Water in an aluminum feul tank

[Nomad Willy]

Most trawlers from the 70s and 80s had "black iron" fuel tanks and "Black iron" fuel tanks seem to last 20 to 35 years. With some water in it how long will an aluminum tank last? What is the downside to SS tanks?

 

[Marin]

Our boat, like all the older GBs, was built with iron tanks. A year before we bought it the owner replaced them (I assume they had started to rust out) with stainless steel tanks. The surveyor who did the buyer's survey in Alameda, CA said that stainless tanks were not approved (at that time, anyway) by the ABYC as being a suitable material for fuel tanks. The problem, it was explained to us, is that stainless quality can vary all over the map. For example, for a long time stainless steel from Taiwan was very suspect. It was inconsistent and often did not meet the standards for the particular stainless alloy.

So the potential for poor quality stainless was (and maybe still is) one problem.

The other problem we were told is that stainless changes properties when heated, as in welding. So the act of welding the seams of the tank can change the properties of the steel, and if the welds aren't done perfectly, the metal along them can corrode, pit, rust or pinhole.

So these two variables, the quality of the stainless itself, and the potential for welds to make the stainless at the weld susceptible to deterioration, is why, we were told, stainless is not a recommended material for fuel tanks. The potential weld problem is made worse if the metal is isolated from oxygen for a period of time, as when fuel sits in a tank for a long time. I assume what the surveyor was referring to was crevice corrosion.

All that said, he told us that he'd surveyed a lot of boats in the SFO Bay area with stainless tanks and in most of them the tanks were holding up just fine.

So based on all of that I would say that stainless is not an ideal material for fuel tanks. Our boat has them by virtue of the previous owner so we'll live with them and hope they hold up for awhile. We use our boat year round and so cycle through our fuel regularly. Plus the new fuel setup has five smaller tanks in place of the original three tanks. So our fuel management is to drain the saddle tanks in pairs, and when one pair is drained empty we leave it empty until the second pair is about 1/3 full, at which point we'll fill the first pair and finish emptying the second pair. So the tanks spend a lot of time empty with air (oxygen) in them which we hope will extend the longevity of the metal along the weld lines.

Bottom line, I think, is don't make fuel tanks out of stainless if you have a choice.

Some of the more experienced shipwrights on the Grand Banks Owners forum also recommend against aluminum tanks. However, I do not recollect exactly why.

The consensus on the GB forum seems to be that iron tanks are very good if the are protected against rust and are mounted in such a way that water will not collect under them and the deck fills are properly bedded so that water cannot get down on top of them.

Also good in their opinions are properly made fiberglass or "plastic" fuel tanks. Apparently fiberglass tanks can be "attacked" by some bio diesel mixtures but if you don't use bio diesel than that's not an issue.

 

[FF]

The "best" fuel tanks are made of monel.

Second best (only because of the limited complexity of the tank) is the roto moulded plastic.

Either has almost unlimited service life , but the cost of the mold restricts the plastic to limited sizes already in production. Multiple tanks can help "fill the space" on an existing boat..

Plastic has special mounting requirements.

 

[Larry M]

RE: Water in an aluminum fuel tank

In 1997 we had two-75 gallon diesel tanks replaced on our last boat.** We were told by the tank manufacturer and the insurance companies surveyor*that the new tanks should be*from marine aluminum grade 5052. Aluminum like stainless steel comes in many grades. I'm not sure water in the fuel tanks would be much of an issue. Maybe we'll hear from someone who has an aluminum boat. I believe most aluminum hulls are made form 5052.

Larry/Lena
Hobo KK42
La Cruz, Nayarit, MX

 

[El Sea]

After cleaning fuel tanks full time, we see water in most tanks. The true problem is what the water is doing to the fuels.

But back to your questions, "With some water in it how long will an aluminum tank last? What is the downside to SS tanks?"

Aluminum tanks usually develop a problem if they are encapsulated in foam and allowed to collect moisture. As a rule when we are called to service a tank, we inquire as to the age of the tank/vessel and if nearing twenty years we recommend a tank replacement. There are rare cases where the normal life can be expanded.

The downside to SS tanks, is like*Marin was stating earlier.

My recommendations to all of our customers are:

1) Keep the fuel tank 96% full or Absolutely empty
2) If you are going to treat the fuel, do so by the size of the tank
3) for gasoline customers - avoid mixing ethanol and non ethanol
4) get a six pack and go boating, a six pack of fuel filters


El Sea / L.C.
44' Thompson (w/FRP Tanks)

 

[Marin]

If you're going to be removing old tanks and installing new ones, if you're going to have the new tanks custom made, it's a great idea to have them feed the fuel line from the very bottom if your boat's configuration makes that possible. That way any water or other crud will be removed from the tank and filtered out, rather than sit on the bottom and get worse.

While our boat has stainless tanks--- not the ideal material as described earlier-- the upside is that whoever designed the system ensured that nothing can get trapped in a tank. The four saddle tanks gravity feed into the day tank via manual valves that are at the very bottom of the tank sides, and the tank bottoms are slightly sloped toward these tank outflow openings. The fuel lines for the two main engines and the generator come out of the bottom of the forward end of the day tank in the bilge, and the bottom of the day tank is sloped slightly toward the forward end.

The system can be valved so the engines feed from one of the saddle tanks on the engine's side of the boat, but the fuel feed is the same line, out of the very bottom of the inboard side of the tank. So the fuel still comes from the bottom of the tank and then through the filter system.

