plumbing on auxiliary fuel tank

[bradpb]

My current diesel fuel tank is too small and I am forced to carry portable tanks on deck. I'd like to reduce the the number of them and have been playing with the idea of adding a secondary tank.

I have not decided if the secondary tank will be a flexible type or a hard shell - certainly the flexible tank would be easier to place rather than struggling with the ridged tank in an already cramped space below the cockpit. If any one has opinions on this let them fly.

The question comes up on how a secondary tank would be plumbed. My understanding is that regulations state a fuel tank can not have a bottom drain on it - so all holes will need to be on the top of the tank . I'm not familiar with the construction of a flexible tank , I assume that there is one hole being used for both fill and drain, and wouldn't require a vent.

Nevertheless, how are typical auxiliary tank arrangements plumbed - I have questions!:
1) Is there a tank selection valve prior to the fuel filter .
2) I assume that the fill hose from the deck can be used for both tanks.
3) looking for thoughts on flexible tanks.
4) are crossovers used - and how does that work if indeed plumbing is disallowed on the bottom of a fuel tank.

I have searched on the internet for diagrams and have come up short - if anyone knows of a resource , please share.

[rwidman]

Originally Posted by bradpb

............. My understanding is that regulations state a fuel tank can not have a bottom drain on it - so all holes will need to be on the top of the tank .............

How did you come to that understanding? That would seem to put both of my tanks in violation. The fittings to the filter are not on the actual bottom but an inch or so up from the bottom on the side.

Fuel won't flow out of the top of the tank unless you pump it.

[a64pilot]

I think he is correct, but I cannot quote chapter and verse.
If I were to install an Aux tank I would transfer the fuel to the main tank and leave all the fuel plumbing alone. I'd also pump it through a filter just in case.
Now I also seem to remember a prohibition of a pressurized fuel line within the hull, but that may be gasoline only prohibition.
I have been considering a bladder tank myself to sit in my bilge under my water tank, this portion of the bilge is always dry.

[bradpb]

ron I can't remember where I read it. I'll see if I can find it. If it isn't the case then I guess it makes life simpler, albeit I'm not that excited about putting a hole in the bottom of my existing tank.

[Frankly]

I installed a 15 gal bladder tank a number of years back to keep from messing with JOD. I installed the bladder in a compartment under the rear berth and because it is above the fixed tank, I just plumbed the bladder drain into the top of the fixed tank with a shut off valve at the bladder. When I need the extra fuel, I fill the bladder and after the main tank is down I open the bladder shutoff and gravity takes over. This configuration has worked really well on the occasions of use (Bahamas trips). I installed a separate deck fill and because the fuel fill is not downhill all the way a vent was also required.

I have wondered if you can really install a flexible tank anywhere below decks and not run a foul of the ABYC or Coast Guard fine print somewhere. I did it anyway and worried initially but the concern has dissipated with time and use.

I installed two GC batteries in the bilges long ago. Worried about that decision initially but just no big deal anymo.

[Geek_Guy]

Interesting discussion. I have an auxiliary tank. The PO plumbed it with 2 valves. I've never used it but assume that the intent was to locate the feed and return to the other tank. I have wondered why you would want that set up when you could just dump the alternative tank into the main. Especially as my alternative is 3 feet above the main.


Dave

[rwidman]

Don't forget that your engine uses less fuel than the pump delivers and the excess is returned to the tank(s). Don't get yourself into a situation where the return is pumping into an already full tank.

[TeddyDiver]

Originally Posted by a64pilot

I think he is correct, but I cannot quote chapter and verse.
.

Boat Building Regulations | Boat Fuel System links to regulations..

BR Teddy

[bradpb]

Originally Posted by TeddyDiver

Boat Building Regulations | Boat Fuel System links to regulations..

BR Teddy

fuel tanks regulations 183.518

Fuel tank openings.
Each opening into the fuel tank must be at or above the topmost surface of the tank.

[sck5]

I put an auxiliary tank on my boat. I left the original main tank just as it was with its own fuel fill and with the fuel return from the engine going to it. The new aux tank had its own fuel fill and a transfer pump to send fuel to the main tank. That way I always filled the aux tank and then pumped the fuel to the main tank so it stayed fresh and wasnt sitting in the aux tank for a long period of time. Transfer pumps are cheap and make it very easy - Just press the button and fuel goes from the aux tank to the main tank until it is full. Then turn it off. Very easy.

