Stern drive boats are much more prone to corrosion than outboard boats and for this reason alone, it is important to pay much closer attention to the stern drive with regular care, inspections and maintenance. Most corrosion damage occurs to stern drives because of a lack of maintenance, or a lack of understanding what maintenance to undertake. To understand why stern drives are prone to corrosion we need to understand galvanism.
WHAT IS GALVANISM?
Aluminium is ideal for marine use except for one major problem, it is anodic to most other metals, and this is where the problems start to occur. Iron, steel and stainless steel, when brought into contact with aluminium, create electricity which then results in corrosion. In other words, if one metal is anodic and has high potential and another is cathodic with low potential, the electrical flow is always from the anode to the cathode. This movement is sometimes called electrolysis but the correct term is galvanism, which is named after the scientist John Galvin. Simply, if two dissimilar metals are combined, one becomes the cathode and one the anode.
If we were able to view the electricity flow from the anode to the cathode, it could be seen that small minute particles of metal were being moved. This brings us to aluminium stern drives that are basically large anodes. To protect the aluminium, zincs are used which have an even higher electrical potential than aluminium. This then means that the zinc is used to protect the aluminium. Stern drive are difficult to keep corrosion-free because they contain other metals, besides the aluminium, such as stainless steel, steel and iron, which compound the problem.
One of the best ways to maintain the integrity of the stern drive is to ensure that the paint coating remains intact. This is easier said than done as stern drives (and outboards) are prone to paint chipping from hitting objects in the water, as well as degradation through marine growth such as barnacles which attach themselves to the drive. Anyone who has tried to remove barnacles knows that they attach themselves with more vigor than the strongest double-concentrated super glue. The barnacles and other marine organisms that attach themselves to the drive affect the integrity of the paint which then promotes the onset of corrosion. If the boat owner is not aware of this corrosion setting in, the drive can incur a lot of corrosion damage within a few weeks.
Galvanism can occur when either of the zincs are no longer effectively making the drive become anodic to the other metals found in the drive or, more often, damage can be the result of stray current. The stray current can come from incorrectly installed electrical equipment, lack of electrical system maintenance and wiring problems. A common problem found on boats is the incorrect installation of a new bilge pump where the wires connected to the pump come into contact with the bilge water. This then puts 12 volts straight into the bilge water and will dramatically increase corrosion. Often, when paint is bubbling up and blistering, there is stray current that is affecting that particular area.
STERN DRIVE PROTECTION
Most stern drives don’t come with antifouling, so it is up to you the owner to do this. Slapping a coat of paint on the stern drive will do little to prevent corrosion unless the surface is adequately prepared. The drive needs to be lightly sanded and because of the irregular shape of the drive, it is a labour intensive project. Any dirt should be thoroughly cleaned from the drive before the antifouling paint is applied.
POST PAINTING CARE
Try and minimise the amount of exposure to the air the drive gets as this will prolong the anti-fouling properties in the paint. With a little TLC, your stern drive will give many years of corrosion-free service.