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Old 03-03-2012, 06:03 PM   #1
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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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Old 03-03-2012, 06:34 PM   #2
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RE: Diesel Fuel Tank Design

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.
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Old 03-03-2012, 06:41 PM   #3
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RE: Diesel Fuel Tank Design

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
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Old 03-03-2012, 07:48 PM   #4
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RE: Diesel Fuel Tank Design

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

*
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.
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Old 03-03-2012, 08:53 PM   #5
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RE: Diesel Fuel Tank Design

Thanks, guys...now I can talk to the fabricator without sounding like a dummy.
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Old 03-03-2012, 10:28 PM   #6
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RE: Diesel Fuel Tank Design

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



*

I haven't quite figured out what all the connections are going to be but there are plenty of them. *
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Old 03-04-2012, 07:22 AM   #7
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RE: Diesel Fuel Tank Design

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.
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Old 03-04-2012, 08:20 AM   #8
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RE: Diesel Fuel Tank Design

Quote:
bshanafelt wrote:
I believe ABS says: no low point drains allowed.
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.
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Old 03-04-2012, 09:53 AM   #9
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RE: Diesel Fuel Tank Design

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.
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Old 03-04-2012, 05:12 PM   #10
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RE: Diesel Fuel Tank Design

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
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Old 03-04-2012, 05:52 PM   #11
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RE: Diesel Fuel Tank Design

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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Old 03-04-2012, 07:30 PM   #12
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RE: Diesel Fuel Tank Design

Quote:
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.
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.*
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Old 03-04-2012, 09:30 PM   #13
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RE: Diesel Fuel Tank Design

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.
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Old 03-05-2012, 05:20 AM   #14
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RE: Diesel Fuel Tank Design

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.
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Old 03-05-2012, 03:27 PM   #15
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RE: Diesel Fuel Tank Design

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..
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Old 06-15-2012, 03:32 AM   #16
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Is a flame arrestor required in your diesel tank vents?
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Old 06-15-2012, 06:31 AM   #17
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Originally Posted by Singleprop View Post
Is a flame arrestor required in your diesel tank vents?

I have never seen or heard required.
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Old 06-15-2012, 08:36 AM   #18
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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.
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Old 06-15-2012, 09:00 AM   #19
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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.
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Old 06-15-2012, 09:32 AM   #20
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Quote:
Originally Posted by bshanafelt View Post
I believe ABS says: no low point drains allowed.
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.
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