Single engine, dual fuel tanks, return to only one. General questions about why.

[DrissZougari]

My single MS390 is setup with supply and return each teed all line have valves on tank side. It also have a cross over between the two tanks with a valve on one side. One important think that everyone should understand is that Diesel Engines burn less fuel that what it is supply to them, so it is very important to know where that return goes to prevent a major spill or a list depending on your boat.
I personal prefer running everything in open position however, my boat will list a little after 5 or 6 hrs cruise. everything will go to normal thanks to the cross over in by the next morning.
having valves on each line may work for other boats that are not too susceptible to listing.

 

[GoneDiving]

I've got one engine, two tanks in regular use and a third in the bow that rarely gets used. Unvavled returned to both tanks. However, one side's copper piping has taken some bumps and flow is restricted to minimal to the port tank.

Consequently, we always tend to build up a list to the stbd side. Don't ask me how I know that over a full day there is enough transfer to empty one 2500l tank into the other and overflow it.

I'll admit I've never left both supply lines open to check if the tanks auto level under way despite the returning to essentially one tank.

A side effect of the lean to the stbd side is that all my other drains (Aircon condensate, roof rain catcher etc) are all on the port side so water drips and puddles can become a pain.

On the to do list are supply and return manifolds with valves as mentioned above, possibly with a small transfer pump for transfer and polishing.

Everything is a work in progress. ��

 

[HopCar]

“ Fuel can be sourced from either tank by changing a valve, but fuel only returns to one of the tanks.“

I can’t imagine any boat plumbed that way. If you fill both tanks and run off the no return tank, you will be pumping fuel out the vent of the return tank in short order.

If the tanks are connected with a reasonable size cross over, no problem. You’re really dealing with just one big tank.

My boat Possum was a single with two tanks. She had a manifold of four valves that would allow me to draw from either tank and return to which ever tank I wanted too. The only reason I had to draw from one and return to the other would be to balance the boat if one tank was really low.

I tried running with all the valves open but discovered that the engine preferred to draw from one tank over the other. I soon had one empty tank and one almost full.

 

[dirtdoc1]

I have a single engine with two tanks but it's set up so I can return to either tank. I have it set up to draw from one tank and return to that same tank. When fuel starts to get low in that tank I can either equalize them or pump fuel from the unused tank to the used tank. I guess you could call one tank the day tank. This is my first trawler and diesel boat so the whole concept messed with my head at first. I added transfer pumps for both fuel and water so I could balance the boat. Later on I discovered that it was useful for when I was taking on fuel or water. I could let it fill in to one tank then use the transfer pumps to fill the other tanks. No moving the hose to the other side, it's great!

 

[MV Caroline]

Caroline, too, has a single and two large fuel tanks Port and Starboard outboard of the engine. I generally feed from both tanks to help maintain balance and I do have a split return. I can manifold to or from either tank or both at the same time. One thing I did was to add fuel return filters to both sides. Because the fuel pump supplies more than triple the amount of fuel used by the engine at cruising speed (2.2 GPH) to the injector pump, I get the benefit of "scrubbing" or "polishing" the fuel while underway. On the hook, my generator feeds from the port tank and the Webasto feeds from the starboard tank.

 

[GB36_OuttaHere]

Enjoyed this read. Thx.

While we're talking fuel lines, valves and crossovers, can we take a quick moment to discuss the tank sight-glass set up of choice? I replaced my two on my 88 GB36 and within 5 years, they are yellowed and hard to discern the level. Even with a flashlight behind it.. Hope this is ok, didn't want to start a new thread.. Cheers, SK

 

[SteveK]

Enjoyed this read. Thx.

While we're talking fuel lines, valves and crossovers, can we take a quick moment to discuss the tank sight-glass set up of choice? I replaced my two on my 88 GB36 and within 5 years, they are yellowed and hard to discern the level. Even with a flashlight behind it.. Hope this is ok, didn't want to start a new thread.. Cheers, SK

Are they more than 1/2" ID clear plastic tubes? I plan on changing mine and putting in some kind of floating colored item.

 

[Cigatoo]

Are they more than 1/2" ID clear plastic tubes? I plan on changing mine and putting in some kind of floating colored item.

I used plain old clear tubing. Turn valve on, ck the level, and turn the valve off. It’s been 2 years and they are still good. I’d be quite happy with 5 years. If they yellow and get hard I will replace. 10 minute job on each tank.

 

[Bryant]

fuel tanks

Hi folks,

I have a question that's not directed at a specific boat, but is something I've seen on a few different boats, and I'm wondering why.

That is, boats with a single diesel engine, and two mirror image fuel tanks, port and starboard. Fuel can be sourced from either tank by changing a valve, but fuel only returns to one of the tanks.

