Stern Drives
The inspection of a stern drive unit can be very difficult, or impossible if the vessel is in the
water. If the vessel is not hauled for a complete bottom inspection which would include the stern drive,
a lot would be left to chance. Often, during the sea trial the stern drive can perform very well and still
be on its last leg. A stern drive can perform reasonably well right up to the point where it is about to fall
apart. Number one, the outside casing is made from aluminum and the inside parts, gears, shafts etc.
are made from various other non corrosive metals. Electrolysis plays a very important role in the
condition of a stern drive. A stern drive can look very good from the outside, but the inside may be an
entirely different story. Usually the condition of the outside casing can give you a pretty good idea of
the condition of the inside. Most stern drives that have been poorly maintained will show excessive
corrosion. The casing literally turns to dust over time. Typically a drive will be equipped with an array of
zinc anodes which should be monitored frequently during the season, as they tend to disintegrate at
various rates.
There are also several grounding wires within or attached to the drive that must be routinely
checked. If the ground wires fail, electrolytic action is enhanced.
I had the good fortune to inspect a vessel with twin stern drives a while back. One drive looked
fairly new and the other was badly corroded. The owner said that he recently had the nice looking one
rebuilt and a new casing installed. I noticed that he surface was not quite as smooth as it should have
been if it was, in fact, a new casing, so I scraped a little paint off in a couple of the rough areas. Not to
my surprise, I discovered areas of epoxy filler. Upon closer inspection I found that the entire lower unit
had been re-finished, filled with epoxy and sanded reasonably smooth, and painted. The lower unit
had not been rebuilt mechanically, only cosmetically. It turns out that the owner did the job himself to
hide the massive corrosion problem and was about to do the second drive when his time came for
spring launch. This is another example of the seller of a vessel misleading the buyer.