I’m often asked for tips on repairing aluminum boats. These boats are usually in the twelve to twenty foot range, consisting of jon boats, aluminum skiffs, fishing boats and canoes. Unfortunately, most are looking for a magic answer.
Aluminum boats have an up-and-down history. At the end of World War II large amount of surplus aluminum and aluminum forming equipment became available as war surplus. Several manufacturers started making various designs of boats from this surplus material.
Huge hydraulic stretchers used to form curved aircraft skins were now churning out parts for boats. Many of these boats looked great and performed well. There was an underlying problem, though. Much of the aluminum sheet and rivets were optimized for aircraft use not marine use. Many of these craft, especially when used in salt water, suffered severe corrosion. This provided aluminum boats a bad reputation for years.
One manufacturer in particular avoided this problem, Grumman Aircraft. They had a tremendous amount of experience in providing flying boats and other naval aircraft used in saltwater environments. Their solution was to use aluminum alloys developed for use in saltwater environments. Grumman Canoes are still being made today, although by a different company, and parts are readily available.
As companies developed an understanding of the proper alloys to use, aluminum boats became more dependable and desirable. Aluminum, of the proper alloy, forms a tight oxide coating that protects the aluminum beneath from further corrosion. Aluminum is also tough, denting and deforming where wood would crack and fiberglass would shatter. Look around any marina and see how much abuse the average aluminum workboat handles.
Today, a wide range of aluminum boats is available. Canoes, flat bottomed and v-bottomed fishing boats as well as small powerboats can be found at almost any marine dealer. Aluminum is also being used in the construction of larger power and sailboats. Aluminum is a good choice for one-off boat building, as no mold is needed.
But I’m here to discuss repairing the more common smaller boats, not the mega-yachts. In general, here are three type of damage requiring repairs: punctures or holes, gashes and cracks. Notice I didn’t mention dents. I would leave a dent well enough alone, as long as it doesn’t leak. Denting stretches the metal and also work hardens it. Trying to pound the dent out only stretches the metal further in a different way. Don’t try to fill it with Bondo or epoxy; it won’t last very long before it falls off.
Punctures or Holes
I’ll also place loose rivets in this category and cover them first. It’s not uncommon for rivets to loosen up over the life of a riveted aluminum boat. When they get loose, they also leak. I’ve seen all kinds of suggestions for using silicone sealer, 3M5200 or JB Weld to seal the loose rivet. They may work for a while but eventually fail.
You can also try to tighten up the rivet by hammering while using a metal bar to back up the rivet. However, the rivet is already work-hardened from the first time it was bucked and you won’t have much success with this process. Also, since you don’t have the proper bucking bar or rivet gun you will probably do more damage to the surrounding aluminum.
The most permanent way to fix a loose rivet is to drill it out and treat it the same as a hole or puncture.
The strongest way to repair a hole or puncture is to use truss or round headed stainless steel machine screws from the outside and stainless steel washers and nuts on the inside. This is especially effective when replacing a rivet that is attaching an internal part to the hull skin. You could replace all the rivets in a boat and have it stronger than before.
However, you will need to deal with the fact that the aluminum hull will corrode around the stainless steel fasteners. To cure this problem, place a thin nylon washer under the screw and the nut. The washers are available in the specialty hardware section of most home improvement stores. Drill the hole slightly oversize and use a little 3M5200 when installing.
If the hole doesn’t involve any structural component, you may be able to seal it with an aluminum pop rivet. 3/16” diameter closed end pop rivets are available in the hardware section of most boating stores. Be sure you get the closed end type, as a standard aluminum pop rivet has a mild steel mandrel in the center and an open end. These mandrels will rust and the rivet will leak. Use a little 3M5200 sealant when placing the rivet for a better seal.
A puncture will usually have the edges of the hole curled slightly inward. Drill the hole out larger so as to eliminate this raised edge and provide a good seal. De-burr and chamfer the edges of the hole. You can then use the stainless steel fasteners and nylon washers or pop rivets as noted above.
Holes can also be caused by localized corrosion. This can be caused by an imperfection in the metal, by leaving a washer or coin laying on the aluminum or even a spill of battery acid. You’ll need to drill out the hole until solid metal is reached. If the hole is too big to use the fastener technique outlined above you will then have to treat the hole like a gash, which we’ll talk about now.
Hit a rock when beaching your boat, hit the corner of your trailer or run over the fluke of your anchor and you may gash the hull of your aluminum boat. Aluminum boats are very strong so it doesn’t happen often, but it does happen. I’ve heard of all manner of methods for making quick and dirty repairs to gashes. Fiberglas and epoxy, Bondo, JB Weld and plain 3M5200 have all been tried and eventually failed.
The surest way to repair a gash is to rivet or bolt a patch over the gash from the outside. The patch has to be made from aluminum sheet about the same thickness as the hull or a little thicker. Use stainless steel fasteners and nylons washers to hold the patch in place. Use more smaller fasteners rather than fewer large fasteners. Clean the surface of the hull and coat it with a thin layer of 3M5200 before bolting the patch in place.
If the aluminum flexes enough it may crack. If you have access to a welder who completely understands welding aluminum boats, you may be successful in having the crack welded (after making sure there is nothing on the other side of the weld location to catch fire). It takes the right technique, the right welding rod and the right equipment to properly repair such a crack.
Simply welding the crack may not solve the problem, though. If the hull or structural member is flexing or working, the aluminum may crack again, this time on either or both sides of the weld. You might be better off bolting on a patch and adding an additional plate from the inside as a backup.
Welding is not recommended for repairing loose rivets. A rivet is used for structural attachments and welding up the rivet will probably not produce a good structural joint. Better to bolt a loose rivet.
Don’t take any of this as an indictment against aluminum boats. Properly designed and built aluminum boats withstand an incredible amount of abuse and neglect. Like all boats, though, the occasional need a little repair and TLC.