Fuel Tank Design Assistance

[Solly]

Just a thought..
Looking at your drawings you want to feed from the front tank and return to the rear with a crossover in between the tanks. I believe the crossover should have shut offs at both tanks in case it starts leaking. Sight tubes have shutoffs top and bottom. If you ever have to shut off the crossover you might have a problem using that side. You can draw fuel from the front but with no vent it will pull a vacuum and shut down at some point. Also what happens if the return fills the rear tank ? Where would it go ?

I'd seriously consider keeping the tanks separate. More connections and all.

Maybe somebody with more experience will chime in on this !

 

[Nick F]

As regards the valving for the sight tubes:

1: at the bottom, use a "PUSH BUTTON VALVE" (see Amazon) so that you cannot leave the valve open by mistake.

2: at the top: Why can one not simply tee the line into the vent of the tank? The tank vent does not have a shutoff valve, so why does the sight tube need a shutoff valve at the top? (I am assuming that the teed connection into the vent line would be above the level of the top of the tank)

 

[FF]

To draw fuel and return the fuel to the same tank is least complex and requires no learning curve.

 

[motion30]

I just did a similar fuel tank replacement as you were speaking. After removing the old steel tanks with a sawzall I mocked up the shape and size of the smaller tanks I planned on from quarter inch plywood to make sure I could get them in place. I used inch and a quarter fittings and ball valves at the bottom of the tanks to connect all together. Actually I had to use three tanks to replace the 148-in long steel tank. I had vent on each tank t together going overboard. It has worked well and I am happy

 

[stubones99]

As regards the valving for the sight tubes:

1: at the bottom, use a "PUSH BUTTON VALVE" (see Amazon) so that you cannot leave the valve open by mistake.

2: at the top: Why can one not simply tee the line into the vent of the tank? The tank vent does not have a shutoff valve, so why does the sight tube need a shutoff valve at the top? (I am assuming that the teed connection into the vent line would be above the level of the top of the tank)

ABYC says no spring loaded valves, since the springs fail when heated by fire and then lets it feed the fire with fuel... By that point, you have a much bigger problem. I wondered about a ball valve with a 2lb weight on the handle to turn it off when you let it go, but I suspect they will have a problem with that too.

I would think sight tubes would be prohibited too, but nooo...

To answer why there is a top of sight tube valve, it is there so if your boat sinks, it won't leak fuel as fast. You never know the orientation it will be when / if it sinks...

 

[mvweebles]

Sight glasses are common in many industrial applications. LDI Industries makes many models. I chose one with a needle valve at top and bottom encased in an aluminum protective housing. They were about $175/ea

Picture is from installation and not hooked up yet

Peter

 

[Caballero II]

Sight tubes

The hassle with site tubes is it requires one to enter the Hell Hole to monitor the reading.

Great in 1900 with paid crew in the engine space, but today it can be way easier .

And other fluids can be monitored .,,,,NOT electric so seldom fails.

Tank Tender

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I have SS saddle tanks on either side of the single Lehman in my GB 36. Sight tubes on both, with gate valves top and bottom so that I can check the fuel during "pre-fllight". If the ER is not a "hell-hole", I wouldn't be without them. Dead easy and cheap, and I don't burn enough fuel to care about how much I've got left except maybe weekly. I don't see the need to involve the venting at the top plumbed to them in any way. They are entirely independent.

I also have side inspection ports (8 in. circular) that don't leak, though, I have never opened them yet (10 years). Wish I had a sump to drain each tank, and wish I had plumbing for polishing each, but that's just me. My excellent engine mechanic poo-pooed the idea as a fad, but I still wish I had that kind of plumbing as well. Fuel supply comes off the side of each tank, down low (1 in.) and returns to the top. I can choose fuel supply from either side tank or both. I'm honestly not sure where the return goes (I'm guessing just to the port side), and it has never come up. Nonetheless, it seems important so that will be my project tomorrow. Flat top tank surfaces are known to corrode from accumulated water and condensation, and I see some of that on the nearly inaccessible top of my original tanks.

Mine are 40 years old now, and I am wondering how I would replace them. I like the idea of cylindrical truck fuel tanks. They've got to be pretty inexpensive given the production volume.

