well on a boat too so I've e-mailed the company for details. Now all I got to do is find it's counterpart that'll allow fuel to be drawn from both tanks equally and I'll have a totally hands free and hopefully idiotproof fuel system.
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Old 20-07-2007, 05:32   #13
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Interesting.
I see that their "Electric Fuel Tank Selector Valves"* are tagged NOT for Marine Use.
* Goto:
Electric Fuel Tank Selector Valves on Velvac Inc.
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Old 23-07-2007, 19:15   #14
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According to the distributor the fuel return splitters are used on a lot of production boats. They're manufactured by GT Development Corp. Specs and more info here....

GT Development Corporation: Products

Looks like it could eliminate a few valves and make dual tanks idiot proof for people like me!
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Old 23-07-2007, 19:49   #15
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Idiot proof things are for idiots...

What if a single fuel line, whether feed or return, gets clogged, or if the draw or return is a little uneven?

What if a fuel line springs a leak?

What then, eh?
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