Diesel Fuel Tank Design

Hi all, I am at the point of my restoration where I must design and fabricate the fuel tanks for the Bristol. The old steel ones are out...they were too far gone to save and very heavy, and I want to go with new 1/4" aluminum tanks, approx. 225 gallons each. Before I go to the fabricator, I have a few questions.*

The old tanks seemed to be gravity fed to the filters; there was no stand pipe on top going down to almost the bottom like in my gas boat. I know I need vents, I know I need fills, I know I need inspection ports, , but how will the single Cummins diesel draw the fuel to itself? I also have heard I should have a sump with a valve to drain sediment?? I removed all the old rotted cribbing and rotted saddles the tanks sat upon and am building new ones out of solid fiberglass, so no more rotting. Does anybody use sight gauges or do you all use sending units to electric gauges?*

Thanks, Steve

Also, if anybody is interested, you can follow along with this refit at our site, www.sibsie.com *

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Corrosion can be an issue with water in an aluminum tank. Ate a hole in one of my motorcycle tanks. For this reason I would think sump would be best so as not to have any water hiding anywhere.I've got integral steel tanks with sight tubes.

I believe ABS says: no low point drains allowed. However, I personally like the lowpoint drains on both my tanks, and each has a valve. When I run the polisher, it draws from these lowpoints and I therefore am assured of catching any water and debris.

Be sure to fab in baffles and inspection/cleanout ports.

You will need a fill connection - match the size of the filler port on the hull of your boat - larger is better.

A vent for each tank. When you install the tank it is a great time to add a fuel spill catch device in the vent line between the tank and the fitting on the hull.

Fuel pickup tube should go down about 3/4" off the bottom of the tank - this way you can use 'most' of the tank capacity if needed.

Fuel return fitting.

Guage fitting - unless you use sight tubes.


make sure you use the 'correct' type of aluminum for the tank material, and install it properly so there is a bit of air circulation around the tank - this can be accopmlished by epoxying plastic strips spaced along the bottom of the tank at appropriate spacing.

The engine has a fuel pump on it, so once the motor is primed and all air leaks are gone, then it runs. For priming, an electric or manual pump in addition to the OEM lever on the motor is advised.


good luck

stevensibs wrote:...but how will the single Cummins diesel draw the fuel to itself?

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I don't think that should be a problem.* Most engines have a mechanical or electric fuel pump on them and this is sufficient to draw fuel from a nearby tank and send it to the injection system.

Our Jurrassic FL120s have a small, external mechanical fuel pump on them.* The fuel daytank the engines normally feed from is in the bilge and the feed to the engines attaches to fittings at the lowest point on one end of the tank.* So I'm guessing the vertical distance from the tank feed on the tank to the pump on each engine is about four feet or so.* But enroute from the tank feed to the fuel pump the fuel goes through a Racor filter.* Then after the pump the fuel goes through a couple of secondary filters on its way to the injection pump.

The engine's mechanical pump obviously has no problem doing this and it sounds like you have a much more modern engine than we do.

By the way--- and since you didn't list it in the connections you said you know you'll need to make-- you will need one more connection to your new fuel tank(s) and that is for the fuel return line from the engine.* As you probably know, more fuel is fed to a diesel like the ones most of us have than it uses, so the excess fuel has to go somewhere.*

In our case the fuel return from each engine can be valved to return to one of two tanks.* We leave it set to return to the tank the engine is pulling from.* I if you set the return to go to a tank you're not pulling from and if that tank gradually becomes full from the return fuel, if you do nothing about it the return fuel from the engine will start flowing out the vent and overboard into the water.

I don't believe the fuel line from the engine has to connect to the actual tank itself.* I believe it can "T" into the tank fill hose, for example, as long as the return fuel ends up in the tank.* But I'm not positive about this so check with someone who is.* The return fuel will be hot, by the way, in case that makes a diffrence to how or where you connect it.

Thanks, guys...now I can talk to the fabricator without sounding like a dummy.

Steven, I will attach some pictures of my tanks new and old and you can see how they were set up. I don't know about an access port. If you need to you can cut one in it but you shouldn't need one. Just another place to leak in my opinion.

This was the access ports I cut into them to clean.



