Fuel tanks

[Ian Farnell]

Hi I am interested in the pros and cons of aluminium fuel tanks. I would welcome your views

[Ike]

The only drawback to aluminum tanks is corrosion. So the most important thing is installation, installation, installation. Follow the ABYC rules for installation. Make sure no water, not even condensation will remain on the tank. Do not mount the tank directly on a flat surface. Mount it above the surface so air can circulate under and on all sides of the tank. If you use straps to hold the tank down do not use any material between that straps and the tank that will absorb water or allow water to collect under the straps.

Do not paint the tank. Aluminum has a natural oxide that forms on the tank. Removing the oxide when preparing to paint allows corrosion to start.

Mount the tanks so it is well ventilated and easily inspected.

A study done by the Coast Guard back in the 90's shows that the average age of an aluminum tank is ten years. Many lasted longer than that. I've seen aluminum tanks that were 20 years old.

All that said I prefer roto-molded plastic. It doesn't corrode, but if you are talking large tanks over 100 gallons, I would go with aluminum.

[longliner45]

dont know of any negitives on aluminum tanks ,,,,most tractor trailors and trucks have them,,,,are you talking diesle? or gas?

[bilgeboy]

I've got aluminum twins on my diesel Bayliner. 19 years old now.

Glad you posted that, Ike, I was considering buffing them up a bit for looks, but I think I'll leave 'em alone, now. That hazy finish is reassuring.

"Powerboat Reports" has been bashing metal over plastic (polyethylene I think). They generally only rate smaller sized boats, so Ike may very well be on to something with the 100 gallon limit.

Why is that, Ike? I enjoyed you post, by the way...very knowledgeable.

Mike

[Ian Farnell]

Thank you all for your replies I am installing four diesel tanks of 40 gallons , should give me a 1000 mile range, but I will only fill all four when on a substantial cruise so that I don't have stale fuel. the boats a 42 foot motor sailor. Would there be any problems with leaving the tanks empty e.g. condensation etc.

[SamSam]

Originally Posted by Ike

The only drawback to aluminum tanks is corrosion. A study done by the Coast Guard back in the 90's shows that the average age of an aluminum tank is ten years.
.

Here's a case where the CG is right. Most are foamed in so that guarantees
uninspectability and a short life. Stray electricity will eat them up fast. The majority are installed and then the deck is built over them with no provision made for replacement so the deck has to be ripped up/removed to replace. Sam

[Ike]

I was the fuel systems guy in boating safety at CG HQ. I also spent many years on the ABYC Fuel & Ventilation committee. Whole books could be written on fuel systems, yet strangely enough there is very little in print. About all there is, is the Coast Guard Regs in 33 CFR 183. 501, ABYC, and NFPA, and SAE, all of which say basically the same thing.

Anyway, is your question why no tanks over 100 gals, or why is Powerboats reports bashing aluminum tanks?

First aluminum vs plastic. (by the way, when I say plastic here I'm not including fiberglass tanks. That's a whole category unto itself. I'm just talking about polyethylene tanks) Aluminum and plastic are both great material for tanks. Plastic probably has the edge but right now it's about 50-50 in the recreational boat industry. Plastic doesn't corrode. If you are building boats in volume (say 100 or more in production run) plastic roto-molded tanks can be quite a bit cheaper and they can be made in almost any shape.

Aluminum tanks can be made in strange shapes too but it's labor intensive. If mounted wrong you can get bad corrosion problems, and alcohol in the fuel can cause internal corrosion.

Plastic tanks can be installed most anywhere as long as you can ventilate the tank (the interior of the tank not the space around it) and route a fill to it. Aluminum has to be in a well ventilated space, out of the bilge water and should be easy to inspect. So should plastic but it's rarely done.

Roto molded tanks have two big disadvantages. They swell up a little when you fill them so you have to leave extra space around them and because of this strapping them down has to be done to leave room for expansion. Two, the industry hasn't yet figured out a way to put baffles in plastic tanks. So if you make them too big you have serious sloshing problems and free surface problems.

That's where metal, particularly aluminum excels. You can baffle metal tanks and eliminate the sloshing problems and mitigate the free surface effects in really large tanks. ALso when you get above 150 gallons the tanks have stricter requirements to meet and metal meets these easier than plastic.

The other problem with plastic is environmental. This may also sound like a safety problem but it is not. That is permeation. Some vapor permeates through the walls of plastic tanks. Obviously this does not happen with metal. The EPA is proposing rules that would limit the amount of permeation to a tiny amount compared to what is allowed now and this would mean a significant difference in the material the tanks are made of, which will raise the cost and result in a different method of building the tanks.

Why is Powerboats Reports bashing Aluminum? About ten years ago we had UL do a study of aluminum tanks, and of course they found a lot of corrosion porblems. We knew they would because boat companies don't install them the way they should. Some tanks failed in less than six months. I talked to the guys at PBR and they took this report to mean that every body should have plastic because it can be seriously abused and still not leak. Not so with aluminum. However, I and many others interpreted the report to mean that you just have to be careful about how you install a metal tank, whether aluminum or steel.

