Have a little time to update on the below deck aluminum gas tank replacement project and what I learned in the process, as I did a lot of reading.
The most common reason that Al. Fuel tanks leak on you is holes in the aluminum caused by either crevice corrosion or electrolysis corrosion. Installation is critical to eliminate or greatly reduce this from happening and get the longest life out of the tank possible.
In terms of crevice corrosion you want the tank to breath and have air all the way around it except on the points where it sits or at the upper attachment mounts. You do not want raw wood to touch the tank as it holds moisture. You do not want any water up against the tank in a small air free space like under padding against the tank. For the upper spacer mounts we used the plastic that gas fuel tanks are made out of except it is of higher density and much stronger and thicker.
As I understand it, plastic that can be glued is not completely fuel resistant and plastic that is completely fuel resistant can not be glued and must be attached mechanically.
For the contact points on the bottom of the tanks we used strips of hard tough plastic that were glued to the bottom of the tank. Every inch of the plastic was covered in 3M Marine 5200 adhesive so as to leave no space for water to get under it and start crevice corrosion. Once the glue had set up we added an extra bead of 5200 around the edges. This plastic protect the tank from crevice corrosion and also adds spacing to raise the tank a little and help keep it up out of the bilge water, although the boat gets very little water in the forward bilge compartment where the tank sits. The padding also protects the glassed boat structure from wear caused by vibration and pounding and absorbs movement/bulging (full tank/empty tank) etc.
We did not want to use the standard rubber pads under the tank as I read that rubber is 80% carbon like the core of some old school batteries and is cathodic. So in a wet bilge with some salt in the water you have current eating at the aluminum. However, we still wanted to have some cushioning for the large heavy tank and also some additional spacing to lift it up a little further out of any possible water. Eventually we settled on some very tough double sided honeycombed pure silicon padding which we cut out of a whole bunch of round kitchen trivets, which were cut down to fit the hard plastic strips on the bottom of the tank and were glued on with liquid silicon. Because some were of a different colour it ended up looking like a six year old's art project. To further eliminate the possibility of electrolysis corrosion, all fittings at the tank for hoses etc are also made of aluminum so no dissimilar metal issues.
We replaced just about everything in the whole fuel supply system and went top end on everything; hoses, inline bronze cutoff valves, racors, squeeze bulbs and hoses clamps etc and the filler cap assembly had already been replaced. The Plastic, silicon padding and very expensive adhesive alone was about $300. the Filler hose was $130. and the 22 high end hose clamps were around $170. plus the taxes. We double clamped when the fittings were long enough. Almost everything from the filler cap assembly to the motors is now new.
The tank itself was made to Transport Canada specifications by a small shipyard which has built Al tanks for Federal Government boats. The plate aluminum is 50% thicker than the original tank and is otherwise a clone of the original tank, but we did make it a half inch shorter to make sure it would fit under the cover plate with the additional spacing on the bottom and possibly different size fittings on top. The tank was basically installed the same as the original tank although some higher quality components were used.
Except for the building of the tank, friends helped with the work and total cost was about $2500. Sounds like a lot but I was talking to another boat owner at the fuel dock who just took his boat in and had his Al tank replaced and his bill was $5600. and I doubt he replaced the whole supply system. In his case he did not have a screw down cover plate so they had to cut out the glassed deck above the tank and then replace the deck.
I would like to think the new tank will outlive the boat and me. Most importantly the boat is safe, I hate the smell of gasoline in the morning.
The most common reason that Al. Fuel tanks leak on you is holes in the aluminum caused by either crevice corrosion or electrolysis corrosion. Installation is critical to eliminate or greatly reduce this from happening and get the longest life out of the tank possible.
In terms of crevice corrosion you want the tank to breath and have air all the way around it except on the points where it sits or at the upper attachment mounts. You do not want raw wood to touch the tank as it holds moisture. You do not want any water up against the tank in a small air free space like under padding against the tank. For the upper spacer mounts we used the plastic that gas fuel tanks are made out of except it is of higher density and much stronger and thicker.
As I understand it, plastic that can be glued is not completely fuel resistant and plastic that is completely fuel resistant can not be glued and must be attached mechanically.
For the contact points on the bottom of the tanks we used strips of hard tough plastic that were glued to the bottom of the tank. Every inch of the plastic was covered in 3M Marine 5200 adhesive so as to leave no space for water to get under it and start crevice corrosion. Once the glue had set up we added an extra bead of 5200 around the edges. This plastic protect the tank from crevice corrosion and also adds spacing to raise the tank a little and help keep it up out of the bilge water, although the boat gets very little water in the forward bilge compartment where the tank sits. The padding also protects the glassed boat structure from wear caused by vibration and pounding and absorbs movement/bulging (full tank/empty tank) etc.
We did not want to use the standard rubber pads under the tank as I read that rubber is 80% carbon like the core of some old school batteries and is cathodic. So in a wet bilge with some salt in the water you have current eating at the aluminum. However, we still wanted to have some cushioning for the large heavy tank and also some additional spacing to lift it up a little further out of any possible water. Eventually we settled on some very tough double sided honeycombed pure silicon padding which we cut out of a whole bunch of round kitchen trivets, which were cut down to fit the hard plastic strips on the bottom of the tank and were glued on with liquid silicon. Because some were of a different colour it ended up looking like a six year old's art project. To further eliminate the possibility of electrolysis corrosion, all fittings at the tank for hoses etc are also made of aluminum so no dissimilar metal issues.
We replaced just about everything in the whole fuel supply system and went top end on everything; hoses, inline bronze cutoff valves, racors, squeeze bulbs and hoses clamps etc and the filler cap assembly had already been replaced. The Plastic, silicon padding and very expensive adhesive alone was about $300. the Filler hose was $130. and the 22 high end hose clamps were around $170. plus the taxes. We double clamped when the fittings were long enough. Almost everything from the filler cap assembly to the motors is now new.
The tank itself was made to Transport Canada specifications by a small shipyard which has built Al tanks for Federal Government boats. The plate aluminum is 50% thicker than the original tank and is otherwise a clone of the original tank, but we did make it a half inch shorter to make sure it would fit under the cover plate with the additional spacing on the bottom and possibly different size fittings on top. The tank was basically installed the same as the original tank although some higher quality components were used.
Except for the building of the tank, friends helped with the work and total cost was about $2500. Sounds like a lot but I was talking to another boat owner at the fuel dock who just took his boat in and had his Al tank replaced and his bill was $5600. and I doubt he replaced the whole supply system. In his case he did not have a screw down cover plate so they had to cut out the glassed deck above the tank and then replace the deck.
I would like to think the new tank will outlive the boat and me. Most importantly the boat is safe, I hate the smell of gasoline in the morning.
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