Surveyor David Pascoe didn't recommend sterndrives for salt water, and his maintenance article explains why: the drive leg is a large piece of cast aluminium living in seawater next to steel and stainless parts. He saw boatyards full of boats with drives removed because of corrosion, and found most of it came down to owners who didn't know how to look after them.
Cast aluminium and its coating
Aluminium is anodic to most metals it's joined to, including iron, steel and stainless steel. Plate aluminium used for hulls contains alloys that protect it; the cast aluminium of a sterndrive can't be made that way. The only protection is the coating. Once the paint is breached and water reaches the metal, corrosion starts. Pascoe says impacts do it, but most often it's barnacles, and the scraping to remove them. He shows a cavitation plate bumped by a forklift with the paint scraped off.
Where to look. The whole drive out of the water: chips, scrapes, bare metal on edges and the cavitation plate, and corrosion starting under the paint.
Anodes
Zinc anodes sacrifice themselves to protect the aluminium, and the drive is "basically" a very large anode itself, so keeping the zincs working matters. Pascoe shows drives coated white with the oxide of rapidly wasting zincs. When the zincs stop working, the drive becomes anodic to the stainless and iron parts.
Where to look. Every anode on the drive and transom assembly for wastage and white oxide; whether they're the right material for the water the boat lives in.
Stray current and galvanic corrosion
Pascoe describes the signs: bubbling and blistering paint, pronounced white spots, and corrosion along angular edges where paint has chipped. He traces it most often to badly installed electrical equipment, poor maintenance of the electrical system, or wiring faults from the builder. Galvanic corrosion shows the same way over years; stray current can cause serious damage in weeks or months. His article also covers 125 volt AC shore power as a source.
Where to look. The paint for blistering and white spots, then the boat's electrical system: shore power polarity, a galvanic isolator, and any equipment added since the build.
Antifouling done badly
Pascoe says neither owners nor yards usually know how to coat a drive properly: it has to be sanded and prepared, and paint slopped on without preparation comes off at the next haul-out and leaves a bigger problem. Ask what's on the drive and who applied it.
Recalls
The USCG recall list includes Mercury Marine campaigns for MerCruiser Alpha and Bravo sterndrives and for Mercury sterndrives. The public pages don't give the defect; check the drive's serial number with a dealer.
What to check on a sterndrive
| Area | What to look for | Source of the concern |
|---|---|---|
| Drive coating | Chips, bare metal, corrosion starting under paint | Pascoe |
| Anodes | Wastage, white oxide | Pascoe |
| Paint blistering and white spots | Stray current or galvanic corrosion | Pascoe |
| Boat electrical system | Shore power faults, added equipment, galvanic isolator | Pascoe |
| Antifouling on the drive | Peeling, poor preparation | Pascoe |
| Recalls | Open campaigns for the drive serial number | USCG recall list |
Writing it up
"Sterndrive leg: paint blistered with white corrosion spots along the lower unit edges and cavitation plate, anodes over 50% wasted. Pattern consistent with stray current or galvanic corrosion. Recommend the anodes are renewed, the boat's electrical system is tested for leakage and shore power faults, and the drive is assessed and recoated." Record the drive model and serial number.