Metal vs. Plastic Fuel Tanks

[thinwater]

Originally Posted by Ike

Interesting thread and lots of good info here. This is a subject that I spent many years on while working in the Coast Guard Office of Boating Safety. Yes I have seen a few plastic tanks fail, but it wasn't the tank. It was usually either a gasket that leaked, or someone (usually the boat owner) accidentally punched a hole in the tank with a screw or drill while installing something else.

Most of what I will say here only applies to gasoline tanks. Actually Cross linked Poly tanks were introduced in the 70's. My first encounter with them was while investigating boat fires in the late 70's. Not one of those were in any way due to a leaky plastic tank and quite a few involved metal tanks. In the 80's we started a research project to test the tanks for permeation and it's affects on the tanks. After soaking them in gasoline in a controlled environment, for five years, the differences in weight, permeation and fire resistance were zero. We also tested them for the affects of alcohol. Again, no change. What does all that mean?

First realize that plastic gasoline tanks have to meet the same safety standards as metal tanks. They have to resist a fire for 2 1/2 minutes without leaking. They are submitted to the fire and then pressure tested at a max of 3 psi, and cannot leak or they fail the test. The same test is true for metal tanks.

Also the USCG had a pretty lose standard (according to the EPA) on permeation, but our concern was not environmental. We didn't want boats blowing up because vapors permeated through the tank walls, so our standard was set to keep it below the Lower explosive limit. All the tanks we tested met this standard, but that is also why you could smell fuel vapors. Your nose is very sensitive to even tiny amounts of gasoline vapor.

In the late 80's we had UL do a long study of issues with aluminum gasoline tanks. I helped with some of the investigations of fires involving boats with aluminum tanks. A lot of information came out of that study.

Aluminum tanks on average last twenty years if properly installed and maintained. We had some fail in less than a year and some that were over 30 years old. But the average was 20.

The most common failure was pinhole leaks in the bottom. This was almost always due to moisture being trapped under the tank with no way to dry out. As an addendum to that we began to see a lot of aluminum tank failures due to phase separation of alcohol from the gasoline in boats that were stored with partially filled tanks. (one marina had over 40 boats with failed tanks in one winter)

In the early 2000's the EPA got into the act and started proposing changes to evaporative emissions and exhaust emissions. I was the liaison officer from the USCG to the EPA on that project for 3 years. The major thing that changed was the construction of PE tanks. The industry had to make them less permeable. They do this several ways but the easiest is a hard liner. The hose people used the same idea on fuel hoses. The added benefit is they permeate far less and don't stink (at least not to my nose)

But to answer the question in the quote, there are boats running around with plastic tanks that are 40 years old or more. I don't know if my boat came with a plastic tank but it got one somewhere in it's life and I have had it 11 years. The boat is going on 47 years old. The previous owner had it 10 years and he said the tank was already there. So that's 21 years or older.

As far as diesel goes I have seen every type of material used for tanks. When I visited Hatteras Yachts in 1985 all their diesel boats had Fiberglass tanks. I have seen aluminum, steel, stainless steel, monel and others. By the way, terneplate has been banned for gas tanks in boats for over 40 years and I have never seen one in a Diesel boat That doesn't mean there aren't some but they are few and far between because of serious corrosion issues.

The primary issue is installation, installation, installation. If the tank is installed right, so that it is kept dry on all sides (including the bottom and top), is inspected regularly and pressure tested annually it may outlast you. Check gaskets and fittings. Accident statistics show that most fuel leaks are at fittings. With diesel, scrubbing the fuel can help considerably and virtually eliminate things growing in the tank.

Great response!


As an API licensed inspector and 35-year refinery engineer, I've seen a lot of tanks in a lot of situations, including seaside and platforms. But boats have their own own twists.


On shore, most failures relate to either accumulated sludge (corrosion under sludge is worse) and ice breaking fittings. Fitting leaks are always a matter of maintenance.

[wsmurdoch]

At Eastman Kodak/Eastman Chemical almost all our tank (all were stainless steel, aluminum, or carbon steel) failures or near failures were caused by chloride induced stress corrosion cracking in the heat affected zones of welded stainless steel tanks, general corrosion, external mechanical damage, and over/under pressurization due to a blocked vent/relief.

