Last year 77,150 boaters bought aluminum boats. They’re light, economical, nearly maintenance-free, easy to repair and almost impervious to damage. The Coast Guard and Navy prefer them for small craft, and many commercial boats are aluminum, as are many recreational craft. Their high strength-to-weight ratio means they can be built lighter and therefore can run faster for a given amount of power, and are easier to trailer. If you’re looking for the ideal boatbuilding material, aluminum could be it. The only problem is that it’s the wimp of the electrochemical schoolyard, being beaten up and corroded away by almost every other metal except for zinc and magnesium.
Want to be a saltwater metal head? Aluminum can seemingly dissolve away in salt water when in the presence of other metals. Builders do everything they can to prevent this, but once the boat is in your hands it’s up to you to keep it alive. Here’s how.
How It Happens
We’re talking about galvanic corrosion. Back in science class you’d say that this is where one metal in an electrically conductive solution (such as salt water) gives up atoms when connected to a dissimilar metal in that same solution. Losing atoms means that the metal is falling apart, or corroding. In the slip aboard your boat, you’d say that this is where your aluminum hull becomes pitted because of a bronze through-hull on a neighboring boat.
The rate of corrosion of a metal on its own is determined by how chemically active it becomes when put in salt water. The more active, the more susceptible it is to corrosion. The less active, the more resistant it is to corrosion. When not in contact with anything else, most marine metals such as aluminum, bronze and stainless steel will corrode away at a reasonably slow rate. No danger there. But connect different metals, one active (aluminum) and the other a lot less active (i.e., a copper penny), in water and atoms will start to flow. And the aluminum will start to fall apart.
The Good News
When not in contact with other metals, aluminum can do quite well in both fresh and salt water, needing only bottom paint to prevent fouling. However, to play it safe, the American Boat and Yacht Council (ABYC) recommends that “aluminum vessels shall have a protective paint coating that provides a high [electrical] resistance barrier between the aluminum and the water.”
Above the waterline aluminum does even better. When continuously exposed to oxygen, it develops a film of aluminum oxide so dense and well bonded to the metal that it prevents further corrosion. That’s why many commercial and military craft leave aluminum bare from the waterline up: There’s no need for protective paints, cosmetics aside. As you can see, building an aluminum boat for salt water takes thought. The right alloys must be used, welding must be done just right, and parts must be carefully assembled.
Fiesta, a Florida pontoon builder that bills its boats as “built for use in salt water,” isolates hardware and stainless-steel bolts from the aluminum with nylon washers. Hull compartments have drains with silicone-sealed nylon plugs that can be opened to drain accumulated moisture. They also employ dedicated mounting brackets for sacrificial anodes, and their hulls and extrusions are made from thicker metal.
Better builders don’t allow crevices or joints that collect water, and avoid upturned brackets and channels that trap water. Moisture, including condensation, must drain away, with no sealed or dead air spaces. An emphatic spokesman for Premier Pontoons said, “We are constantly doing pre-emptive detective work to stop corrosion before it starts.”