To my inexperienced mind, it seems like there would be more potential for "losing track" and either running out of fuel (from one tank) or possibly overfilling the other one (draw from one; return to the other), and/or inability to balance boat.

So my question is this:

--- Why would this have been done in the first place? Is there a benefit I'm not aware of?

-- If it was done for production ease or cost, then I would suppose it's something an owner could change on their one particular boat. Is this more complicated than adding fuel line and valves? Anyone done it? If more complicated, what are the considerations?

-- Any other thoughts I don't even know to ask about?

(Reason I'm not being more specific is that I have seen this on more than one brand and model of boat as I've been boat shopping, and I want more "general concept" answers vs. answers for one specific boat/model.)

Thanks,
Frosty

My fuel system is set up so that the tanks equalize. In other words, they feed each other to maintain essentially the same fuel level in each tank.

 

[HopCar]

You guys with fuel level sight tubes might want to switch to Tygon instead of clear vinyl.
https://www.usplastic.com/catalog/item.aspx?itemid=23487&catid=864

 

[Frosty]

can we take a quick moment to discuss the tank sight-glass set up of choice?... Hope this is ok, didn't want to start a new thread.. Cheers, SK

These threads discuss sight tubes (if you haven't already read them):

https://www.trawlerforum.com/forums/s6/fuel-tank-sight-glass-material-24177.html

https://www.trawlerforum.com/forums/s32/fuel-tank-sight-tube-valves-54397.html

https://www.trawlerforum.com/forums/s6/fuel-sight-tubes-7095.html

https://www.trawlerforum.com/forums/s6/sight-tubes-diesel-tank-1309.html

 

[SteveK]

You guys with fuel level sight tubes might want to switch to Tygon instead of clear vinyl.
https://www.usplastic.com/catalog/item.aspx?itemid=23487&catid=864

But why? That stuff starts off yellow.

 

[HopCar]

But why? That stuff starts off yellow.

Vinyl hardens and will eventually break. Tygon is designed for fuel.

 

[Frosty]

Thanks to everyone with a single engine and two tanks who has commented on their fuel tank return setups. I'm still reading each new post.

My thought now is that although it can certainly be worked with (many clearly do), one engine with P & S symmetrical feed tanks that only have a return possibility to one tank is not ideal.

That's what I suspected but thought there might be some hidden benefit (not really seeing one though at this point).

If I ever find a trawler (not the ideal time right now ), and it has this setup, I may ask for some input on changing it (because you know, there's always so little to do on a new-to-you boat )

In the meantime I'll still be reading any new contributions on the topic. Thanks

Frosty

 

[Pete Meisinger]

Balance and Range

pete

 

[Bill Neilson]

My situation is a bit unique. On my Mainship 390, the 150gal port saddle tank was leaking. I closed it off completely including the return line. I pumped the remaining diesel into the good tank using a 12v fuel transfer pump. I have been operating one 150gal tank ever since. Luckily, my generator is feed from the good tank. I don't have plans to replace the bad tank. The boat will offered at a good price when I'm ready to sell it.

 

[jleonard]

My Albin was return to one side only, but had an equalizing line at the bottom of the tanks. Since the Lehman returns so little fuel the boat always stayed level.
My old 1978 Mainship was another story. The original Perkins 160 returned "some" fuel and only to the port tank. Si it was pretty much run the port side for 4 hours, then switch and run starboard for 2 hours and the level would equalize.
However, when I repowered with the Cummins 270 it used an inline Bosch "P" pump that circulated 45 gallons per hour and consumed about 5 at cruise. I knew this ahead of time so I plumbed a return to the starboard side and built a return manifold that I could direct. So it was run starboard and return the same for X hours, then switch both valves to keep the boat level.
That was sort of a pain, but I got used to the routine.
The plus side was that my fuel stayed pristine because it was always getting filtered.

 

[SteveK]

Jay. Was the return manifold open to return to both sides at same time, but you had to direct when one side became fuller. During this time was there also a crossover at bottom?

 

[jgwinks]

Many boats don't actually return to the tank. My Concorde has twin Perkins 6.354's and twin tanks. They return to the supply lines upstream of the filters and lift pumps. No return lines or crossovers at the tanks. One of my previous sailboats had a single engine and dual tanks, the return went to the secondary filter, so the return line was only about six inches long. That would be a cheaper install, fewer lines and connections. Seems like to would be harder to bleed the air out, but I haven't noticed that.

 

[Frosty]

Balance and Range

pete

Is this in response to my "I couldn't really think of an advantage to the setup [I asked about] but thought there might be one"?

I don't see how it could give more range since the size of the fuel tanks is the same either way; and it seems pretty obviously less advantageous for balance (tho' you can clearly work with it).

So could you extrapolate a bit? I'm missing something in your reply.