Bill

[thinwater]

^^ Bill: I guess if I think back...
* Several destroyed by bad vents (I actually watched a 30,0000 gallon tank sucking in--A workman had been directed to plug the vent because and over flow had come out there!). That was always one of my first inspection points. Surprisingly often vents were removed, blocked by birds or bugs, or modified.
* Explosion. One was was destroyed because the vent had been moved to near ground level and there was a spark nearby, and the other because the diesel delivered had some gasoline (about 5%) (static was the ignition source).
* In one case a copper float was resting on the bottom, which created a cell.



So lots of different reasons, many of which can happen on boats. I've inspected thousands of tanks.

[Alan54]

Originally Posted by wsmurdoch

To the left is a 1988 Pacific Seacraft 34 0.090 aluminum fuel tank that developed a pinhole leak in 2018. It was repaired on a dock in the Bahamas with JB Weld putty, West System epoxy, and glass fiber cloth. To the right is its 2019 0.250 5052 aluminum replacement.


30 years is a long time. Aluminum lends itself to odd shapes. Aluminum is owner repairable.

Bill

Hi Bill, I'm new to this forum. I also have an 1987 PSC34. I'm out of Kemah, TX. I'm trying to get a new Aluminum fuel tank and came across your post and pictures on the pinhole tank and new black coated one.

Where did you get that one built? PSC or an outside vendor?

[wsmurdoch]

Best Fab Inc built the tank, shipped it by truck to New Bern, and we installed it there ourselves.

The story is in my wife's blog, irish-eyes-to-the-bahamas.blogspot.com The March 19, 2018 and February 15, 2019 entries.

The Honda generator and its attached 5 gal gasoline jug along with the patched dock are from hurricane Florance's 2018 damage to our dock neighbor's boats and to the dock itself which was still under repair when we left for the Bahamas in January of 2019. When we got back in June, we had electricity at last.

[Alan54]

Bill,

I got hold of Best Fab and they sent me a print of the PSC34 (standard keel) fuel tank. Im pretty sure its the correct one along with dimensions, but on the print it states 36.5 Gal? my tank is a 32Gal??

Alan

[wsmurdoch]

They make two different PSC34 fuel tanks. One like mine for the standard draft keel (36.5 gal) and another for the shoal draft keel (32 gal).

Before I signed the approval prints, I pulled my tank and measured it. The only modification I made was to remove the four brackets because my tank is held in place with two hose covered pieces of all thread over its top. I kept my original fuel gauge.

[gonesail]

i like plastic tanks. but aluminum is good for custom shapes which is usually what you need for a boat.

[Alan54]

Im pretty sure I have a standard keel looking at a recent picture of my boat and referencing the brochure??

[Bowdrie]

Back in the old days, Stainless Steel was considered as the new Miricle Metal that would solve most all of the problems of corrosion.
It would be perfect for diesel tanks on boats, right?
Of course the diesel fuel back then was High Sulfur/Higher Sulfur, and More Sulfur, (hey, Sulfur is an accelerant in the combustion process, more is better, right?).
The failures of SS tanks for diesel were traced to the welding/welds.
The welding process affects the metal and any water in the tank would start to react with the Sulfur to make Sulfuric Acid.
The acid would attack the welds, (where the composition of the SS was changed by the welding).
With today's ultra-low sulfur diesel much of the problem went away, but not all.
We now know that the welding needs to have the inert gas flooding not only the side where the TIG torch is being used, but on the other side of the weld also.
Once the tank gets assembled close to the point of being a "tank" then it needs to have its interior flooded with gas while it's being "buttoned up".
The highest quality Aluminum tanks are also done that way, gas on both sides of all the welding.

[carlos1w]

Originally Posted by oleman

I see the plastic tank as a fire hazard but also the entire plastic boat also.

The day a major country builds a warship from plastic I will come onboard.
My choice of material in 2019 is monel.

