connecting 2 tanks.... best option?

so as a followup to this thread http://www.trawlerforum.com/forums/s3/bladder-smaller-poly-tanks-40487.html

I ordered 2 50 gallon diesel tanks https://www.easternmarine.com/moeller-50-gal-below-deck-fuel-tank-032550

that's about the biggest tank I can get into the engine room without major trouble... and I can get 3 in there for the factory 150 gal capacity if I need to. this is all on the PORT side only for now.
So my question is: should I attempt to connect the tanks at the bottom to make one 100 gal fuel tank, or keep them separate?
If I connect them it's going to be complicated to add a fitting to the bottom of a Polyethylene tank... from my research I'll have to install a nut on the inside of the tank and possibly cut an inspection port to do so.
And if I keep them separate do I install a second deck fill? or have a selector valve for each tank and a Y?
also if they are separate I'll have to figure out the fuel pickup situation.
any help/advise is appreciated while I brainstorm and wait for tanks to come in.
Thanks

also wondering if I could get the inside part of a bulkhead fitting through the fuel sender hole, or if anyone has had success doing this... I could imagine maybe using bailing wire to run the fitting down to the hole I've drilled in the bottom.

a third option is maybe have one tank a day tank and one an AUX tank, with an electric transfer pump?

Just put a shutoff on each tank at the top feed and run one tank to a tee in the line from the second tank.

Each tank should have a shutoff anyhow, and the engine will only draw from whichever one you open.

"Each tank should have a shutoff anyhow"

While its not a requirement for pleasure boats the Coasties demand a tank fuel cut f that can be controlled from outside the vessel for commercials .

Not many bucks for a safety that might save the vessel, or the usual huge fine if you put a teaspoon of diesel overboard and create an oil sheen.


Easy to do , just an outboard push pull cable when replacing tanks.

The shutoff I believe is a ABYC suggestion (read insurance survey worthy) anyhow....

And it will allow separate uses of the 2 tanks.

AND if located to be accessed out of the engine room....like Fred posted.... BONUS.

The only time I saw twin replacement tanks they were either 50 or 55 gallon and they spaced about 1 foot apart (from memory) and they had a large, maybe 1 1/4 inch pipe connecting the bottoms, with a shut off in between.
Each tank had its own deck fill.
Both sides were set up this way.

What's your plan for fuel return? If you isolate the two you'll need to have dedicated valved return lines as well. I didn't see a return port mentioned in the specs.

If he has Lehman 120s, can't tell from pics in other thread...

They return so little fuel, I just plumbed them right back into the fuel feed lines at my fuel manifold with appropriate valves,zinced andey have worked for over 2000 hours that way just fine

psneeld said:

If he has Lehman 120s, can't tell from pics in other thread...

They return so little fuel, I just plumbed them right back into the fuel feed lines at my fuel manifold with appropriate valves,zinced andey have worked for over 2000 hours that way just fine

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Thanks for this! I was wondering if that would somehow interfere with the operation of the lift pump, or of the suction from the lift pump would interfere with the operation of the injection pump.
And yes they are FL120s

Jeff F said:

What's your plan for fuel return? If you isolate the two you'll need to have dedicated valved return lines as well. I didn't see a return port mentioned in the specs.

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As the other poster said, I may just plumb the return back I to the feed. It's a FL120

jleonard said:

The only time I saw twin replacement tanks they were either 50 or 55 gallon and they spaced about 1 foot apart (from memory) and they had a large, maybe 1 1/4 inch pipe connecting the bottoms, with a shut off in between.
Each tank had its own deck fill.
Both sides were set up this way.

Click to expand...

That would be a good option for me and simple IF I can figure out how to reliably make a 1" + outlet in the bottom of a polyethylene tank.

psneeld said:

Just put a shutoff on each tank at the top feed and run one tank to a tee in the line from the second tank.

Each tank should have a shutoff anyhow, and the engine will only draw from whichever one you open.

Click to expand...

This is probably the simplest method and a second Y on the engine side of this for the return.

From my understanding reading around here and the internets, it's better to not have any fittings low on the tank to prevent fuel leaks into the bilge and being pumped overboard by the bilge pumps. Am I wrong on this?

Check out fuel systems in a big truck or MD market like a freightliner m2-106 with cross lines. I could fill one tank and it would balance out the level as well as handle the return flow.

TowLou said:

Check out fuel systems in a big truck or MD market like a freightliner m2-106 with cross lines. I could fill one tank and it would balance out the level as well as handle the return flow.

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None of the medium duty or commercial trucks I drove had that system. I've seen it though. Dumping 100 gallons of fuel on the ground doesn't seem as bad to me as dumping 100 gallons into a bilge and then overboard.

ben2go said:

From my understanding reading around here and the internets, it's better to not have any fittings low on the tank to prevent fuel leaks into the bilge and being pumped overboard by the bilge pumps. Am I wrong on this?

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There are differing opinions on this. I like to have a drain point at the lowest point on the tank. Preferably on a sump, but my present setup has the tanks sloping down to the drain point.

