How to Install an Aluminum Fuel Tank
. . . . So you Don't Have to Do It Again
David Pascoe, Marine Surveyor
Many people blame
aluminum as being a bad material for fuel tanks. Actually, it's not. All
materials have their strengths and weaknesses, and the problems with aluminum
tanks are problems of proper installation, not the material itself. Properly
installed, aluminum tanks will usually outlast the life of the boat.
Crevice Corrosion
Here's the answer that many of you asked for on how to install an aluminum
fuel tank so that it doesn't corrode and leak again. Fuel tanks don't corrode
because they get wet; they corrode because at some point something is in
contact with the tank that traps water between it and the tank. Like the
original foam that it was installed with. Aluminum is
self-protecting, so long as the surface gets adequate air
exposure. Severe corrosion is always caused by water plus a lack of oxygen.
Metallurgically, this is known as "crevice corrosion," and the key is to
eliminate all the points (crevices) that trap water. As long as
the aluminum has a good air flow around all surfaces, contact with water will
not damage it.
New Deck
For replacing tanks foamed under the deck, between stringers, or tanks just
sitting on a bare plywood deck, you need to build in a new deck. This can be
done by thoroughly glassing over good quality plywood, being sure to
thoroughly seal the edges of the plywood so it doesn't suck up water. Use mat
if you like, but be sure that it's completely wetted out, and use two layers
for the top surface. it's best to glass the edges in a second operation, after
the two surfaces have been done. Wait until the resin kicks off before doing
the final roll out.
Note: You may need to add frames under the deck so that it
doesn't sag, depending on the amount of the span between stringers. If the
tank is more than two feet wide, we'd recommend this. Install the frames
before glassing. Also, if it looks like the deck is going to collect water in
the center, it's a good idea to drill a few 1/2" drain holes, coating them
with resin or epoxy so they don't rot.
Mounting Deck
The best method for mounting the deck is to fasten heavy, fir ledger
strips to the side of the stringers. Make sure the height of the ledge strips
is tall enough that they are not going to split. it's probably best to cross
bolt through the stringers rather than using screws, if possible. Remember
that this has to withstand the weight of the fuel with the boat slamming, so
you need to make them strong. Then heavily fiberglass or epoxy the ledger
strips (before installing) so they don't rot and set the fully glassed deck on
top. Counter sink your deck attachment screw holes about 1/4" and then fill
the counter sinks with epoxy or 5200 after the screws are set.
Set Tank
As shown in the illustration below, you are going to set the tank on top of
1/4" strips of plastic about 2" wide and spaced about every 12". it's best to
place the strips transversely to the length of the tank. We recommend Haysite
or any of the fiber reinforced plastic sheet. The strips should be cut to a
length exactly 1/4" shorter than the width of the tank. We don't want the
strips sticking out the sides and collecting water.
Next, you will need at least two tubes of 3M 5200 adhesive, and you will
literally glue the plastic strips to the bottom of the tank. Apply the
adhesive so that the entire surface of the plastic is coated with 5200 and
will not leave any gaps or crevices for water to get into. Once the strips are
pressed into position, make sure that the ends are equally 1/8' from the edges
of the tank. Wipe off the excess 5200 that squeezes out, making sure that the
joint between tank and plastic strip is completely sealed. Don't leave any
globs of material.
Allow 24 hours for the 5200 to set up before setting the tank in place.
Next, apply two 1/4" wide beads (like a stream of tooth paste) of 5200 along
the length of each plastic strips that are now glued to the bottom of the
tank. The strips do not get fully bedded because tanks expand and contract as
they are filled and emptied. If the bottom distorts, we want the plastic
strips to loosen from the deck, not the bottom of the tank. If the later
happened, we'd be back to our crevice problems again. So we want just enough
5200 on the bottom of the strips to create some suction to hold the tank in
place.
It will take two people to carefully set the tank straight down into place.
You don't want to have to move or adjust its position once you set it down.
Use wood shims on the inside of the stringers if necessary to guide it into
the right position, shims that will be removed after it's in place. The 5200
is so strong and will create so much suction that you not need any other
method of securement. Don't worry that ballooning of the tank will break the
seal. The weight of the fuel in the tank will pretty much hold the tank in
place.
That's it! Now it won't matter if the tank gets wet because there are no
crevices to trap water and cause crevice corrosion.
Click for bigger graphic.
Points to Consider
There will be situations where this method will have to be modified.
Aluminum tanks are not expensive, so if you need to make modifications to the
original shape, be prepared to do so. Just remember that the objective is to
avoid creating points that create crevices and trap water. Here are few other
points to consider;
- Never allow water absorbent material such as wood to come
in contact with the tank.
- Do not use rubber strips: rubber contains carbon, which
is cathodic to aluminum and will cause galvanic corrosion.
- It is best to use only stainless steel pipe fittings for
the fuel supply. Do not mix a variety of metals. Do not
use steel or galvanized.
- Make sure that whenever the tank is sitting on is a stable
surface; always use the hull stringers and never the bottom
of the hull.
- Make sure that straps or whatever securing devices you
use don't cause crevice corrosion.
- Tank should not be so deep in bilge that it's going to
be in constant contact with bilge water.
First posted on May 23, 1998 at David Pascoe's site:
www.yachtsurvey.com.
Page design changed for this site.
Last reviewed 11/28/98
| 