Obviously this sort of fuel feed will not work in every situation. But if your boat uses saddle tanks in the engine room you may be able to have the tanks, fuel plumbing, and filter system designed to feed from their bottoms of the tanks if you're having custom tanks made.

 

[Marin]

With regards to Eric's original question, I've taken the liberty of copying some comments about fuel tanks, specifically aluminum tanks, from Bob Lowe and Mike Negley, the two most knowledgeable and experienced shipwrights on the Grand Banks owners forums. While they are stating their own opinons, these guys truly are "gurus" and what they say is worth at least giving consideration to. Bob Lowe doesn't like aluminum fuel (or water) tanks at all. Mike Negley likes aluminum fuel tanks IF they are properly made and installed.

Comment #1, Bob Lowe

GB used iron for the fuel tanks for good reasons, it is the best material suited for the job. The problems many experience with these tanks is not due to the materials used, it is due to poor setup (no drains at the bottom of the tank) and poor maintenance at the deck fill and exterior of the tank.

Aluminum is a problem waiting to happen. Bayliner used aluminum and experienced many failures in short time.

SS is problematic at the welds.

Fiberglass has a mixed history and is labor intensive.

Plastic is fuel proof, but for large fuel tanks that require baffles to control the live load in a seaway, is problematic.

In short, if you have to have new tanks build, use iron and build them right with baffles, a drain and preferably a sump to collect water and debris. Set the deck fill properly in good bedding and keep it watertight.

We replaced Dreamer's fuel tanks with iron shortly after we got her. Although they are about 23 years old now, they are like new inside and outside. They were built with sumps and drains.

Comment #2: Bob Lowe

I am not a fan of aluminum tanks, having seen too many failures over the years. There is absolutely nothing wrong with black iron as the original tanks were built. I highly recomment adding a sump with drain plug and make sure your fuel feeds are at the bottom of the tank (not the sump).

Comment #3, Mike Negley

I think there are several reasons why SOME [aluminum] fuel tanks have been a problem. First, the selection of the right alloy (only 5- and 6000 series are marine grade) with the 6000 series being by far the best. Second, incorrect welding techniques and third, failure to keep the exterior of the tanks clean and free of dirt and particle deposits (as is so often found in an engine room)which prevent access to oxygen. Aluminum tanks should not be set on wood frames or butted up against wood and insulation should never be attached. Another factor is that aluminum is not as strong as steel or stainless steel and if not properly posisitioned on its cradle, could be subject to stress corrosion. Certainly stainless is stronger and more corrosion resistant with Monel even more so, but stainless, being an alloy, is still subject to corrosion since it contains dissimilar metals. The pitting on a stainless shaft is most often caused by barnacles which restrict the free flow of oxygen in the water.

Comment #4, Mike Negley

Aluminum is right next to zinc in the Galvanic Series with stainless in the middle and bronze toward the more noble. In other words, given the right conditions, aluminum makes a fine sacrifical metal. When I discussed aluminum fuel tanks, the fuel inside the tank is not a satisfactory electrolyte, but if you lay a copper penny on the top of the tank and let seawater drip on it for a while, it will eat right thru the tank. Keep the tank clean on the outside and away from more noble metals and it works fine.

 

[FF]

"Although they are about 23 years old now, they are like new inside and outside. They were built with sumps and drains."

This is one key , IF there is a sump for the water to collect in , it can (hopefully easily) be removed.

A DE Emulsifier will help get the water into the sump, not the common stuff in the boat store.

The other key to iron tanks is the ability to keep them from sitting in water and not having water sit on their tops. Paint helps.

With proper clean outs , so the ashpaltine that sticks to the internal tank walls , you have a setup that is almost as good as it gets.

Monel would care less about deck leaks , but its expensive.

 

[sunchaser]

As said, standard steel if properly installed will last for a long time in diesel boat applications. It certainly does in above ground applications for earth moving* equipment (I've worked with Cat equipment built in the 50s whose tanks were OK)**and for over the road trucks.

This summer I heard something interesting from a big boat engineer who*is associated with the PNWs big yacht "engineer's organization." He says there is mounting concern that some Al alloys or welding rod are not holding up with the newer diesel fuels. The culprit could be some bio diesel manufacturing processes, ditto the lower S fuels or over the counter additives that are filling Walmart's shelves. Fact or fiction I don't know but I am aware of a large yacht*with 25 year old Al tanks which have recently started leaking.


-- Edited by sunchaser on Monday 8th of November 2010 10:33:47 AM

 

[Bendit]

Pioneer's aluminium tanks are 30 years old.

We had a problem with the port tank along a weld line where water*caused pitting on the inside of the tank which eventually became a pin-hole leak. The boat had a history of sea-water in the tanks due to a crazy fuel-filler and breather design, since remedied.

An old engineer told me how to fix the leak - with a ball-peen hammer. Just peen over the weld where the leak is. Fortunately the leaky area*was fairly accessible and it has been OK for 4 years now.

I now use an additive which claims to remove water, and the bug, from fuel.

 

[RickB]

Bendit wrote:We had a problem with the port tank along a weld line where water*caused pitting on the inside of the tank which eventually became a pin-hole leak.

Unless you saw generalized pitting over a large portion of the tank bottom, that was crevice corrosion, not pitting.

The weld either had a crack or an overlap that provided a home for the process. Considering that peening was effective and the leak was on a weld seam, this is the most likely scenario.

*

 

[Bendit]

You are probably right. It was very localised.