 

[stephenbandi]

We have a 1979 Marine Trader Europa 40. I am replacing 2 at 200 gallon rusted out black iron tanks with 8 at 50 gallon stock polyethylene tanks (2 groups of 4 tanks). Each tank cost $325. The old black iron tanks are being cut out of the boat and the plan calls for the new tanks to be joined together for equalization and venting via spin welded fittings. Would be interested in any comments.

 

[Marco Flamingo]

ABYC says no spring loaded valves, since the springs fail when heated by fire and then lets it feed the fire with fuel... By that point, you have a much bigger problem. I wondered about a ball valve with a 2lb weight on the handle to turn it off when you let it go, but I suspect they will have a problem with that too.

It might be the "push button" types that ABYC doesn't like, although I've seen them sold for that purpose by an EU supplier. I can't imagine that there would be a problem with this type of spring loaded ball valve. It would be in the closed position if heat effected the spring.

And you are right. If the push button spring failed because of heat, it would be because the inside of the valve was in excess of 700 degrees Fahrenheit. By that time one would one would be engaged in other more pressing issues.

My tanks are stock with exposed PVC tubing for site glasses and no valves at all. I think even a push button would be a safety improvement.

 

[Portage_Bay]

My concern would be the size of the connections and vents. As mentioned up thread the size of the interconnections should equal the size of the fill pipe, assuming one fill pipe per set of connected tanks. Venting on many boats, especially recreational boats tends to be too small. The displaced air cannot exit fast enough leading to a slight over pressure in the tank(s). When that happens you can get a geyser of fuel erupting from the fill pipe.

We have a 1979 Marine Trader Europa 40. I am replacing 2 at 200 gallon rusted out black iron tanks with 8 at 50 gallon stock polyethylene tanks (2 groups of 4 tanks). Each tank cost $325. The old black iron tanks are being cut out of the boat and the plan calls for the new tanks to be joined together for equalization and venting via spin welded fittings. Would be interested in any comments.

 

[Nick F]

We have a 1979 Marine Trader Europa 40. I am replacing 2 at 200 gallon rusted out black iron tanks with 8 at 50 gallon stock polyethylene tanks (2 groups of 4 tanks). Each tank cost $325. The old black iron tanks are being cut out of the boat and the plan calls for the new tanks to be joined together for equalization and venting via spin welded fittings. Would be interested in any comments.

This looks like an interesting option Stephen. I discounted multiple polyethylene tanks because I was in the belief that custom connections were not possible. Some questions:
1 whose tanks are you considering?
2 are these fabricated tanks or one-piece molded tanks?
3 are you confident that the spin-welded fitting will be strong enough to withstand the loads from the connected fittings/hoses?
4 Is the tank manufacturer going to do the spin-welding?

Thanks,
Nick

 

[stephenbandi]

I bought Moeller tanks that get by at the back of the engines (with the transmissions removed for service anyway). Link below. I think they are one-piece molded tanks. I am confident that the spin welds will be strong enough. We are doing the spin welding on site. We are using lower connections the size of the fill intake line and venting all four tanks per side to a manifold that will then vent out the boat at a single location. I have also been thinking about creating a fill manifold that would let fuel flow evenly to all four tanks as we take on fuel. We plan on cutting the old tanks out of the boat later in the week.

50 Gallon Permanent Below Deck Boat Fuel Tank |

 

[stephenbandi]

Agree with your comments. The plan is to have 1-1/2 interconnections, a vent manifold that connects all four tanks to a single vent out the side of the boat. Still being debated is the idea of a fill manifold that would allow fuel to flow from the fill pipe into all four tanks at the same time.

 

[Nick F]

My $0.02:
I would not install a fill manifold - your 1-1/2" bottom interconnects are quite generous. Personally, I think 1" would be sufficient.

On further thought, I am asking myself how would the fill manifold work:
How will you "divide" the flow before the fuel enters the nearest tank? Is there not going to be one tank directly below the filler? In this case, would the fuel not tend to go primarily into this first tank, until it was full and the liquid level rose to the height of the fill manifold? To avoid this you could put restrictions in the individual tank fills in order to promote sharing - but restrictions would defeat the whole purpose, which (I presume) is to minimise refuelling time.
Am I missing something? Thoughts?

