Bedding Aluminum Fuel Tanks in my Fiberglass Tomcat

J

John S

New member
Hello all,
I am replacing the leaking fuel tanks (58 Gal each) on my 20 year old C-Dory Tomcat.
There is a tank in each "pontoon". I don't want them to set directly on the fiberglass bottom of the boat.
The manufacturer originally had them setting on large goops of 5200.
I had the tanks out 8 years ago for other work and reset them on strips of rubber, the rubber completely deteriorated when the fuel leaked out of the tank.
The contact area is not flat and it is uneven, I need to shim up the inner side of each tank 3/4" to level the tanks.
The contact area is very minimal with bilge drainage running under the tanks bow to stern.
The tanks are 42" in length, 30" Wide and 12" high
The tank manufacturer (who does not install tanks) suggested uhmw adhered to the bottom of the tank.
I'm looking for ANY good suggestions, advice or practices on tank installation.
I don't want to have to do this over again!!
Thanks, John
The wood in the photo shows the contact point for the tank edges.
The second photo shows the rubber strips I used previously
hull.jpg
strips.jpg
 
Any carbon containing compound will react with the aluminum tank--and eventually fail. You found that out the hard way. Silicon rubbers are OK--no C.

The problem with UHMW (Ultra-high-molecular-weight polyethylene) is that it is very slippery, even more so than HDPE (or Starboard) BONDiT is about the only adhesive which will work with it, and I am not sure about how good a bond you will get with the boat hull/metal. Ideally you want to set the tanks on a platform, so water will go under, and then use straps to hold them in place. PVC lumber would be a reasonable candidate--you can contour the PVC to the bottom of the inside of the hull, and tab it in place with epoxy. Then the top surface conjured to the tank. Be sure there are "limber holes" to allow water to move well from under the tanks. You can made a cardboard pattern, and then make several 1 x XX pieces of the PVC, glue it up. You could also use 5200 to adhere the tank to the lumber. A caveat is that the PVC lumber can react with gasoline and become soft, so you want to be sure there are no leaks. Alternately you can cover (wrap) the PVC with fiberglass, or use wood, saturated with epoxy and covered with fiberglass. Some have used fiberglass battens to support fuel tanks.

The new tanks should be cleaned, then properly primed and epoxy coated to keep the aluminum from oxidation. Properly done, the tanks should be good for a lifetime. A caveat is that the PVC lumber can react with gasoline and become soft, so you want to be sure there are no leaks. Alternately you can cover (wrap) the PVC with fiberglass, or use wood, saturated with epoxy and covered with fiberglass. Some have used fiberglass battens to support fuel tanks.

An excellent article on boat gas fuel tanks: . by David Pascoe.
No matter what you decide to do, study this article.

Never foam in an aluminum tank! If you make a platform, you may want to use different dimensions than the original tank--get more fuel, make it easier to secure in the boat--put in some "tabs" to screw the tank down on the platform.
 
I would glue HDPE/UHMW strips to the bottom of the tank & keep the tank as high up out of the water/bilge as possible, or use plastic tanks to avoid the electrolysis your having (Alu. turning to white powder) Tanks made of 5086 alu. are less prone to that also.
Have a good one!! :thup
 
Thanks Bob and TSTURM for responding
Bob, thanks so much for the detailed response. I also read the article and it was valuable. Glad to see you’re still on here offering up quality advice. I’ve been gone for a while.
I have 0 clearance to work with vertically getting this tank in and out of this compartment. I literally have to pry the filler neck past the structure above.
On the positive side there is a fiberglass bracket glassed to the forward bulkhead to hold the tank in place. Large splotches of 5200 on the rear of the tank adhere to the rearward bulkhead to deter movement and remained very effective (and troublesome to remove) both times I’ve had to remove the tank.

I think I will go with the plastic strips both you and TSTURM spoke of. I looked for the sealant you mentioned “Bondit” I’m not seeing it readily available in Alaska and Amazon is out of it. I will run the strips the full length of the tank on both the inside edge and outside edge near the contact point with the hull. I had already ordered some UHMW, but will NOT use it now.
I need adhesion so this tank doesn’t end up directly on the hull. I can order HDPE if you think I have a better chance of adhesion with it ?.?
PVC lumber is available at Home Depot, I’m pretty sure, hate to hear that it is fuel dissolvable. I didn’t see the plastic mentioned in the article being widely available.
Any other thoughts?
Thanks John
 
John S":3m09jc4n said:
Thanks Bob and TSTURM for responding
Bob, thanks so much for the detailed response. I also read the article and it was valuable. Glad to see you’re still on here offering up quality advice. I’ve been gone for a while.
I have 0 clearance to work with vertically getting this tank in and out of this compartment. I literally have to pry the filler neck past the structure above.
On the positive side there is a fiberglass bracket glassed to the forward bulkhead to hold the tank in place. Large splotches of 5200 on the rear of the tank adhere to the rearward bulkhead to deter movement and remained very effective (and troublesome to remove) both times I’ve had to remove the tank.

I think I will go with the plastic strips both you and TSTURM spoke of. I looked for the sealant you mentioned “Bondit” I’m not seeing it readily available in Alaska and Amazon is out of it. I will run the strips the full length of the tank on both the inside edge and outside edge near the contact point with the hull. I had already ordered some UHMW, but will NOT use it now.
I need adhesion so this tank doesn’t end up directly on the hull. I can order HDPE if you think I have a better chance of adhesion with it ?.?
PVC lumber is available at Home Depot, I’m pretty sure, hate to hear that it is fuel dissolvable. I didn’t see the plastic mentioned in the article being widely available.
Any other thoughts?
Thanks John
Click to expand...

Kachemak gear shed Homer. A belt sander & 36grit belt, clean it rough it up & glue it :thup
 
Got the 2nd tank pulled out that wasn’t leaking, but is still getting replaced. My previous bedding job for that tank still looked great. Neoprene strips were all intact. The neoprene is forgiving with my slightly uneven bottom area where the tanks set. Neoprene sheet with a high durometer rating is widely specified, sold for this purpose and available locally, so it’s what I’m going back with.
Thanks again for the input guys.
 
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