This bottom outlet valve is normally closed, and opened only to drain water, transfer fuel to the other tank, or check the level of either tank with the shared sight tube. This bottom drain point can also supply fuel to the engine in an emergency if the levels drop below the pickup tubes.

My fuel lines and fittings are heavy walled stainless steel rated at 6000 psi. A bit of overkill, but I think this system has much less risk of leaking fuel than a system with top fittings only but rubber or plastic fuel lines. You can still empty a tank into the bilge on top fitting systems if you develop a leak in your return line.

ben2go said:

None of the medium duty or commercial trucks I drove had that system. I've seen it though. Dumping 100 gallons of fuel on the ground doesn't seem as bad to me as dumping 100 gallons into a bilge and then overboard.

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The freightliner I drove the M2-106 had a 40 gallon tank on driver side and a 60 gallon tank on the passenger side. Every other day I'd top off the passenger side and how they ran the fuel lines between the tanks it would balance out between the two with a cross system.

"From my understanding reading around here and the internets, it's better to not have any fittings low on the tank to prevent fuel leaks into the bilge and being pumped overboard by the bilge pumps. Am I wrong on this?"

Simple to drain water from the lowest point in the tank.

The valve can be plugged as a second safety and a drain pipe installed only when checking for water & crud.

Personally I would not cross connect the tanks so a bad load of fuel could be kept isolated till cleaned.

AusCan said:

There are differing opinions on this. I like to have a drain point at the lowest point on the tank. Preferably on a sump, but my present setup has the tanks sloping down to the drain point.

This bottom outlet valve is normally closed, and opened only to drain water, transfer fuel to the other tank, or check the level of either tank with the shared sight tube. This bottom drain point can also supply fuel to the engine in an emergency if the levels drop below the pickup tubes.

My fuel lines and fittings are heavy walled stainless steel rated at 6000 psi. A bit of overkill, but I think this system has much less risk of leaking fuel than a system with top fittings only but rubber or plastic fuel lines. You can still empty a tank into the bilge on top fitting systems if you develop a leak in your return line.

Click to expand...

Your tanks are probably stainless as well them
I'm using polyethylene so I'm ot sure this applies.

Think about how you will service the tanks in the future or if something goes wrong? Valves to isolate, valves to drain water and crud and valves for the sight tubes. I would also add clean-outs, so you can look in there easily in the future. Its easy during installation and not so easy later. Also, my preference is a gravity feed vs. a dip-tube on the top. Ball valves, not gate valves and MAKE SURE the ball valves you choose are compatible with the threads on the fittings on the tank, avoid like the plague tapered and straight thread mixes. Last but not least, go to a hose shop and have proper fuel lines made up, no press-fit-hose-clamp crap.

For a cruiser "proper" may have different concepts.

Some end makers like Aeroquip make end fittings for both fuel and hyd that are owner reusaible.

A good set of box wrenches , and a vice help , but the hose can be replaced when desired.

Folks going to a hyd. shop might want to ask for swivel fittings on BOTH ends to male R&R a snap.

I'm still in the process of sorting out all of the systems on out 'new to us' 1988 CHB Sundeck. It has four 110 gal fuel tanks, two on each side (two aluminum replacements, two orig iron). Each has a fill cap on the side decks, and as I was told, all four tanks are connected - which must be correct as the sight tubes are all equal - took several hours for them to level back up after I added 75 gallons each to two tanks on the port side. I have discovered a valve on the cross line (from port to starboard tanks), and will check next to see if I can hopefully find valves between each side tank. Ah, the joys of discovery on an older boat - but thankfully this boat is pretty original ie not a lot of DIY work has been done on the boat.

I've read connections on the bottom of a fuel tank are forbidden, at least for gasoline, not sure about diesel. I wanted to do the same with my gas engine but that's a no go. Seems to me the only option is separate feeds with a selector valve, or a transfer pump.

Forgot to mention - ours are diesel engines (Lehman 135's). Don't think there is any problem with low connections with diesel fuel tanks. And I suspect I'll get the opportunity to change out those other two iron tanks some day - but for now they appear solid with no leaks and no rust on top

jimfrens said:

I've read connections on the bottom of a fuel tank are forbidden, at least for gasoline, not sure about diesel. I wanted to do the same with my gas engine but that's a no go. Seems to me the only option is separate feeds with a selector valve, or a transfer pump.

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That may be where I got the "no low connections" stuff from, my days of working on gasoline inboards.

Don’t forget that you’ll also need a vent line for each tank.
You can tee into one common vent line at a high point but they will all need to breathe individually.

AusCan said:

Don’t forget that you’ll also need a vent line for each tank.
You can tee into one common vent line at a high point but they will all need to breathe individually.

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Definitely. Thanks

FoxtrotCharlie said:

Forgot to mention - ours are diesel engines (Lehman 135's). Don't think there is any problem with low connections with diesel fuel tanks. And I suspect I'll get the opportunity to change out those other two iron tanks some day - but for now they appear solid with no leaks and no rust on top

Click to expand...

Well the problem for me might be trying to install a low connection on a polyethylene tank.
I'll probably go the 2 seperate top feed with a T and 2 shutoff valves for now method.