These are the new tanks



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I haven't quite figured out what all the connections are going to be but there are plenty of them. *

I think it would be a mistake to try to design your own fuel tanks unless you have a lot of experience in this field. Working with a company that's in this business would be a better idea. Tell them what you have and let them design the tanks, that's their business. They should know the legal or code requirements and they should know what works well and what doesn't. You can ask about sumps and valves, etc.

I will suggest this though. Sight gauges would be a nice feature to have even though you will probably want an actual electrical gauge that reads at the helm. Electrical fuel gauge systems are notoriously inaccurate, especially on boats. A sight gauge gives a very accurate indication of the level in the tank.

If an extra $1K or so is not an issue, an electronic fuel monitoring system would be the ideal. You tell it what you put in, it measures what is used and you know the difference.

bshanafelt wrote:
I believe ABS says: no low point drains allowed.

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The American Bureau of Shipping could care less about your fuel tanks.

The American Boat and Yacht Council and the USCG are not particularly fussed either.

You can have as many drains as you like.

I might add that you want a straight shot from the fill point to the tank and a good vent.Many fuel docks have High flow-rate pumps That will make you cuss like a sailor if any restrictions in your design prevent rapid fueling, not to mention the mess to your boat and the environment.

Swampu, aint it fun tearin up your boat? lol. Thanks to you and everybody else for the input. And Ron, thanks as well...I surely am going to seek advice from the tank fabricator. But in my many years of boating I have found one thing to be constant. There are a great many skills relative to boat building/maintenance that I possess, most all learned the hard way. But when I need a contractor to do somethong for me, I better had know my stuff or they will #@&* me unless I do!
Thanks again guys. Very appreciative of all the knowlege/experience in here....

steve

I just got a price quote for alum. tank 60" long, 20" X 32" meeting all marine specs at $2250. from a shop in Barnegate. N.J. They have a very good rep. and I felt the price was fair. Replaceing 2 500Gallon steel tanks with 4 tanks side by side. Make sure water can not sit on top of tank. Drains on the bottom are the best way to keep water out.

Ron T wrote:
I just got a price quote for alum. tank 60" long, 20" X 32" meeting all marine specs at $2250. from a shop in Barnegate. N.J. They have a very good rep. and I felt the price was fair. Replaceing 2 500Gallon steel tanks with 4 tanks side by side. Make sure water can not sit on top of tank. Drains on the bottom are the best way to keep water out.

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That seems about right, my tanks were 8' long 24"x24" two total and I paid 5k for both. *Make sure they pressure test them before delivery.*

Most points have been covered.
But be sure you have accessable (large) inspection ports, well made and bolted they will not leak.
Sumps , very important,all water will settle here and be easy to drain off.
Engine will draw its own fuel so no need for boost pump.
Ensure you have enough baffles to stop surge in tanks.
Self closing (spring loaded) lever cocks on your sight glasses are the way to go.
Don't forget to calibrate your tanks as you fill them for the first time.

I(f you do not have a sump and pump the minimum should be a low point drain.

A good valve and a plug in the valve will satisfy most surveyors.

I would create an emergency method of actually running off the bottom of the tank drain.

Usually 5%-10% is left by the pickup tube , you may need that fuel someday.

For a tank 60" x 20" x 32", approx 165 nom gallons, we usually quote $10 p/gal when estimating.* Ron, you may want to check out Best Fab in Bartow Florida for their pricing. Their fab is top notch, but their business creates a back log of work..

Is a flame arrestor required in your diesel tank vents?

Singleprop said:

Is a flame arrestor required in your diesel tank vents?

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I have never seen or heard required.

I would definitely include the inspection port, sight glasses, a sump with drain valve.
The red dye in off road diesel will stain the sight tubes so they can't be read after a few years. Put a valve at top and bottom and they can be changed in 10-15 minutes, on my boat.
Steve W

Put Fiberglass tanks in my boat as Diesel will not bother glass, Also its easy to change add of alter the tank with out to much fuss, Along with the standard electric gauge i also have a fuel manager, Tells total fuel on board, Fuel burnt, Fuel burning per hour, fuel per trip i think it has 4 trips you can log, very cool piece of tech, It displays GPH burn when on so you can find the sweet spot for economy, push one button tells you how much fuel you still have onboard. Sounds like over kill but each system i have is set up redundant, As in Aircraft makes me feel like a breakdown in something is not a show stopper.

bshanafelt said:

I believe ABS says: no low point drains allowed.