 

[Frosty]

Many boats don't actually return to the tank. My Concorde has twin Perkins 6.354's and twin tanks. They return to the supply lines upstream of the filters and lift pumps.

That's interesting. I'm not actually clear on where the returns usually go (figured it was into the tank through a fitting but don't actually know for sure); but perhaps "your" way could make retrofitting a second one easier.

 

[jleonard]

Jay. Was the return manifold open to return to both sides at same time, but you had to direct when one side became fuller. During this time was there also a crossover at bottom?

I did not have a crossover line between the two tanks.
If I was feeding from the port tank I was also returning to the port tank and visa versa.

 

[C lectric]

Many engines over the range of builders are plumbed either way.
It depends upon the engine and how much fuel they send back. Engines that flow substantial quantities of fuel more than they burn usually send the excess back to the tank. Those that are this way use the excess fuel to cool AND lube the injection pump and injectors. They are plumbed to send back to the tank. The tank then acts as a heat sink and radiator.
If there is air in the return fuel the tank will allow it to dissipate.
Cummins, Cat, DD and others are this way.

There are engines that do not send excess back to the tank but they usually do not flow much more than is burned. The Lehmans are often this way and it would appear the older Perkins are. Often small engines are this way.

The catch is if there is air entrained in the fuel it will stop the engine much more readily because the air cannot escape via the tank. It heats the fuel more which may or may not be a problem. It can make troubleshooting more difficult.

 

[Frosty]

Many engines over the range of builders are plumbed either way. [Then all the info about return to tank vs. return to line]

C lectric: Super informative. Thank you!

 

[SteveK]

Balance and Range

pete

Balance- One tank either side to balance the boat loading?
Range- Two tanks extend range when only so much room for one tank off center?

 

[Transaxial]

There are a lot of different ideas for fuel systems for sure. My 40' Palmer has a single 6D14 Mitsubishi and 2 - 250 gallon tanks just behind the engine room. It was plumbed with a valve and line off each tank to a T before the filters on the engine. The return was a single line to unique "manifold" that dumped fuel to both tanks or if there was a list, it dumped into the high side. Have never seen one like it but it was just a length of 1 1/2" Al pipe with the ends welded closed and mounted up near the top of the tanks and between them. The single return line was tapped into the center of the 2 ft long 1 1/2" pipe. Then at each end of the 1 1/2" pipe a fuel return line was piped to the opposite side fuel tank. There were valves on each tank but I ran with them open so that say you had a list to port, the fuel in the 1 1/2" pipe would flow to port side and out into the hose that went to the starboard tank. This automatically (the true simple definition of automatic!) ran the return fuel to the high side tank which balanced the boat at level and then the fuel would return to both tanks keeping it level. I still had valves on the outlet from each tank so if one tank was disabled I could still run on the other. Lots of options there. I also added a home made polishing system that had a valve and a hose coming off the bottom drain port of each sump can (about 5" diameter and 6" long) and running to a selector valve before the 12 V pump that sucked through the 900 Racor and pushed through the big Cat 1R - 0755 6 micron off a Cat 777 rock truck. The return of the polish system could flow to the 1 1/2" self leveling manifold, or be directed to the tank of choice by closing one of the return valves. By having a valve on each of the ports in the tanks I could do lots of things like run on one tank, run on both tanks, empty one tank completely into the other if below 1/2 full while filtering all the fuel, then filter all the fuel back into the other tank. This allowed complete emptying and filtering of all the fuel in both tanks to be sure everything was clean. I had a selector valve from the polishing pump system connected to the engine supply line that could be opened to prime the engine filters which was a nice service feature. It seems you can let your imagination run on these fuel system features as long as it doesn't get too complicated to keep track of. There is more possibility for screw ups as you get too many things happening at once. I like the simple two tank feed and self levelling return setting on my boat for most days.

 

[Trimjb]

My boat has 1 diesel engine and 2 fuel tanks. No fuel tank cross over.
Fuel tank selector valve is a 6 port manual valve which shifts between tanks. Both supply and return to tank selected.

 

[HopCar]

My boat has 1 diesel engine and 2 fuel tanks. No fuel tank cross over.
Fuel tank selector valve is a 6 port manual valve which shifts between tanks. Both supply and return to tank selected.

Like this?
https://www.groco.net/products/valves-seacocks/fuel-valves/fv-6-port-series

 

[Trimjb]

Like this?
https://www.groco.net/products/valves-seacocks/fuel-valves/fv-6-port-series

Yes, that's the one.

 

[FF]

The shown valve would be best choice as the danger of fuel returning to a full tank and blowing fuel overboard is gone, as is the danger of overheating the fuel by returning it to a feed line.

A series of dangers removed , NO knowledge required to keep in balance,,cheap insurance.