[boatpoker]

CFR's do not have any standards for diesel fuel in pleasure craft. Check ABYC Standards for how to properly install any kind of diesel tank.

ABYC Standards are not mandatory but many (most) insurance companies will require compliance.

[TrentePieds]

Standards or no standards, why would anyone even contemplate having any kinda opening in a fuel tank - or any other kinda tank - anywhere but in the "lid"? You want to have absolute assurance that when things go sideways the stuff in the tank, whatever it may be, will STAY in the tank. Most particularly you'll want to have absolute assurance that the stuff in the tank,- whatever it may be, will not wind up in your bilge. Ergo: You'll want to have to pump it out of the tank to get at it.

This conversation started with a query about connecting discrete tanks. Remember that gravity is rather indefeasible! If tanks are at different levels, and connections are at the bottom of the tanks, what's in the higher tank will inexorably wind up in the lower tank. You don't want that! Remember what they told you in high school about "open, connected vessels". Have openings only in the "lids" of tanks. Use a transfer pump to get stuff through an "inverted U" rising higher that the top of the topmost tank from one tank to the other.

TrentePieds

[mitiempo]

Openings in the bottom of fuel tanks are not allowed (ABYC) for gasoline but are allowed for diesel. Same as sight glasses are allowed for diesel if properly installed. Common on many commercial boats such as fishing vessels.

When I had a new diesel tank built I had a threaded fitting installed in one lower corner. There is a valve as well as a threaded plug installed so it cannot be accidently opened. If I ever have to drain the tank for cleaning it will be simple to remove the plug and do so. The normal fuel line comes from the tank top as does the fill and vent.

[Pete7]

I would choose a second rigid plastic tank and connect a fuel pump in like this:

Facet Red Top Style Fuel Petrol Diesel Pump Kit Universal 12V Electric HRF-027 | eBay

The thought of a flexible fuel tank leaking diesel just doesn't bear thinking about. I looked at a 33ft yacht that had a fuel tank leak, oh dear what a mess and the boat stunk. The ply bulkheads had absorbed the fuel and I wasn't sure the boat could ever be properly cleaned up. I just walked away.

Pete

[seabreez]

Originally Posted by bradpb

My current diesel fuel tank is too small and I am forced to carry portable tanks on deck. I'd like to reduce the the number of them and have been playing with the idea of adding a secondary tank.

I have not decided if the secondary tank will be a flexible type or a hard shell - certainly the flexible tank would be easier to place rather than struggling with the ridged tank in an already cramped space below the cockpit. If any one has opinions on this let them fly.

The question comes up on how a secondary tank would be plumbed. My understanding is that regulations state a fuel tank can not have a bottom drain on it - so all holes will need to be on the top of the tank . I'm not familiar with the construction of a flexible tank , I assume that there is one hole being used for both fill and drain, and wouldn't require a vent.

Nevertheless, how are typical auxiliary tank arrangements plumbed - I have questions!:
1) Is there a tank selection valve prior to the fuel filter .
2) I assume that the fill hose from the deck can be used for both tanks.
3) looking for thoughts on flexible tanks.
4) are crossovers used - and how does that work if indeed plumbing is disallowed on the bottom of a fuel tank.

I have searched on the internet for diagrams and have come up short - if anyone knows of a resource , please share.

My opinion is to keep both tanks separate. I run a tug and last year an engine shut down and I was in a dangerous situation. While underway we found out one line had blocked with a glob of silicone. Go figure where that came from. I was able to manoeuver with one engine and finally got going on two. The tug had two fuel tanks connected in parallel. Now both are separate with individual water / fuel separators and return line and valves. I operate less stressed now. For the fitting at the bottom of the tank... both our fuel tanks are plumbed under the tanks. I like this because "no" water remains in the tanks to cause rust. With a top pickup, you always have some (a bit) of water that will some day cause worries. The materials tanks are made of varies also and have both pros and cons. We run steel tanks and have opened the inspection covers and found that they were in very good shape at the bottom... no water... no rust.