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

[Frosty]

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

 

[Brisyboy]

Logical question ..........in most cases the fuel tanks are linked (or can be) so if the return fuel only goes to one-no big deal as the level between the two eventually equalizes.

Even though our boat has 2 engines the idea is the same. There is a tank either side - outboard of each engine. There is a valve panel which gives the option of each engine drawing from a separate tank and both from a single tank, either one. There is also a pipe which runs direct from one tank to the other -it has a valve on it- which, if open allows the tanks to equalize

 

[DavidM]

I had a MS Pilot 34 with that tank and fuel setup. Each tank teed into a common line that fed the engine. This acted as an equalization line and the tanks stayed pretty even. I never had to touch it and worry about it.

When I refueled I would always put maybe 5 gallons more in one side vs the other, but that was no big deal.

Easy to plumb and easy to use. But it did give you the option of isolating a bad tank, like when I mistakenly put water into the fuel tank .

David

 

[SteveK]

One would expect a switching valve for the return as well if only using one of two tanks.
With a single engine and two tanks would one not be more inclined to draw from both tanks to help keep them at same level? The returning to one may not matter so much.

 

[HeadedToTexas]

My boat is plumbed that way, but we have no crossover valve. To move fuel from one tank to the other, we must draw from one and return to the other. That brings in the possibility of overflowing a tank though, so be careful.

 

[Conrad]

Our boat has the one engine two fuel tank setup and no crossover line. I leave both supply valves and both return valves open and the tanks balance themselves.
I'd always assumed that the pickup and return tubes were top mounted with the bottoms of the tubes reaching a point slightly above the bottom of the tank. Varying head pressures would keep the tanks in relative equilibrium. But as was suggested earlier in this thread, maybe all it takes is the joining of the supply and return tanks before and after the engine.

 

[Stout]

2 Fuel tanks tee’d together ?

We have had two single diesel engine boats with 2 tanks, the outlets are tee’d together and the engine fuel return is only plumbed to a single tank. The generators are fed by the tank the return line goes too. So far it has worked out well and is simple to operate.

 

[alormaria]

Mine is setup like this: Single engine, two tanks with balance tube, return line to one. After a day's run I have a slight list. By next morning it is gone. Boat's too.

 

[High Wire]

I had a MS Pilot 34 with that tank and fuel setup. Each tank teed into a common line that fed the engine. This acted as an equalization line and the tanks stayed pretty even. I never had to touch it and worry about it.

When I refueled I would always put maybe 5 gallons more in one side vs the other, but that was no big deal.

Easy to plumb and easy to use. But it did give you the option of isolating a bad tank, like when I mistakenly put water into the fuel tank .

David

My Monk is plumbed the same way. No separate cross tie line. It does have a return line that tees off to both tanks. Level always stays within an inch.

 

[Frosty]

Hmm, maybe my brain just thought the symmetry of two tanks, two feeds, two returns seemed "better" but for no real reason.

Or... would there be advantages to two returns?

Is there a good reason not to have them (so return to only one side)?

I've only had diesel boats with single engine/single tank (or gas outboards so no returns) --- so I've never had to think about it before.

 

[HeadedToTexas]

Our boat has the one engine two fuel tank setup and no crossover line. I leave both supply valves and both return valves open and the tanks balance themselves.

Suppose I could do that too, though there is quite a bit more fuel line to and from the port tank as the Racor and valves are all on the starboard side. I suspect that would mean more fuel would be drawn from and returned to the starboard tank, but it would nice if they just evened things out on their own.

I quickly grew to appreciate all the valves because it let me move fuel into the port tank to offset the weight of my batteries on the starboard side to keep the boat level. A short term fix looking for a long term solution.

 

[Lou_tribal]

Hmm, maybe my brain just thought the symmetry of two tanks, two feeds, two returns seemed "better" but for no real reason.

Or... would there be advantages to two returns?

Is there a good reason not to have them (so return to only one side)?

I've only had diesel boats with single engine/single tank (or gas outboards so no returns) --- so I've never had to think about it before.

Not sure I understand what you mean with two returns? With a single engine you will have 1 return line from the engine so either a valve to select which return tank, or a single return to one tank, or a T fitting returning to both tanks.
I have the later setup, so two tanks, 1 feeding valve to select one or the other or both tank, 1 return with T to both tanks. Fuel is slightly more coming from port than starboard but not a big difference. One important thing is not to fully top any of the tanks so not to risk any overflow in any of them.