The UK Royal Navy lost an aluminum destroyer (?) the Sheffield to fire after an exocet missile struck it. So metal may burn too 😬

[Bowdrie]

Originally Posted by carlos1w

The UK Royal Navy lost an aluminum destroyer (?) the Sheffield to fire after an exocet missile struck it. So metal may burn too ��

An ancient saying in the military;
"If it moves, salute it, if it doesn't move, paint it".
Between WWI and WWII, the Navies were quite keen on having everything well painted and spotless.
When real combat started it was quickly found that a couple of decades of paint build-up was not helpful to damage control efforts when a fire got started.
Aluminum conducts heat very rapidly.
Did the British forget that in the intervening years since WWII?
Was there a lot of paint build-up in order to "cover-up" the aluminum and keep all bright and cheerful looking?
I don't know.

[NYSail]

Last boats both had aluminum tanks….. had no issues. New boat has diesel, water and holding tanks. So far no smells from any (2005) and all seems fine. Only issue with the plastic fuel tank is the screwed on fittings for sender, return and fuel pickup. On my fuel tank there are no backing plates so the screws have to tenderly be tightened or they will strip the threads in the plastic….. my fuel sender leaked when I first bought the boat when the tank was full and in bouncy conditions. Looks like the p/o removed it a few times to try and get it to work properly and never got the refit right. I stopped the leak, however will eventually add backing plates to this fitting to eliminate this issue all together.

Greg

[dlj]

Originally Posted by Bowdrie

Back in the old days, [snip]

The failures of SS tanks for diesel were traced to the welding/welds.
The welding process affects the metal and any water in the tank would start to react with the Sulfur to make Sulfuric Acid.
The acid would attack the welds, (where the composition of the SS was changed by the welding).

I'd be interested in knowing what years are "back in the old days".

There have been advances in stainless manufacturing. What you are describing is likely due to a phenomenon called "sensitization". It is pretty much eliminated in modern stainless steels used in tanks.

I'd also be interested in more info on the problem of cracks in SS welded tanks - i would guess that either it was inferior welding technique or sensitization.

I have 40 year old stainless tanks in my boat and there are no signs of any issue. I do have an unusual design, I have two saddle tanks feeding a pony tank. The pony tank remains full at all times as it's fed from the saddle tanks. Of course if I run low enough on fuel then the.pony tank could be less than full. But I never run my fuel that low. The feed and return from the engine go into the pony tank. Condensation issues are minimized through this design.

In another life, I did a lot of long term research in polymers (among many other materials) long term stability. There are formulations that will last 15 years no question. Of course many would last much longer, but it could be shown that 15 years was reliable at - I think it was 6 sigma - at a minimum. In any case at whatever sigma it was, it permitted the publication of a 15 year life. There was work being done to push that to 20 years.

Compare that to stainless steel - there was no published limit imposed on product related to that material. That means the expected lifespan was beyond 50 years.

dj

[Bowdrie]

Originally Posted by dlj

I'd be interested in knowing what years are "back in the old days".
There have been advances in stainless manufacturing. What you are describing is likely due to a phenomenon called "sensitization". It is pretty much eliminated in modern stainless steels used in tanks.
I'd also be interested in more info on the problem of cracks in SS welded tanks - i would guess that either it was inferior welding technique or sensitization.
Compare that to stainless steel - there was no published limit imposed on product related to that material. That means the expected lifespan was beyond 50 years.

dj

Perhaps I was unclear.
No, the base metal did fine.
It was the welding/welds.
Perhaps poor technique, perhaps wrong choice of filler rod, perhaps choice of shielding gas for the welding, (gas on both sides of the bead as it moves along IS superior,) whatever, (I'm not a welder,) but any sulfuric acid formed attacked the welds, and fuel back then had a lot more sulfur than today's stuff.
"Back in the old days"?
Ok, maybe ancient in the way many might think.
In the '50s>'60s, although I've seen tanks from Tiawan boats that were built in the '70s that had leaky welds. I'll grant that the Tiawan SS was not always up to par compared to US product.
Anyway, SS tanks got a bad rap for diesel usage, perhaps undeserved, but there was a learning curve.
As an aside, a friend of mine, (pro welder,) runs a welding/fabrication crew on the North Slope, he says they always run gas on both sides of the welds. When they're joining pipe and fittings there is always some gas being bled thru the interior, keep the oxygen away from the molten metal on the back side.
A company up in the PNW, that builds custom tanks does the same, even in aluminum.
When the tanks were built for "Tally Ho", the guy who welded them pointed out that gas was kept on both sides for the welding.