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Old 18-06-2022, 00:20   #1
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Fuel tank feeds and return

Fuel tank question. I’m buying a new boat and she has two, 280 litre fuel tanks on port and starboard sides. The port tank has been half filled with water, which I suspect was to create ballast so that the elderly owner didn’t have to fill up both thanks.

I intend to reinstate that tank as well as put some fuel units / gages on. I suspect the ‘return’ line is currently going to the single starboard tank.

My question is, in reinstating the port tank, where in the system should the ‘feed’ to the engine be and where should the ‘return’ go to?

Would I be okay having a hf500 fuel filter on each tank and then taking the feed between the two filters, so that fuel is coming from both tanks? If so, where should the return go?
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Old 18-06-2022, 02:10   #2
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Fuel tank feeds and return

You feed and return to the same tank with changeover valves. You do not normally draw from both tanks simultaneously
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Old 18-06-2022, 03:05   #3
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Re: Fuel tank feeds and return

In power boats drawing off two tanks at once is common. Not so much on cruisers but balance is critical when you catch air. Four tanks are pretty common and lowering fuel in the aft pair first. Returns are required to each tank. So that’s the extreme true gas guzzlers with 2-3 motors.
In a sailboat two large tanks one empty could change the boats pointing ability. Enhance prop walk. Air must return to both tanks. Some fuel systems have an accumulator to draw pressurized fuel from but no air into. If the tanks are set up to run together like aircraft of hi perf marine they will have at least one cross-over hose if not two.
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Old 18-06-2022, 03:13   #4
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Re: Fuel tank feeds and return

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Originally Posted by goboatingnow View Post
You feed and return to the same tank with changeover valves...
Indeed.
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Old 18-06-2022, 04:19   #5
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Re: Fuel tank feeds and return

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Originally Posted by Rumrace View Post
In power boats drawing off two tanks at once is common. Not so much on cruisers but balance is critical when you catch air. Four tanks are pretty common and lowering fuel in the aft pair first. Returns are required to each tank. So that’s the extreme true gas guzzlers with 2-3 motors.
In a sailboat two large tanks one empty could change the boats pointing ability. Enhance prop walk. Air must return to both tanks. Some fuel systems have an accumulator to draw pressurized fuel from but no air into. If the tanks are set up to run together like aircraft of hi perf marine they will have at least one cross-over hose if not two.


As the owner of four motorboats I’ve never seen multiple draws simultaneously , you can’t have return fuel going to the wrong tank as it could overflow and balancing pipes are not allowed in the RCD
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Old 18-06-2022, 04:54   #6
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Re: Fuel tank feeds and return

You're using diesel so you can take from the bottom of the tank (leave a tiny space for sludge). The return goes in wherever you wish, top or bottom. Then, assuming that you are willing to be careful with setting your valves, you have a valve in both lines to both tanks, four in all. One feed and one return valve are open whenever you run the engine. In the usual case, you take and return to the same tank. You can, if you wish for reasons of ballasting, take from one and return to the other. Monitor your fuel gauges if you do this. Usually, you select a tank based on it being on your heavy side.

Opening both tanks at the same time is not recommended. The high side could flood the low side, and you have no control over your ballasting. In addition, should you have a fuel problem in one tank, you should be able to cut it off and only use the "good" tank.

I did once forget to open a return with this system. That created back pressure in the feed line below the fuel filter, popped the feed to the genset, and flooded the engine room with diesel. That's not a pleasant discovery.
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Old 18-06-2022, 06:15   #7
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Re: Fuel tank feeds and return

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Originally Posted by Rustic Charm View Post
Fuel tank question. I’m buying a new boat and she has two, 280 litre fuel tanks on port and starboard sides. The port tank has been half filled with water,
First thought ... I'd be a little leery of stagnant fresh water and outright afraid of stagnant salt water sitting sitting in a steel or aluminum tank for a long period.

I have two aluminum tanks that I draw from simultaneously with a valved cross over pipe at the bottom. I have one return line from the engine that is t'd to the top of each tank

My fuel feeds are from the bottom of the tanks so all moisture and sludge goes to the filters and never sits on the bottom of the tanks.
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Old 18-06-2022, 06:58   #8
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Re: Fuel tank feeds and return

The setup in my boat with two tanks I think ideal for me as it allows maximum redundancy and is quite idiot proof.

Pickup and return lines from each tank go to one tank selector valve. The valve has four tank connections in two pairs and a pair of connections to the engine. When turning the handle on the selector valve it will connect just the port tank pickup and return, just the starboard pickup and return or shut off both tanks.

This way the return always goes back to the tank where the fuel is pulled. I have dual gauges and it's not complicated to alternate between tanks to keep the load balanced.

The output from the valve then feeds a dual Racor so whichever tank is selected goes to the same filter system. This way if one filter is clogged you can switch to the other but still feed the engine from either tank, not possible if each tank has a separate filter.
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Old 19-06-2022, 02:00   #9
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Re: Fuel tank feeds and return

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Originally Posted by skipmac View Post
The setup in my boat with two tanks I think ideal for me as it allows maximum redundancy and is quite idiot proof.

Pickup and return lines from each tank go to one tank selector valve. The valve has four tank connections in two pairs and a pair of connections to the engine. When turning the handle on the selector valve it will connect just the port tank pickup and return, just the starboard pickup and return or shut off both tanks.