L

 

[DDW]

Mine has two tanks, feed valves teed together, return valves to each tank which lets you do what you want but allows user error. My other boat has two tanks, but a 2 spool valve that switches both feed and return with a single lever. Less flexible, but much more user friendly. It would not be expensive (in boat terms anyway) to fix whatever you have to whatever you want. The most expensive part would be adding a port to a tank that didn't have one.

 

[Simi 60]

Ours has two tanks and is plumbed to run wherever I want.
Single return line on ceiling into t piece going to both tanks.
Both tanks feed the filters
All ins and outs have valves
Boat always balanced

 

[Jeff F]

If you don't have switchable returns then you can't effectively isolate tanks. Essentially you're treating the two tanks as one big tank. To my mind that's a bug, not a feature. I guess each approach has its merits, but I want to be able to shut off one tank and run off the other if needed. That requires directing the return to the tank I'm drawing from.

 

[Frosty]

Not sure I understand what you mean with two returns? With a single engine you will have 1 return line from the engine so either a valve to select which return tank, or a single return to one tank, or a T fitting returning to both tanks.

The bolded part is what I meant. In other words, the boat models I'm thinking about (don't own one yet but have seen this on more than one type of boat), ONLY allow you to return to one of the tanks (whichever one they set it up as when building the boat and never the other one); but you can DRAW from either tank.

Headed to Texas: I bet you have hit on the "why." Now that I think about it, all of the boats I've seen that do this return the fuel to the tank closest to the Racors and the valves, and the "far away" tank cannot ever be designated as the one to return to. Not saying this is why it should be; but probably why it "is."

DDW: The spool valves sound interesting.

If you don't have switchable returns then you can't effectively isolate tanks. Essentially you're treating the two tanks as one big tank. To my mind that's a bug, not a feature. I guess each approach has its merits, but I want to be able to shut off one tank and run off the other if needed. That requires directing the return to the tank I'm drawing from.

I think my mind works like yours. I like the idea of more control, and the symmetry of being able to do things whichever way.

Thanks everyone for responding (any any future ones too). My take away from this is as follows:

1) The two tanks, one return destination can work fine, and requires fewer valves and less fuel line running around. I think you could overfill the tank being returned to though when running off the other tank (right?) so there is still possibility for error. In other words, you have less control but not sure you have less responsibility?

2) With two return destinations you have more control, more options, and a bit more fuel line. You need to think about it a bit so you have things going where you want them to.

3) The spool valves that move both at once take away some responsibility (always draw and return to same tank), but maybe remove a little bit of control too.

Thanks again,
Frosty

 

[HeadedToTexas]

Makes sense to me.

 

[Arc]

I have to feed valves and 2 return valves. I open the side I want and close the other. I was told trying to use 2 tanks at one time was a bad idea. I never tried it.

 

[Frosty]

I have to feed valves and 2 return valves. I open the side I want and close the other. I was told trying to use 2 tanks at one time was a bad idea. I never tried it.

That is a new thing to think about (drawing from two tanks at once). But what I was asking about (perhaps could have said it better) is boats that allow you to choose which tank to draw from, but only ever allow you to select one tank to return to (not choose one, but only allow you to return to the one tank they decided to plumb that way when they built the boat).

 

[Frosty]

I just thought of something that is probably obvious to everyone else Maybe someone above mentioned it and it went right past me.

So let's say I come into possession of one of these boats that has a supply from either tank, but you can only return to one specific tank and not the other one.

The return fuel line probably goes into the tank near the top, right?

So on the tank that was not built with a return, there would probably be a "missing" port that the other tank has for its return line.

This would depend on access, but could one put a Tee in the vent line right at the tank and return to that?

This would not be a Detroit, so let's assume for now not a "firehose" of return fuel.

(This is all only theoretical at this point as I don't even have the mythical boat yet.)

 

[rgano]

My single engine Mainship Pilot came to me as Frosty described in the first post, return for main engine to port and bottom-fed supply lines connected with tee to the main engine. The small generator draws and returns to the stbd tank.

If I fill one tank and the other is say, half full, the fuel will transfer from the high to low side to a point and then ceases flow at some point leaving a noticeable list. This has been discussed ad nausea amongst Pilot owners - it is what it is.

So what happens if a tank where the only main engine return becomes unusable (name you poison here)? You are screwed because this engine at cruise returns about 11-12 GPH (75-gallon tanks).

I came from a twin engine GB with four tanks and the flexiblity to move fuel from anywhere to anywhere via a pump or using main engines' returns, and I was totally unhappy with the Pilot's original system. It was obvious to me it was a "management" decision to plumb it this way because the return port on the top of one tank was there in plain sight unused. Same for the dopey fuel level senders.

