multi tank fuel system design

[naturewaterboy]

Where can I find good info on how to design and plumb a gasoline fuel system with about 4 or 5 tanks? I'm replacing my one 220 gal. aluminum tank with multiple plastic tanks.

I'm rebuilding the boat (34 Silverton Convertible), and switching from inboard power to outboards on a bracket. I may need to change the location of the tanks to shift the CG. I'd like to find some good design info on the best way to hook up multiple tanks.

[TeddyDiver]

I'm planning same thing with diesel powered motorsailor so there's certainly some similarities and dissimilitaries. I'm using one of the tanks as a "main tank", not so that it would be the biggest, quite the opposite, but all fuel lines to the engine would go from this tank. Other tanks will be "feeding" this main tank throw a separate pump/filter on demand (manually controlled).

[rasorinc]

Here are some regs for you http://www.uscgboating.org/safety/boatbuilder/fuel/fuel.htm

[marshmat]

I like the kind of setup Teddy describes: a 'day tank' that the engine draws from, and you transfer fuel to it manually through a pump/filter from the big tanks.

With the addition of a return manifold, this setup could also be used for a fuel polishing system.

NWB, if I were setting up a system like that, I would want to put a conventional pickup in each tank, and run individual lines from all of them to a single manifold that has a valve for each. This manifold is connected to the fuel pump and filter, which output either directly to the day tank, to the engine lines, or to another manifold by which you can decide where to send the fuel.

Since a 34' Silverton will likely spend a lot of time going fast and chugging back the gas, and probably only has a few hundred miles of range, you might want to skip the day tank and draw directly from the mains.... but there's a lot of benefit to keeping it so that you can fill the tanks separately, and draw from them separately. After all, bad gas is one of the most common outboard killers.

[naturewaterboy]

Thanks, Rasorinc, for the link. That's some very good info. I'll have to go back thru it when I'm ready to layout my system.

I was thinking like you, Teddy, about using one tank as the main and have a single fuel transfer pump, and electrically operated valves to transfer fuel from each of the auxillary tanks. I may need to have 5 tanks, and I was hoping there was a simpler way to connect them all. Having to remember to transfer fuel from 4 tanks - and having to have 5 fuel gauges isn't my idea of a good design.

[Stumble]

Marsh,

The problem with your set up on a boat that chugs down the fuel is that because you are going to only fill up at one place you will either get all good or all bad gass. It is a good set up on a boat that has enough range that it will be carrying fuel from multiple stations, but otherwise it just increases the likelyhood of getting bad gass everywhere, and here is why.

If you have one large tank that is always being added to and taken from the sludge will tend to settle towards the bottom, and be relatively dillute compared to the total amount of fuel in the tank. As you go to smaller multiple tanks you are more likely to run one untill it is dry then switch to the next. What this does is ensure that the fuel pump will try and suck out the last bit of sludge from the tank before you swap over. But since you will be going through a small tank during a day run you are almost guaranteed to pull some skanky fuel. Of course when you switch over you get good fuel for a while untill the same thing happens again.

Now lets say you get back to the dock and top off the boat before you put it away. You top off the two tanks you burned, and the other two that wern't used continue to go bad (you know fuel has a shelf life right?).

Now the next time you run the boat you run off of the same two tanks you used before, and get the same sludge off of the bottom of the first that you got the first time, switch to the second and call it a day. Top off the boat, and leave it for a week or so till the next run.

By now you are starting to see where this goes... There are two tanks that really never get emptied, and the fuel continues to age, while at least one tank gets sucked almost dry every time. So when you go on a long run those tanks are topped off with skunkie fuel, and the day becomes a disaster instead of a fun ride.

Even by rotating the tanks properly you use you still wind up sucking down one tank and pulling up garbage every time you go out. And rotating has it's own problems in that you are not able to pull from the tank that allowes you too balance the best, but have to pull from the one with the oldest fuel.

This is actually an easier fix on diesels than gass engines since they have a fuel return line that allowes you to mix fuel or move it to a different tank as you run. This could be done with a gass engine but requires installing two way delivery pumps and manifolds.



My recomendation is to try and keep is simple. Try to use the fewest number of tanks you can, and try to keep the fuel delevery system that was original to the boat. If you have to change I would probably use a day tank that is fed from two holding tanks. This allowes you to only need a P/S manifold. which could be switched even while running.

[naturewaterboy]

I agree, keep it simple. I would like to put one or two tanks back in, but I want to put plastic tanks in, and they don't come large enough. I took out a 220 gallon tank, I'd like to have at least that much. Also, if I had 5 tanks, does that mean I need to have 5 fuel fill caps?

[Stumble]

Nature,

I think you just need to look for some larger plastic tanks. This sight http://www.incaproducts.com/site/products_fueltanks.aspx has plastic tanks over 200 gallons, and will custom design one if you want (though I imagine that would be expensive.)

And yes if you have 5 holding tanks you need 5 fuel caps.

