PS Advisor January 2006 Issue

PS Advisor: 01/06

A victim of stray current corrosion, this saildrive-type unit is well beyond repair.
Saildrive Corrosion
I purchased a 1983 Sweden 41 in September, 1999. Since that time I have had to purchase three saildrives because of corrosion. I have installed galvanic isolation, zincs, and an expensive electric galvanic isolator. Iíve now been told by the boatyard, that my third saildrive is corroded beyond repair. These run about $5,000, so the thought of putting in a fourth to have it corrode away is not an option. What can be done? What is the history of saildrives in salt water? What can be done to protect them from corrosion? Why are they built of aluminum? Any suggestions or assistance will be enormously appreciated.

Barbara Nylund
Corte Madera, CA

Saildrives (a generic term that refers to sailboat outdrives from various makers), along with thousands of inboard-outboard sterndrives and other sail-oriented propulsion units are made of aluminum primarily because itís well-suited for manufacturing and keeps the units affordable. Aluminum is one of the least noble metals, which means that it is anodic to, or it will corrode when it remains in contact with, nearly every other metal while itís immersed in an electrolyte, seawater in this case. This is known as dissimilar metal or galvanic corrosion.

The primary advantages of these units are a reduction in vibration (quieter running), fixed alignment and less underwater drag (thus their appeal to racers), However, because of corrosion problems and maintenance issues associated with these units, we much prefer inboard propulsion for cruising.

The engineers at Volvo and other manufacturers spend a great deal of time studying ways of thwarting corrosion in these units. The primary protection against galvanic corrosion is the use of zinc anodes. Other types of corrosion involve stray DC current, or stray current corrosion. Where galvanic corrosion may take months or years to cause damage, stray current corrosion can damage underwater metals in days or even hours. If you eliminate these problems and keep up with maintenance (including religious zinc renewal), you should get about 10 years of service from these units, though weíve seen some powerboat stern drives last as long as 18 years. You should note that neither your zincs, nor your galvanic isolator will provide adequate protection against a bad case of stray current.

We suspect stray current in your case (or perhaps an incompatible copper-based paint on the unit that would cause galvanic corrosion), but itís difficult to know without an on site analysis. An experienced electrician or corrosion technician who is certified by the American Boat & Yacht Council should carefully inspect your vessel for electrical faults. Next, the technician should determine the level of anodic protection afforded your saildrive by performing a reference electrode test using a silver-chloride reference electrode. With the results of these tests, it will then be possible to determine why your saildrives have wasted away so rapidly and frequently and you can decide what will be your next step.

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