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Old 11-04-2010, 07:15 PM   #7
Marin
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RE: Water in an aluminum feul tank

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.
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