It was an afternoon job (and not even all afternoon) to run a hose from the non-return tank to a wye valve off the main engine fuel return where the other tank was joined up. Now the system allows me to select where the return fuel goes. I can level the tanks very quickly by shutting off the high tank's return. In order to make this work easily by avoiding me having to lift the hatch to check the sight gauge, I had to be able to read the fuel level in either tank at the console, but the stupid boat came with only one fuel tank sender connected to the single Faria cluster gauge. There was a sender with unconnected wires in the other tank. It was a simple task to run a wire from that unused sender to a new double throw switch on the console to be able to read either tank level.

Nowadays, I leave the two return valves open, and things seem to stay level. Of course, settling the boat in the boat lift cradle chocks forces the tanks to equalize after any uneveness accumulated durng a run, something they would never do completely during the year and a half the boat sat in the water.

 

[Frosty]

My single engine Mainship Pilot came to me as Frosty described in the first post...

If I fill one tank and the other is say, half full, the fuel will transfer from the high to low side to a point and then ceases flow at some point leaving a noticeable list.

Yes this is the exact sort of thing I was thinking of (plus you brought up a couple of additional points).

So did you have to add any fittings or ports to the "new" return tank? Does fuel return through the supply line (that doesn't sound right). I suppose it might make a difference if a tank was bottom supply or dip tube supply.... right?

 

[rgano]

Yes this is the exact sort of thing I was thinking of (plus you brought up a couple of additional points).

So did you have to add any fittings or ports to the "new" return tank? Does fuel return through the supply line (that doesn't sound right). I suppose it might make a difference if a tank was bottom supply or dip tube supply.... right?

I unscrewed the plug in the tank fitting and screwed in a brass pipe to barb fitting. Easy Peasy.

Fuel returns through the return fuel line from the main engine. I just cut the rubber return fuel line and stuck in the wye valve, original line to the port tank into one side and a new hose to the stbd. BTW, my particular wye consists of one input nipple and two outputs, ech output with a valve of its own.

Top feed tanks cannot equalize without and cross-connection line going from bottom to bottom. But if you have that equalizing line, why bother with the top feed? Just tee into the crossover and use that to feed a single engine. More complex for twins.

Although not in your original post, I added the second Racor in the photo. Hard to imagine feeling safe and secure with the original fuel delivery and return system, but hey, it's mass production.....

 

[Spinner]

My set up is the same. Engine draws from both tanks, return goes to one tank. When I record fuel at the end of the day, they vary by only a few gallons... it works.

 

[mvweebles]

My Willard 36 with Perkins 4.236 75 hp was plumbed with a single return to port tank. It was workable, but not optimal as any list had to be managed via balancing supply lines.

As part of tank replacement, I went with groco fuel selector valves.

https://www.groco.net/catalogsearch/result/?q=TS-350-T

Peter

 

[AusCan]

A wide hard chined boat wouldn't be ok to keep tanks equalized, but not so on a narrow beam, round bilge boat like mine.

This type of hull lists with minimal imbalance.

With a crossover line open and a slight list, the fuel (or water) will start running down to the lower tank, making to the list more pronounced and more fuel/water transferring to the lower side. A compounding problem.

 

[FF]

The boat assembler knew what engine was selected for the boat.

Some return so little fuel that it would hardly be notices to have a gallon or two extra on one side after a days run.

For sure the boat was not built for a Detroit , but many auto or farm implement engines return almost nothing,so why bother?

 

[Frosty]

With a crossover line open and a slight list, the fuel (or water) will start running down to the lower tank, making to the list more pronounced and more fuel/water transferring to the lower side. A compounding problem.

That's one of the issues I read about from owners. Oftentimes a majority of the seating on the boat was on the (only possible) return tank side, making it worse.

 

[C lectric]

To me the dual feed , single return was done by the builder to save a few bucks for parts and labour. I'm not really knocking it as lots of those decisions are made for all of our boats for all kinds of things.

No matter which way they go there will be a problem show up at some time such as the time I forgot to open a feed valve but the tank was receiving return fuel and overflowed. Just glad I caught it as soon as I did.
I always close one feed valve when refueling and in 35 yr that was the one time I was in a bit of a rush to get off the fuel dock as I was in the way of a much larger vessel.
Just a lack of attention as I have dual feeds and dual returns on a single engine.

 

[markpierce]

Have one engine and four fuel tanks: two to port and two for starboard. Have two manifolds: one for sending fuel and one to return. Have flexibility to draw from any tanks and return to any. Unless there is a special reason, I draw from one tank at a time and return to same. Do monitor boat lean to switch tanks.