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Many people blame aluminum
as being a bad material for fuel tanks. Actually, its 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.
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. Metalurgically,
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.
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 its completely wetted out, and use two layers for
the top surface. Its 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,
its a good idea to drill a few 1/2" drain holes, coating them
with resin or epoxy so they don't rot.
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. Its 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.
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". Its best to place the strips
transversly 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 its 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.
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 wherever 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 its going to be in
constant contact with bilge water.
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