This way the return always goes back to the tank where the fuel is pulled. I have dual gauges and it's not complicated to alternate between tanks to keep the load balanced.

The output from the valve then feeds a dual Racor so whichever tank is selected goes to the same filter system. This way if one filter is clogged you can switch to the other but still feed the engine from either tank, not possible if each tank has a separate filter.
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Old 19-06-2022, 05:06   #10
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Re: Fuel tank feeds and return

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Originally Posted by goboatingnow View Post
You feed and return to the same tank with changeover valves. You do not normally draw from both tanks simultaneously
This! It is much preferred to use a single two channel valve to do the switching to avoid possible human error.

If it is possible to draw from one tank and freed another, then some day you WILL OVERFLOW A TANK, and that is messy, dangerous, and likely to dump fuel in the ocean.

It is required by every boatbuilding standard.
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Old 19-06-2022, 06:15   #11
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Re: Fuel tank feeds and return

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This! It is much preferred to use a single two channel valve to do the switching to avoid possible human error.

If it is possible to draw from one tank and freed another, then some day you WILL OVERFLOW A TANK, and that is messy, dangerous, and likely to dump fuel in the ocean.

It is required by every boatbuilding standard.
With a bottom crossover pipe there will be no overflow aas the are essentially the same tank when rigged this way and will automatically equalize.
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Old 19-06-2022, 06:23   #12
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Re: Fuel tank feeds and return

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With a bottom crossover pipe there will be no overflow aas the are essentially the same tank when rigged this way and will automatically equalize.
I may be wrong but was told by a very competent surveyor in the US some years ago that any connections to the bottom of a fuel tank were highly discouraged if not prohibited. This came up in a survey on an older boat that had a sight gauge on the tank IE a clear hose connected vertically to fittings top and bottom so you could visually see the fuel level.

The reason of course is any break or leak in a fitting on the bottom of the tank would then dump the entire tank contents into the bilge.
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Old 19-06-2022, 06:30   #13
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Re: Fuel tank feeds and return

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Originally Posted by skipmac View Post
I may be wrong but was told by a very competent surveyor in the US some years ago that any connections to the bottom of a fuel tank were highly discouraged if not prohibited. This came up in a survey on an older boat that had a sight gauge on the tank IE a clear hose connected vertically to fittings top and bottom so you could visually see the fuel level.

The reason of course is any break or leak in a fitting on the bottom of the tank would then dump the entire tank contents into the bilge.
There are no such restrictions on diesel fuel systems in US Code of Federal Regulations Title 33 or 46, Neither are there such in ABYC, ABS, CE, RCD nor Transport Canada TP 1332 ... Gasoline tanks are another story altogether.

As far as sight tubes, they are specifically permitted under ABYC as long as they are fitted with ball valves as mine are.
Send me an email address and I'll be happy to send you ABYC and Transport Canada Standards in pdf files. I have all the others but in hard copy only.

ABYC H-33 Diesel Fuel Systems ...
33.5.8 A means to determine fuel level or quantity shall be provided.
33.5.8.1 If a sight gauge is used,
33.5.8.1.1 it shall be equipped with a shutoff valve at the top and at the bottom of the gauge, and
33.5.8.1.2 a warning label shall be placed adjacent to the sight gauge, and
33.5.8.1.2.1 the warning label shall comply with the requirements of H-33.16.2.
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Old 19-06-2022, 07:09   #14
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Re: Fuel tank feeds and return

Quote:
Originally Posted by boatpoker View Post
There are no such restrictions on diesel fuel systems in US Code of Federal Regulations Title 33 or 46, Neither are there such in ABYC, ABS, CE, RCD nor Transport Canada TP 1332 ... Gasoline tanks are another story altogether.

As far as sight tubes, they are specifically permitted under ABYC as long as they are fitted with ball valves as mine are.
Send me an email address and I'll be happy to send you ABYC and Transport Canada Standards in pdf files. I have all the others but in hard copy only.

ABYC H-33 Diesel Fuel Systems ...
33.5.8 A means to determine fuel level or quantity shall be provided.
33.5.8.1 If a sight gauge is used,
33.5.8.1.1 it shall be equipped with a shutoff valve at the top and at the bottom of the gauge, and
33.5.8.1.2 a warning label shall be placed adjacent to the sight gauge, and
33.5.8.1.2.1 the warning label shall comply with the requirements of H-33.16.2.
Thanks for the clarification. Perhaps he was referring to a gasoline system. It was almost 50 years ago when I was selling boats and my memory is pretty fuzzy that far back.

Regardless I do think connections on the bottom of a any fuel tank do present a risk of dumping the fuel into the boat and I prefer to avoid them. Yes, well maintained and regularly inspected the risks are negligible but not zero.
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Old 19-06-2022, 08:02   #15
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Re: Fuel tank feeds and return

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Thanks for the clarification. Perhaps he was referring to a gasoline system. It was almost 50 years ago when I was selling boats and my memory is pretty fuzzy that far back.

Regardless I do think connections on the bottom of a any fuel tank do present a risk of dumping the fuel into the boat and I prefer to avoid them. Yes, well maintained and regularly inspected the risks are negligible but not zero.
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