[rasorinc]

this might be a helpful link. stan
http://www.incaproducts.com/site/default.aspx

[TeddyDiver]

You being in Florida so propably there's not a big chance to get bad fuel I think, so there's not so much benefit of the multiple tanks in this regard. Depending how far you take your tours theres annother advantage to consider. In case of a bad weather there's a lot benefit if you can adjust the VCG of your boat. Having (larger) tanks in both end of a boat gives a good chance to make the difference when it's really choppy..
Of course I meant LCG not VCG..

[marshmat]

I agree with what you say in post #6, Stumble, if the boat is going to be sucking back the fuel quickly and have a relatively limited range.

I got the impression from the first post that NWB already had a number of small plastic tanks and was wondering how to hook them up. You can always draw from several tanks simultaneously if you feel the need to draw them down at the same rate, effectively serving as a single big tank. But yes, it would be simpler to install just a couple of big tanks.

[tom28571]

While there is nothing wrong with following the standards of the USCG regs, they specifically do not apply to boats powered by outboards. They are intended for inboards where fuel tanks and engines often occupy the same spaces or may be vented commonly.

In outboard boats you are mainly left on your own except for tank materials as to whether the tank is enclosed or vented overboard. The red plastic tanks are good only for exposed locations and the other colors, mainly gray, are for enclosed locations. Not certain about colors other than gray.

[Ike]

Tom is partly correct. The USCG rules for fuel systems are not required for outboard powered boats, But, the ABYC Standards for gasoline fuel systems incorporate the USCG regs and apply to both inboard and outboard powered boats. I you get a survey for insurance or sales purposes ( A condition and value survey) the surveyor is going to use ABYC standards. So follow then USCG rules anyway.

The INCA link is a good one and they do make larger tanks.


Here are some more
Kracor Inc - Rotational Molders of Marine Tanks http://www.kracor.com/pages/marinecatalog.html
Moeller Marine Products http://www.moellermp.com/
Marine Fuel, Holding,Water Tanks by Custom Marine Tanks http://www.custommarinefueltanks.com/

[El Sea]

NWB,

Being in Florida, you are subjected to ethanol. This fuel is problematic as you may very well know. The choice of multiple tanks pose no advantage if you consume a large amount of fuel per hour. If you take on bad fuel you can only hope your filtration cleans it up. Stored fuel if not treated will start going bad within two months.

I clean fuel tanks and the findings today are very alarming. We clean tanks for the Department of Interior, The US Coast Guard and a number of individual tanks. The issues we see is what lies waiting on the bottom of your tank. A fuel polishing system is good but is no gaurantee. The new fuels have been stripped of the lubrication needed for the fuel systems that operate at high pressure and a fuel polishing system removes even more of the lubrication.



If I was to design a fuel storage system I would:

A) have a low point sump where the water could be drained off
B) a deck fill that was not flush with the deck, but elevated to minimize water intrusion
C) a vent located a minimum of 12 inches above the deck, not below the gunnel or rub rail

These recomendation would be for a basic gas or diesel system. However diesel would require return and additional custom details would take place.




El Sea,
"What's in your tank?"

[naturewaterboy]

Thanks for the links! I looked at Moeller before - but I didn't look at the OEM tanks - they do have a 157 gallon tank in their OEM list - looks like it has baffles molded in. The Inca Products tanks that I looked at (99 to 182 gal) do not show any baffles. Aren't baffles really needed in a 100+ gallon marine fuel tank?

[naturewaterboy]

Ike,

Is there a new standard for plastic fuel tanks? I remember seeing something about a new lower allowable permeability on them.

Definitely will get USCG approved tanks. Sure don't want to be out at sea and have a bilge full of gasoline.

El Sea - good points on the fuel fill above the deck - I plan to build up an inch or so to get my filler cap well above the deck to keep water out. As far as the vent goes, I don't know that I can get a vent up above the deck. My vent is now in the side of the hull - probably not a great place - even though it is about 3 1/2 feet above the waterline. I suppose I could get a line up into the side of the cabin - that would put it high enough to avoid any seawater.

[naturewaterboy]

Is there a book anywhere on marine fuel system design? I still have a lot of questions about my fuel system. What are the best vents to use? I need to turn some sharp 90 degree turns in my fuel fill - can I use copper or where can I find brass sweeps? I bought two 99 gallon XLPE tanks from Ocean Link, a Moeller distributor. I want to be sure I get them plumbed right. Anyone suggest someone I can contact for help on this?

[Ike]

Is there a new standard for plastic fuel tanks? I remember seeing something about a new lower allowable permeability on them.


Yes, there is. But the EPA is still working on it. But as far as you go, you can buy whatever is available. The standard is not retroactive. The lastest proposal is for charcoal canisters on the tank vent line to collect vapors. The current USCG standard for permeation is 1.2 grams of fuel loss in 24 hours per cubic foot of net compartment volume (if it exceeds that the compartment has to be ventilated. It's a safety standard, not an environmental standard.) The EPA had proposed 0.5, the industry countered with 0.8 and so it goes. I don't know exactly where it's at right now.

[naturewaterboy]

Thanks, Ike. I'm installing the Moeller tanks now. I would like to install some large radius (anything but a sharp 90) sweep fittings in my fuel fill line to avoid stressing the plastic fill fitting on the tank, and keep from putting too much bend in the hose. Any idea where I can find some brass 1.5" OD fittings, or is any other material acceptable?