And to answer Ian's question I would leave the tanks empty. The practice has been to fill the tanks before storage to eliminate condensation in the tank. But todays fuels have additives like MTBE and Alcohol, and When fuel sits for a long time you get phase seperation and these things seperate out and begin causing all kinds of problems especially corrosion of aluminum tanks from the inside out. So its best to empty them. This is hard to do. Gas tanks are not allowed to have a drain so it's very difficult to drain them. If you chose to fill them you can get stabilizers and co-inhibitors to put in the fuel that prevent the fuel from seperating.

[Ike]

Don't get me started on foamed in tanks!!! I'll get kicked off the forum!

[Ian Farnell]

Thanks Ike, you sure know your tanks ,after all this perhaps I should go for stainless steel? The plastic option in the UK appears to be very expensive which is the reason for looking at aluminium, any views on stainless? I've got to say this forum is fantastic.

[SamSam]

Originally Posted by Ian Farnell

Thanks Ike, you sure know your tanks ,after all this perhaps I should go for stainless steel? The plastic option in the UK appears to be very expensive which is the reason for looking at aluminium, any views on stainless? I've got to say this forum is fantastic.

We were redoing a 40' powerboat and the guy had two 3'x3'x8' stainless diesel fuel tanks custom built to fit just so for some horrendous amount of money and we had no sooner installed and plumbed them when some official 'authority' happened by and said something like "You know you can't do that , don't you?" At least he didn't wait until we put the deck in. He pointed out some code that limited stainless fuel tanks to 20 gallons and only to be made cylindrical, like a water heater tank on its side. Something to do with welding stainless made the metal brittle and prone to fatigue cracks in the areas of the welds. That was 10 years ago and they might have changed the regs, but you should find out first. Sam

[Ike]

There's stainless and then there's stainless. Not all Stainless Steels are created equal. Use a 300 series. ABYC, ISO and others recommend 316L or 317L. These are the best for the marine environment. However, that's only part of it. Now for the rest of the story. Stainless steel is subject to crevice corrosion, particularly in the welds. ABYC for years has recommended that SS tanks be 20 gals or less and cylindrical with domed ends. That is to minimize the welds and surface area of the tanks.

Well, I'll probably get a lot of disagreement on this because it's very controversial, but I think you can use bigger SS tanks if you are very careful in how you install them. They are commonly used on drag boats in the USA. But here's the rub. The are typically mounted out in the open where if they get wet they will dry almost immediately. That's the secret to SS tanks; keeping them dry, and of course on boats that's not an easy task. The Nickel Institute in Canada has done a lot of research on stainless steels and found that if they get wet AND STAY WET they corrode. If they dry out right away they don't corrode or corrosion is kept to a minimum.

So, even more so than aluminum they have to be mounted so that all surfaces won't collect moisture, so they are well ventilated, and easily inspected. Again, all surfaces, especially the bottom should be easily inspected and ventilated. Frankly, SS is not commonly used on Recreational boats because people simply don't maintain their boats the way they should, and it's expensive. However, if you are having a one off tank made, the costs would probably be about the same for AL or SS. SS is used mostly in specialty applications. The most common are plastic and Aluminum.

By the way, when making any metal tank make sure you don't use disimilar metals like brass for the fittings. It'll result in galvanic corrosion and eat a hole right through your tank. I've seen tanks that had pinpoint holes all over them from metal filings that fell on or into the tank and weren't cleaned up.

It's an old cliche' but a fire or explosion will ruin your whole day!

[Ike]

Sorry Sam, I was working on a reply and didn't see your post. Many surveyors, and other authorities confuse regulations with voluntary standards. In The US the Federal Regulations must be complied with by boat manufacturers. They do not apply to owners. However there is a big liability issue, as we all know, so it makes the voluntary standards almost mandatory. Most insurance companies won't insure a boat if it doesn't pass a survey and the surveyors use ABYC standards.

The 20 gallon domed tank thing is an ABYC standard. The Fed Regs don't say anything about it.

Now, elsewhere this may be different, particularly in England. I would make it a point to check the RCD standards before I did anything to make sure I was in compliance. In the EU it's ISO. (and in about 70 other countries as well). Italy has it's own rules but they aren't much different.

[Ike]

Ian, I went back and reread your post. The rules with diesel in the ABYC, ISO and RCD are pretty much the same as gas. There are some differences and one is stainless, but the welding has to be TIG. If you use a heavier guage steel, 1.9mm, you can go bigger than 20 gals. But they give a very specific welding procedure in ABYC. Again check the RCD, see what they say about it.

[Ian Farnell]

Thanks Ike I will do that and thanks to everyone else for your contributions

[longliner45]

How about a brones tank?