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Old 09-02-2021, 13:00   #1
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Diesel tank fuel return

We had to abort our Pacific crossing in 2015 because a black iron diesel tank cracked. On return to Mexico, we had a local weld us up a set of plastic tanks which held together for our trip to Hawaii to Alaska to Olympia. Last summer, one of those tanks split a seam and diesel fuel spilled into the bilge. What a mess. I'm pulling those tanks out and replacing with US made aluminum tanks.

Three questions.... should the return on the tank have a pickup (drop?) tube or can the returning fuel just fall in from the return fitting on top of the tank? Splash or no splash?

and... Do I need to put two vent openings on each tank, one on the port side and one on starboard to take into account the heel of the boat when motor sailing?


and...

Do you have a recommendation as to where to have my new tanks made in the Pacific Northwest? I've got some ideas but welcome suggestions.

Picture is of the old iron tank that split.....

Thanks!
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Old 09-02-2021, 13:07   #2
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Re: Diesel tank fuel return

Return to the top. This gives the fuel extra opportunity to cool off. You draw nice cool fuel off the bottom. Diesels like cool fuel.
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Old 09-02-2021, 13:42   #3
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Re: Diesel tank fuel return

Best practice for return lines (but rarely seen) is to enter the top of the tank, and then use a short elbow to direct the flow against the wall of the tank. This is called a "splash leg." This greatly reduces foaming from a spray of the fuel falling to the surface.

It is also a good idea to put a small hole in the splash leg elbow as close to the top of the tank as practicable. This serves to break any potential syphon if the tank is really full.

If you do feel that dip leg must be used, be sure that you have a valve on the return line where it enters the tank because fuel will syphon out of the tank in the event of a hose failure or any filter maintenance, and you will need a way to stop that.
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Old 09-02-2021, 15:25   #4
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Re: Diesel tank fuel return

I have had a couple custom marine fuel tanks made by Coastline Equipment in Bellingham and was very satisfied. https://coastlinemarinetanks.com/
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Old 10-02-2021, 09:23   #5
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Re: Diesel tank fuel return

What engine are we discussing? Volvo, for one, has very specific details on the returning fuel arrangement. Joe D
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Old 10-02-2021, 09:57   #6
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Re: Diesel tank fuel return

Originally we had a Perkins 4108. Now replaced by a Beta 43 (Kubota).
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Old 10-02-2021, 10:07   #7
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Re: Diesel tank fuel return

Our steel tank was replaced with aluminum also.

The best part about the change was adding a clear acrylic (diesel-safe) inspection hatch plate.

We use this every time we fill the tank to alert the other person (who is running the fuel nozzle or jerry jugs) when the tank is full. It prevents any spillover at the deck fill. Also a great way to check for water or debris in the tank without removing the hatch plate.

The return line tube goes all the way to bottom and is angled against the wall. It should be cut off so the fuel is allowed to cool before it is sucked back into the intake line. However, we haven't had any problems with fuel intake in over 1500 engine hours.


The tank (located just aft of our mast) only has one vent, which goes from the top of the tank (not shown) all the way aft to the stern. With that distance, not sure how effective it is, so we are careful to fill the tank slowly.
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Old 10-02-2021, 10:15   #8
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Re: Diesel tank fuel return

The Beta 43 returns a small amount of fuel at very low pressure. Returning to the top of the fuel tank will work fine. Be sure to follow the fuel hose return arrangement in the installation / owner's manual concerning required geometry. This is an important detail.

We have also [usually when upgrading from an Atomic 4 gas engine to diesel power ] spliced into the fuel tank vent hose using brass hose barbs and a 3 way T fitting. Joe D
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Old 10-02-2021, 10:18   #9
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Re: Diesel tank fuel return

Yes, definitely Coastline. We have been pleased with their products for many years.
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Old 10-02-2021, 10:40   #10
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Re: Diesel tank fuel return

Given the rise of biodiesel and associated bugs, is it worth thinking about how to drain the tank (and crap) now by putting in a drain tap at the bottom which can be easily reached?
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Old 10-02-2021, 10:52   #11
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Re: Diesel tank fuel return

ABYC says all holes into a marine fuel tank should be on the top surface only. Joe D
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Old 10-02-2021, 10:52   #12
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Re: Diesel tank fuel return

Quote:
Do I need to put two vent openings on each tank, one on the port side and one on starboard to take into account the heel of the boat when motor sailing?
One line with a loop in the vent line as high up as you can get it is sufficient. Check with Kubota as to how far you can heel over and still safely run the engine. A lot of engine will not lubricate well at over 20* to 25* angle of heel. The spec may be in your owner's manual.
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Old 10-02-2021, 11:17   #13
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Re: Diesel tank fuel return

Quote:
Originally Posted by Sailor647 View Post
Our steel tank was replaced with aluminum also.

The best part about the change was adding a clear acrylic (diesel-safe) inspection hatch plate.

We use this every time we fill the tank to alert the other person (who is running the fuel nozzle or jerry jugs) when the tank is full. It prevents any spillover at the deck fill. Also a great way to check for water or debris in the tank without removing the hatch plate.

The return line tube goes all the way to bottom and is angled against the wall. It should be cut off so the fuel is allowed to cool before it is sucked back into the intake line. However, we haven't had any problems with fuel intake in over 1500 engine hours.


The tank (located just aft of our mast) only has one vent, which goes from the top of the tank (not shown) all the way aft to the stern. With that distance, not sure how effective it is, so we are careful to fill the tank slowly.
I am jealous of that setup! On my boat the diesel tank is built in well below the chart table with no access at all. I am worried about what is going on inside that tank. For that reason I installed a conditioning system (dual Racor filter, electric fuel pump, magnetic filter - apparently that kills bacteria...) and run this system to circulate when the boat is out of service. Let's see...
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Old 10-02-2021, 12:20   #14
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Re: Diesel tank fuel return

I have a return line from the injector pump to one of the two inlet ports on the primary filter. This leaves only four holes in the fuel tank, one for the fuel fill line from the deck, one for the fuel pickup to the primary filter, one for the fuel tank vent and one for the opening for the fuel tank gauge float. This has worked fine on the same engine for thirty years.
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Old 10-02-2021, 13:09   #15
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Re: Diesel tank fuel return

Don't forget to valve the return lines as well as the suction lines if multiple tanks. It's unlikely all will have the same back pressure so you could find fuel being drained from one tank and returning to other(s) with less resistance.

Don't ask me how I know that many fuel pumps return multiples of what they actually use so have the ability to transfer the entire contents of one tank into another on a long day's run. Copious overflow and weight shifting is not fun. 😁

Of course this can also be an advantage should you want to shift fuel, polish tanks or transfer weight.
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