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ABS could care less what you do to or on your boat unless you hold a class certificate issued by that organization.

ABYC standards do not mention low point drains on diesel tanks. There is no USCG or other prohibition on low point drains or connections on diesel fuel tanks. You are confusing diesel tanks with gasoline.

I can't recall ever seeing a diesel tank on a small boat that did not have fittings at or near the bottom of the tank for drains, sight glasses, or fuel outlets.

Singleprop said:

Is a flame arrestor required in your diesel tank vents?

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ABYC standard (H-33.13.3) requires a flame arrestor "that can be cleaned, unless the vent system is itself a flame arrestor." A little study will explain what that means and how it can be accomplished.

If you charter your boat then 46CFR182.455 applies and includes more detail on the screen and mesh required.

Yes Rick i had a coast guard inspection and it was explained to me. that the fuel,lowpoint drains, water seperators, and even the battery bank can all be in the bilge on a Diesel boat,None of the electrical items have to be spark proof, Where with Gas its another story. The pick up needs to be 3/4 of a inch from the bottom i was told that its a bad idea to pull your fuel from the bottom of the tank But Lowest point drain ( Oh yes ) good thing to have,

Late to this, but Nigel Calder has some good ideas on design and materials,see "Cruising Handbook" pp213-215. He does not like s/steel or aluminum tanks.At page 208 he deals with fitting a "fuel sampling/draining line" at the lowest point of the tank,ie lower than the feed line, with a manual pump to draw off water & sediment. I was sure he had a diagram of a tank with an extra sump lower than the tank floor,but can`t find it.
My diesel man successfully drew off the water/sediment from screw in plugs,without valves,he didn`t fill the bilge with diesel, as I would have, must be a skill to it. I`m guessing the reason valves don`t get fitted is because they would get stuck.BruceK

RickB said:

ABYC standard (H-33.13.3) requires a flame arrestor "that can be cleaned, unless the vent system is itself a flame arrestor." A little study will explain what that means and how it can be accomplished.

If you charter your boat then 46CFR182.455 applies and includes more detail on the screen and mesh required.

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Interesting. Never thought that a system could be designed to be a flame arrestor.

Has anyone looked into how a vent system can be designed as a flame arrestor?

Well I re-read all the posts regarding the design and fabrication of my two diesel tanks and after speaking with loads of people I've decided on fiberglass tanks. I will have spring-loaded bronze ball valves at low point of tanks with plugs for added security; 2" straight fills; large inspection ports on top made of G10 FRP 3/8" thick, sitting on Nitrile gaskets; sight gauges with more spring-loaded ball valves and chemical resistant PVC tubing; baffles also made from G10 FRP 3/8" thick; stand pipes, fuel return lines and polishing lines all from ESI as well as the Racor filters and fuel manifolds; (which I just received); fittings on top for topside venting; and all resin will be vinyl ester or epoxy, haven't decided which yet. Have I forgotten anything?

Put Fiberglass tanks in my boat as Diesel will not bother glass,

True , but if the gov demands some percent of "bio fuel" the GRP tanks may not stand up to it..

Talk to the tank builder FIRST!

For FRP tanks, Epoxy lined is the way to go. Check with a few boat builders or suppliers who use Epoxy lined FRP tanks for diesel to insure right Epoxy is used. They are not all created equal.

A properly designed and installed steel tank will outlast you and your boat.

I agree about the steel; the ones I took out lasted 30 some years then failed. But I'm worried about rust on the INSIDE...I am planning this boat's long cruise on a single Lugger engine with a ESI fuel polishing system but I've heard horror storie in here about rust. Aluminum is defo out. Am I wrong to worry about steel?

Steve, here is my take:

Well designed steel tanks are perfectly fine provided you keep the water from pooling on the outside and can drain/empty the tanks completely. A 3 stage filter system ala Tony Athens will protect you from most dirty fuel issues and problems. Most important, fill up from a known source wherever possible.

On new tanks, no matter what material was used, a complete cleaning before the first fill is essential. Internal construction debris is all too common.