Fuel tank idea - comments?

Chinookie

Chinookie

GreatGrady Captain
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Hi, thought I would run an idea by the forum for comment. I need new fuel tanks for my 228. The main tank is shot and the aux is running on borrowed time. The original drawings show .125 is to be used for the 92 gallon main tank, and .09 is to be used for the aux. My understanding is that when pinholes appear, they will be bottom and/or top of the tank. Is that a true statement? if so, does it make sense to specify using a thicker material like 3/16ths on the top and bottom skins only? There would be no point of using 3/16 for the tank sides and baffling because they wont develop pinholes and that will save tank build costs.
 
The reason for the pin holes is corrosion from galvanic activity that is generated when the aluminum comes in contact with another conductive material in the presence of moisture (mainly sea water) as an electrolyte.
Often the tanks sit on structural supports, bulkheads or stringers in addition to whatever is used to hold the tank in place like topside straps, or tank brackets. Moisture often gets trapped between the supports and the tank surface. Depending on what materials were used an mounting surfaces and/or insulators, corrosion can occur.
In summary, the methods and materials used to mount the tank and protect the tank coffin from trapping moisture are more important that the tank thickness, Yes, thicker material will take longer for the corrosion to go through the tank wall but it won't stop it.
 
To add onto what Seasick said... another common cause for corrosion in this area is when water is allowed to sit on the aluminum. Or, more specifically, when aluminum does not have contact with the air. When Al has good oxygen, it will self protect itself.

So, the solution, as noted above, is to build a mouse trap than Grady did. You can epoxy coat the tank... you can ensure the installation doesn't allow any gaps/cracks where water will sit or where the AL won't have air movement around it.
 
Take a look at my thread below. My advise is to have quality tanks made at a certified tank builder. SP Tanks is too far away from you. I'm sure there are West Coast builders.

Use 0.125 aluminum. Have the tank epoxy primed. Install nitril strips with 3M4000 to insulate the tank. Corrosion will occur where water is trapped against the tank. Install new deck ports with 3m4000, clear grease on o-rings and consider caulking them with silicone. This will keep water off the top of the tank. Make sure you deck is solid (not full of water) and screws are properly caulked.

Hookup1

Thread '268 Islander Fuel Tank Replacement'

Decided that this is the year to replace my 25 yo fuel tanks. No leaks. I plan to keep the boat. I'll document the process here.

Pulled the hatch. This hatch was rebuilt a few years ago with Coosa board. Looks good and feels solid. Hatch was set with 3M 4000 - kept everything sealed. Some difficulty cutting with blade to get it up. Not sure what it will go down with. Deck ports were caulked in from the underside as well. Kept the tanks dry.

1 Rebuilt hatch.jpeg

I also put plastic over the fill, sending unit and pickup. This deflects any water away from the busy area of the tank.

2 Tanks uncovered.jpeg...
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All good pints above, nothing to add.
I wrote it already in other treads, if the top of the fuel tank would be slightly curved with a flat spot for fuel sender plate then the water dropping from the pie plate above to sender unit would still drip there, but not poodle there as it would run away to the sides and away from fueltank top.

I am still thinking about how to make a "gutter" below pie plate to deviate the water.

However coal tar the tank seems the best way to preserve a aluminum fuel tank for longest time, even if no coating at all would be best if fuel tank would be 100% dry.

Chris
 
For my tank in my 209, I took a bit from the aviation field. I used zinc chromate from a Pilot supply house, then epoxy coated the entire tank. I even added some glass to the edges for extra protection. Then I epoxied plastic strips to the bottom and ends of the tank for air to circulate around it. It might be overkill, but I don't ever want to pull the center console again. Ha Ha.
 

Attachments

Painting fuel tanks even with epoxy doesn't always protect the tank. Some tank builders will tell you not to paint them at all. The reason is the paint doesn't stick well to aluminum. Zinc chromate is a toxic paint that does an excellent job of preparing the tank. For my tower that was sandblasted I used AwlGrip etch primer before painting it (working great). My tanks were straight epoxy painted by the tank builder (looks pretty but latent defect).

Key thing to consider is making sure the plastic strips stay attached to the tanks. If there is an air gap it invited corrosion. I won't happen immediately even if not perfect. Make sure that nothing contacts the tank on sides or bottom. Tank should be "insulated" and float in the fuel coffin. I usdsed nitric strips and 3M4000.

You should be fine though.
 
Question for the group: does fuel tank corrosion always happen from the outside-in? in other words, could there be also corrosion on the inside of the tank due to moisture condensation (moisture intake related to use of E10 gas versus ethanol-free gas). And related question, does it make sense to use only ethanol-free gas to reduce moisture condensation on the inside of the tank?
 
Fuel tank's are a replaceable maintenance item. You can't make them last forever. 25 years is a reasonable goal. At that point you just don't know.

It always pays to use ethanol free gasoline. It's far better for your engines. The tanks will take care of themselfs.
 
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