aluminium Fuel tank questions

[bud37]

So , after reading the thread about the fuel tank replace I can't help but wonder about the reason for the tank failures.....is it bad Al metallurgy, grounding electrolysis, fuel changes or just funky installation practices??

We have been looking for a new boat for a while now and these tanks are probably the thing I look at the hardest as to ease of replacement if necessary ( don't have a million in the bank, lol ).So what say you folk....

I replaced the 40 year old galvanized tanks in our current boat this past year with plastic because maybe they can deal with the new fuels better....

[Ramsport47]

The problem with most fuel tanks is, they're mounted outboard of the engines usually under the bilge vents. This allows water to get underneath the tanks and stay there, corroding the aluminum. Most boat builders, Carver included, leave and air gap underneath to help prevent this, but it really just helps the water stay there. It doesn't matter if the boat is in salt or fresh water, the result will eventually be the same

[bud37]

Something that I have noticed in the various pics of these tank failures is the holes are on the bottom and appear to be random, not concentrated in any one spot....would be interesting to get a piece and have it looked at by a lab to see if it is corrosion from the outside or from the inside leading to this.......I just don't remember hearing about tank failures years ago, maybe there was.....just sayin..

[tomschauer]

I am in the (long) process of purchasing a 1998 Carver 355. Yesterday I was looking through the original owners paper work and found they had the aux tank replaced in 2002 for a cost of almost 5k on a 4 year old boat that was only used in fresh water. Many of the tank failure threads I have read seem to be around the same year.
I have had many other older boats (one SeaRay 1971 a friend of mine still has it) and never had a tank problem. I currently have a 1990 Proline 17' center console with an aluminum tank that is almost always wet due to be mounted low and center, and it is solid.
this would lead me to believe either Carver specified thinner wall tanks for a couple years to cut costs or their supplier cut corners with thickness and or quality of the aluminum?

[anothernewb]

Here's my experience with fuel tanks. My 2000 350 Mariner had the starboard fuel tank replaced in 2007 and after I purchased it I had to replace the port side this year. So one factory tank lasted 7 years, the other 16. From what I could tell, the tank I replaced had corrosion from the outside in. After cutting the tank open I couldn't see any signs of corrosion from the inside. The material used from the factory was .120 thick. Between the tank and the wooden platform it was mounted on there was rubber strips to create an air gap. In my opinion the failure was from moisture being trapped between the tank and those rubber strips. Why one tank only lasted 7 years and the other lasted 16, I dunno. What I did do to prevent future problems was I painted the bottom of the tank with an aluminum primer containing zinc chromate and a top coat on top of that.

[drosteo]

I have a 2157 Montego which I assume has an aluminium tank. I am getting strange reading on my fuel flow sensor. When I attach a new piece of hose to the brass fuel outlet on the tank and suck fuel through it does not come easily. It has a resistance and then let's some fuel through with a farting/gurgling noise. Also I can't then blow the fuel back down the hose into the tank again.

My questions are
Does anyone know if what I've described regarding resistance of fuel coming out is normal?
Also is this brass elbow a one way valve?
Is it a metal stem or rubber hose inside the tank attached to this?

It is very hard to remove this elbow and I'd rather not if everything described above is normal. Air flow in to the fuel tank is clear. Was getting fine and stable readings on my Lowrance fuel flow sensor but now I'm getting readings that jump all over the place. Could be the sensor or could be the outlet on the fuel tank.

Thanks in advance

[km1125]

There should be a valve right at the tank before it hooks to the hose. That is called an anti-siphon valve. You must draw strong enough to overcome this valve in order for gas to flow. Some electric pumps can't do that but all mechanical pumps can. It also means you can't have any air leaks between that valve and the fuel pump, because the fuel pump will suck air into the system. You can usually remove that valve separate from the elbow that goes into the tank. You can clean the valve but some prefer just to replace it because they are not that expensive (maybe $20?).

They are a safety device to prevent a broken hose from being able to siphon gas out of your tank and into your bilge.

Usually the "dip tube" is some kind of metal or hard plastic. That goes from the elbow at the top down near the bottom of the tank. They usually don't go all the way down though, so there's always an inch or so of fuel left that the tube can't reach

[Viper]

+1
If the anti-siphon valve is working properly, it shouldn't take much to overcome it to extract fuel. Requiring any more vacuum than 2 inHg increases the chances of vapor lock so the valve should open before reaching that point. A dirty valve is the most common reason for valve problems.