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Old 03-17-2013, 04:07 PM   #1
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Fuel Selector

Assuming a vessel is sitting level port to starboard, with port and starboard fuel tanks, should you have the fuel selector set to take fuel from both tanks to maintain trim?

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Old 03-17-2013, 05:01 PM   #2
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A parallel plumbed system will always have some bit of assymetry which will cause most of the fuel to be drawn from one tank. And the same lack of symmetry will send most of the return fuel to one tank, but not necessarily the same one.

I prefer to run on one tank for a while until trim gets noticeable and then switch to the other one. That way you are sure which tank is supplying the fuel.

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Old 03-17-2013, 05:05 PM   #3
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David, thanks for reminding me why I draw fuel from only one tank at a time.

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Old 03-17-2013, 05:12 PM   #4
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Originally Posted by djmarchand View Post
A parallel plumbed system will always have some bit of assymetry which will cause most of the fuel to be drawn from one tank. And the same lack of symmetry will send most of the return fuel to one tank, but not necessarily the same one.
But on my boat, as fuel levels approach the half way mark, the list is much less pronounced. ?????
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Old 03-17-2013, 05:18 PM   #5
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This is what happens when loading tanks on only one side:

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Old 03-17-2013, 07:48 PM   #6
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I use fuel from both tanks and I return the excess to both tanks. It's just simpler that way. When the boat sits for a couple hours, it levels itself out.

This works for me but a different boat might behave differently.
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Old 03-17-2013, 09:37 PM   #7
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While I *can* select to to and from tanks on my manifold, I never do. I draw from both and return to both. It always seem to burn almost even from both tanks. If one gets a little ahead, the other catches up shortly with no intervention from me.
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Old 03-18-2013, 10:38 AM   #8
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Having accidentally had fuel return to the "wrong" tank, I am always VERY careful now to ensure that flow is only in and out of the same tank. Even on a long run (24 hours) you are only talking about burning something like 60 gallons (500lb) of fuel. Enough to cause only a slight trim change for a heavy trawler. Much safer to control the flow rather than assume gravity is doing it for you. Plus, if return flow ever does cause one tank to get more flow than the other, then the situation can possibly get out of hand. The preferred tank will conceivably fill at a faster and faster rate (because that side of the boat is tilting down due to increased fuel weight) until it may overflow!!
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Old 03-18-2013, 10:44 AM   #9
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Isn't the return just a small fraction of what is sent? In other words, wouldn't the return on a 60-gallon day be just a few gallons? I have never measured it. Would be interested to know what the numbers are.
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Old 03-18-2013, 10:49 AM   #10
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No. Depending on the engine the fuel return is usually much more than the fuel burned. The fuel flow overall is high to keep pressure up and to provide cooling. Your engine specs should indicate the fuel flow vs burn relationship.
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Old 03-18-2013, 11:01 AM   #11
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I draw from and return to the same tank, but have gravity transfer valves that can be opened to adjust trim if necessary. After winter, I'm usually lighter on the side my heater draws from. I simply dump my water from the opposite side (1,200 pounds) and open the transfer valves until the levels are equal. Having large outboard potable tanks makes it simple.
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Old 03-18-2013, 11:38 AM   #12
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No. Depending on the engine the fuel return is usually much more than the fuel burned.
That's certainly the case on my boat which is why I route the return line to the tank I draw from. Doing this, I know I won't be sending fuel to a full tank, resulting in a spill from that tank's vent.
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Old 03-18-2013, 11:51 AM   #13
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I always draw and return to one tank.

If you have gravity transfer fuel to the "low" side it will become lower and then more will transfer ............. and on and on. But usually not much list results but what if there was another force involved like a strong crosswind. That ma amplify things considerably and then what if you were running down a narrow channel in that strong wind and suddenly ran out into a very rough bay. That amplified downwind list will not be an asset in the sharp beam seas in the bay.

The above is hypothetical of course but given enough time something like that will happen. Kinda like a single engine quitting. I personally like to have control over such matters and my very slight list w full tanks invites me to start drawing from the low side.

My fuel manifold system is very similar to Marks but not so pretty and new looking for obvious reasons.
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Old 03-18-2013, 11:55 AM   #14
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My situation:
Starboard-Motor with starbord-Maintank,
Port-Motor with port-Maintank.
Keeltank es the trim-depod, from here works the Fuelpump o gravity to the 6 Tanks-also retorn.
In total i have 6 tanks and over 5000 Liter diesel when the tanks are full.
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Old 03-18-2013, 01:15 PM   #15
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While I *can* select to to and from tanks on my manifold, I never do. I draw from both and return to both. It always seem to burn almost even from both tanks. If one gets a little ahead, the other catches up shortly with no intervention from me.
There's a lot to be said for the "KISS" (Keep It Simple, Stupid) principle.
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Old 03-18-2013, 01:40 PM   #16
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Our two Fords normally feed from the 60 gallon day tank that is on the centerline of the boat. While the fuel return from the Ford Dorset is so little as to be almost non-existant, it is normally fed back to the day tank.

Te day tank is filled manually by using valves and gravity from the four saddle tanks. So we can affect the lateral trim of the boat if necessary using the saddle tanks. So far this has not proved necessary.

The engines can feed from and return to any tank(s) on the boat so in the event of a problem with the day tank feed the engines can be valved to other tanks. But since all the tanks including the day tank feed by gravity from their lowest points there is little or nothing that can interfere with the fuel flow other than a frozen valve or a hose problem.

As Ron mentioned earlier, there is much to be said for the KISS principle when it comes to the fuel system on a boat.
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Old 03-18-2013, 01:53 PM   #17
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When travelling where fuel could be suspect...many operators will only fill one side at a time and run from one side. That way if the fuel is bad, you have options...

That said I have never gotten bad fuel in the states or Bahamas for over 50 years of boating. But I have heard horror stories including a friend of mine that flamed out both helo engines between the fuel truck and the runway due to 20 gallons of water!!!
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Old 03-18-2013, 02:13 PM   #18
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When travelling where fuel could be suspect...many operators will only fill one side at a time and run from one side. That way if the fuel is bad, you have options...

That said I have never gotten bad fuel in the states or Bahamas for over 50 years of boating. But I have heard horror stories including a friend of mine that flamed out both helo engines between the fuel truck and the runway due to 20 gallons of water!!!
I'm in your camp. I have never had bad fuel but heard stories so I stay on the safe side and fill one tank at a time and run and return to one at a time. This way I have one "clean" tank to switch to if things go south.
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Old 03-18-2013, 02:24 PM   #19
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Obviously those that have a day tank are in a different boat...

But hopefully it isn't so large that it couldn't be quickly emptied or bypassed if contaminated (but in most cases a good filtration system to it is the answer anyway so problems should be rare to non-existent)
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Old 03-18-2013, 02:55 PM   #20
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Assuming a vessel is sitting level port to starboard, with port and starboard fuel tanks, should you have the fuel selector set to take fuel from both tanks to maintain trim?

Tom
In addition to the fuel selector lines, I have a connection between port and starboard tanks which I normally keep open to prevent overfilling of one tank in case that the return line drains to the other tank ( which easily could happen after dome dumbo ( me) puts one of the stopcocks in the wrong position)

And yes I do have a day tank, but only use it directly after refueling and if in doubt in heavy weather and for checking fuel consumption - the day tank mainly adds to complexity, creating chances for new mistakes ( like keeping the return line switched to one of the main tanks ...)
In my experience if you create a possibility for stupid mistakes, eventually you make them.

In my boat the open connection line does not result in significant listening, but that will not work if you need different levels to maintain the trim.
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