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Old 10-02-2021, 13:39   #16
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Re: Diesel tank fuel return

The return line can just end at the top tank penetration. There is no 'need' for any inspection ports. I see them as a leak source, but put in as many as you want. I built my own diesel tank. Not hard, even with the unusual trapezoidal shape. I just made a 'core' out of 3/8" exterior grade plywood and covered it in several layers of fiberglass and steelflex epoxy. Make sure you use epoxy impervious to fuel. The top was solid glass where my penetrations were. I didn't put in an inspection cover, but the hole for the fuel level sensor was big enough to see into the tank pretty well.... Maybe $150 in total cost for 25gal tank.
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Old 10-02-2021, 15:01   #17
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Re: Diesel tank fuel return

We just had a 42 gallon day tank installed. Got rid of the crappy 12 gallon day tank, made much better use if the space, have dedicated ports for all needs and one spare, located our new Purasan unit on top of it.

It was an expensive proposition but it was very well done by the yard in NC. Prepared the CAD drawings and the yard did the detailing.

Space limitations meant all protrusions needed to be in the inspection plate, which was installed once the tank was in place. This allowed me to maximize the tank size and still get it through a doorway.

1 engine pick up
2 engine return
3 tank fill
4 espar heater
5 vent
6 spare

I had not thought of siphoning, the valve is a good idea.
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Old 10-02-2021, 15:37   #18
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Re: Diesel tank fuel return

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Originally Posted by HeinSdL View Post
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...
A magnetic filter that kills bacteria? Is it supposed to mess up the magnetite in their beaks that they use in migrating from continent to continent searching for the proper breeding grounds? Messes up the electrical pulses in the bacteriaís central nervous system?
Hospitals have dire needs in killing or controlling bacteria, especially since now the antibiotic resistant species are multiplying. Iíve never heard of magnetic means to control bacteria.
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Old 10-02-2021, 15:47   #19
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Re: Diesel tank fuel return

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Originally Posted by swvoltz View Post
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!
I would go plastic in a well made tank. I've had aluminum corrode a hole in the bottom pretty fast. But it will last 10 years maybe.
The key is not having supports that can stay wet where it touches the tank.
One vent is fine.
The aluminum tank maker in Bellingham is good. Not sure they do large inspection plates though...?
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Old 10-02-2021, 16:26   #20
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Re: Diesel tank fuel return

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Originally Posted by out42 View Post
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?
They outlawed this in the EU on the grounds it was a safety issue !
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Old 10-02-2021, 16:50   #21
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Re: Diesel tank fuel return

The return line cannot be connected to a pickup tube that extends below the top of the tank, other than the elbow mentioned, which is a great idea.
If you did have a closed tube from top of tank to the bottom for the return fuel, that line would build up pressure in the trapped air inside it, as you fill up your tank. That would create a back pressure in the return line, probably not a good idea!
One vent per tank, if it is located at center of tank top panel or highest position if tani top is not level,, should be all that is needed.
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Old 10-02-2021, 17:08   #22
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Re: Diesel tank fuel return

How do you build up pressure if it is vented?
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Old 10-02-2021, 17:23   #23
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Re: Diesel tank fuel return

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Originally Posted by hpeer View Post
How do you build up pressure if it is vented?

It's a very low pressure so my comment was mostly theoretical.

I was only referring to pressure in the return line, not the body of the tank. Assuming a return line connected to a tube that runs to bottom of tank, that tube is itself not vented, unless you do drill a hole in it near the top. Nor is there a vent anywhere in the return line.

Before the tank is filled, the level of fuel inside the return line tube is the same as on the outside, say near bottom of a nearly empty tank. As tank is filled, the fuel will also enter the bottom of the tube. Fuel level rising inside the closed tube will compress air in the tube and return line.

Since there is probably a lot of air in the entire length of return line, I guess the pressure increase would be small, and I'm sure the pressure provided by the engine in it's return line would be way bigger.
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Old 10-02-2021, 18:04   #24
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Re: Diesel tank fuel return

When you get your new tanks, coat the inside bottom and a few inches up the sides with a diesel tank coating. That way the tanks will last the life of the boat.
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Old 10-02-2021, 18:47   #25
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Re: Diesel tank fuel return

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Originally Posted by Key Wester View Post
ABYC says all holes into a marine fuel tank should be on the top surface only. Joe D
This is not true. If it were, replacing large tanks on trawlers would not be possible. Large tanks are routinely split into two (or more) smaller tanks. There is no way to join these tanks except at the bottom.

There is a school of thought that a AP at the bottom outlet of a tank is a good idea to drain water and crap.

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Old 10-02-2021, 19:03   #26
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Re: Diesel tank fuel return

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Originally Posted by Lepke View Post
When you get your new tanks, coat the inside bottom and a few inches up the sides with a diesel tank coating. That way the tanks will last the life of the boat.
That is a good suggestion, I should have thought of that. Airplanes use it and Boat Life sells it, but at a premium price. Duh!
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Old 10-02-2021, 21:41   #27
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Re: Diesel tank fuel return

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Originally Posted by swvoltz View Post
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!
For what it's worth, welded plastic is always going to be an issue sooner or later.

One piece moulded plastic tanks are another thing - all of ours on board are (fuel, water, black holding, grey holding - six in total), and they seems very, very solid.

As to the fuel return - just to the top (you don't want to be drawing through it, and ideally with a bend to direct fuel against the side, minimising splashing and therefore foaming.
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Old 10-02-2021, 22:21   #28
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Re: Diesel tank fuel return

I have a tank deep in my keel. I have a problem with air getting into my fuel system and I have to bleed my engine after it has been off for days or weeks. A boat diesel mechanic told me that running the return tube as described in this Beta installation manual would solve my problems. I'll have to try it some day!

https://betamarine.co.uk/resources/O...HE-OM/#page=32

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Old 10-02-2021, 23:48   #29
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Re: Diesel tank fuel return

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Originally Posted by swvoltz View Post
Originally we had a Perkins 4108. Now replaced by a Beta 43 (Kubota).


The Beta 50 (same engine) manual has some pretty specific instructions on the return tube. See page 19 of the manual.

In summary, return tube is drawn below the surface of the fuel and some effort is made to prevent drain down, something that will be difficult with your setup.
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Old 10-02-2021, 23:53   #30
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Re: Diesel tank fuel return

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Originally Posted by jmschmidt View Post
I have a return line from the injector pump to one of the two inlet ports on the primary filter.

This is not recommended as you lose much of the cooling effect provided by the diesel as you feed the hot diesel straight back into the system.

Beta also cautions that it makes it harder to eliminate air bubbles, which makes sense.
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