Stephen - back to the spin-welding - will you do this yourself? How do you learn this? Do you need fancy equipment? Did you consider thru-hull type fittings as an alternate for the bottom connections?

Nick

 

[stephenbandi]

Nick,

I have been uncertain about the need for the fill manifold and your $0.02 may have helped me decide. It would have been purely to speed up refueling and your "entering the nearest tank" argument is the problem that would have had to be overcome. May not be worth it, particularly since it may not work.

The tanks do not allow for double sided access for installing the bottom interconnects. I originally planned to use single side access thru hull fittings but after looking into spin welding decided I could do this myself. Using a router and a special bit ($100-$175 depending on the size of the fitting) the process is straight forward and I have worked with plastics some in the past. I am still deciding, but if I go spin weld I plan to do a couple practice installs on some old polyethylene and then go for it on the tanks. Worst case is I destroy a $325 tank but the best case is a really strong permanent seal.

 

[Nick F]

I have never heard of "single side access thru-hull fittings" - sounds interesting - can you tell me more?
Thanks,

Nick

 

[stephenbandi]

This link is to USI-RV.com for what they called the blind return.

This link is to a company that sells the spin weld fittings and router bit for that process.

https://vintagetrailersupply.com/spin-weld-fittings-vts-344/

 

[BCK]

Interesting subject. I am in the middle of the same project...1977 34' CHB with old twin 150 gal. leaking steel tanks. I have cut them out and have four aluminum tanks welded up making approx. 100 gal. per side. I am now trying to figure out the fittings...I hope I have not ordered/designed tanks that will be a problem to install. I have allowed a 1 1/2" link from fwd tank to aft tank similar in location to MVWeebles post #3. I was thinking of two 90 L's facing each other, then hose barbs and a short length of A1 hose connecting the two tanks. BTW, 1 1/2" A1 hose is very difficult to find! Then the actual fitting material...is SS OK? For the sight tube, I have a 3/8 npt hole near bottom and near top on front face...I was thinking, again, out with 90 L's facing each other then ball valve at bottom only, then barbs and that tygon tubing connecting the two. But by the time I install the 90, ball valve, then hose barb, my sight tube will start essentially several inches off bottom...unless, on the bottom, I use a 90 with ball valve facing outboard/horizontal, then 90 up with hose barb...argh! Help!

 

[BCK]

...oh, and installing ss fittings into aluminum tanks and getting them facing the desired direction...This makes me nervous. 4 wraps with the pink tape (PTFE), then tighten, but how much can I tighten to a, make the seal and b, further tighten to desired direction (especially on an 1 1/2" ss 90 into the aluminum? Anyone use aluminum fittings? NPTF/dryseal?

 

[Marco Flamingo]

The blind tank fittings only state that their use is for water in the videos I saw. I'd be concerned about using them for fuel if the manufacturer never states that they can be used for that.

Also, the Moeller tank is probably a generic below deck tank for gas/diesel. Gas tanks now have to meet EPA regulations for evaporative emissions. This is usually accomplished by adding a black nylon lining in a poly fuel tank. It gives the tank a brownish tinge like shown in the picture rather than plain white. I would talk with Moeller to make sure that there are no issues with spin welding on a nylon lined tank.

 

[Portage_Bay]

...oh, and installing ss fittings into aluminum tanks and getting them facing the desired direction...This makes me nervous. 4 wraps with the pink tape (PTFE), then tighten, but how much can I tighten to a, make the seal and b, further tighten to desired direction (especially on an 1 1/2" ss 90 into the aluminum? Anyone use aluminum fittings? NPTF/dryseal?

Preventing leaks at threaded connections, especially when you need to 'aim' the fitting in a particular direction can be very frustrating to say the least. When I've had to do that I pay a LOT of attention to the threads. Any visible problems at all on either piece and they won't get used. I really don't like mixing aluminum and stainless. I've just had too much trouble with that combination. I can't prove it but I think the stainless is too hard. And stainless has a tendency to gall. I prefer copper fittings with aluminum. I'd use high quality steel before I'd use stainless. My goal if I must really wrench on it to stop a leak is to preserve the expensive part, in this case the aluminum tank and it's threaded ports.

For sealant I've used teflon tape in it's various thicknesses. Liquid PTFE. Rectorseal. X-pando. I haven't found anything that is fool proof yet. In general I've had better luck with the liquids and pastes than tapes. They seem to fill imperfections better.

A few things in the for what it's worth column. My previous boat had aluminum tanks installed in 1995.

• The owner and tech at that time used copper fittings to thread into the tank fittings. I couldn't tell what sealant was used but when I sold the boat in 2018 there were no leaks.
• I had some work done about 2 yrs ago on the present boat. The tech epoxied the fittings in. I was skeptical but he claimed to have had great success with the technique. So far so good.
• The PO of this boat seems to have used a lot of a product labeled "VersaChem prime seal gasket sealant type 3 aviation" judging by the appearance and the partial containers left around. It's tough stuff and all the work he did on stainless to stainless that we did not disturb remains leak free.

 

[Nick F]

My humble apologies, but regarding the earlier recommendation to use copper fittings with aluminum, I absolutely disagree. This is a very bad electrolytic combination. Fittings being in contact with the aluminum tank must be carbon steel, stainless steel or aluminum. Not brass, not bronze, not copper.

Personally, my gut tells me that aluminum fittings would be softer than steel (carbon or stainless) and less likely to suddenly go very tight, so aluminum could make it easier to get your final orientation.
Note that galling generally only occurs between same metals, so galling of stainless/aluminum would not be an issue. Can aluminum gall against aluminum? Don´t know - I have not heard of this.

I agree with the use of a liquid sealant - probably the best choice for this low pressure, orientation sensitive application. As stated by my learned friend, thread quality is important for leak-free NPT connections.

 

[Portage_Bay]

Nick F,


No need to apologize. I'm aware of the dissimilar metals issue between Al and Cu. I understand your recommendation against it. And without having seen it in use long term would make the same recommendation. I can say is it worked for 23 yrs on my previous boat and was still working when I sold her. She had two tanks with connections in the engine room with very little risk of water, let alone salt water getting near the connections. Two tanks in the lazarette where they did experience some exposure to water and salt water.


If dissimilar metals are the concern then I'd stay away from stainless as well. Al and SS will build a, for lack of a better phrase, corrosion bond that is impossible to break loose. I don't understand what it is, just seen it many times.


There is a tendency in the pleasure boat world to make stainless the go-to material for everything. It's not always the best choice.


I have no experience with Al pipe fittings and little experience with Al fasteners.

 

[Caballero II]

Fuel tank sight gauges

Lots of great advice. Looking at your plans, I'd expect a single fuel return port. As tiltrider mentioned, a manifold is needed to switch returns between P and S.

Also the top port for a sight tube should be thought of as a vent - you need unpressurized air above the top of the fuel. So probably a port on the top of the tank better supports the sight tube. One sight tube would do for both tanks.

My tanks have a box sump at the end. If you've got the clearance its a great feature. You could have it in one tank to draw from for polishing/draining, and it also gets the sight tube low enough to measure low tank levels.

In this picture the fuel draw is the top fitting, and the bottom one is from the sump.View attachment 114915

In response to this fuel gauge thread, here are a couple of pictures of my set-up. The gauges are glass with a background color stripe to highlight the fuel height in the tube. I would guess you could source these from McMaster-Carr, but you might have to google. They are plumbed with a gate valve top and bottom, so you have to open both top and bottom valves to get a current reading. I only open them to get a reading, then shut them both again. Notice the all-thread that sort of acts like a guard and the lock washers. They have never caused a problem, and I'm glad my mechanic added them.

To plumb the bottom one with a full fuel tank, he closed the vent and applied suction to the top with a shop vac to hold the fuel in while he plumbed. I think brilliant.

Sorry the picture loaded sideways. Maybe the editor can turn them so the gauges are oriented up and down?

 

[Giggitoni]

I’ve found the photo should be in “landscape” mode to post correctly.

 

[mvweebles]

I asked Steve d'Antonio a while back for recommendation on sealant for diesel fuel fittings. He said he's been using Leak Lock by Highside Chemicals for years.

https://www.highsidechem.com/leaklock.php