BoatUS has received few reports about corrosion damage on single-prop MerCruiser outdrive models and none involving Volvo outdrives. Volvo Penta is the only other manufacturer producing outdrives with two propellers. Most of the Bravo III complaints in our files involve 1998, 1999, 2000 and 2001 models.

If there is a formula for success in dealing with MerCruiser it is to follow their instructions to first have an authorized mechanic inspect the boat at its normal mooring place. This will help the mechanic to determine if stray current from the dock wiring system or from neighboring boats is the source of corrosion.

According to MerCruiser’s definition, “Stray current corrosion is caused by leakage of current into and through the metal drive components, exiting for a water path to ground. Stray current corrosion is commonly a result of connecting a boat to shore power.”

On the other hand, galvanic corrosion, which is a reaction between two dissimilar metals, seems to be at the crux of the MerCruiser Bravo III problem. Bravo III’s design features two large stainless steel props, an aluminum housing and aluminum drive components.

MerCruiser’s bulletin explains, “the simplest example of galvanic corrosion, and the most applicable, is an aluminum lower unit with a stainless steel propeller.” Just like with the Bravo IIIs.

Owners just discovering crumbling skegs and drive housings are keeping their fingers crossed that damages will be covered by MerCruiser’s three-year corrosion warranty. They have good reason. Others who are not so lucky report spending well over $5,000 for replacement outdrives. MerCruiser will not help if damages are caused by outside sources and only on rare occasions will they help if damages occur after the warranty expires.

The engine maker has a few recommendations for lessening the chance that corrosion will happen. Using the right type of sacrificial anode is crucial. Merc produces aluminum anodes for use on boats kept in salt water and magnesium ones for use in fresh water. Trouble is, not many boat owners and few MerCruiser mechanics knew about these special anodes until a few months ago. Many boat owners learned the hard way that zinc, the traditional anode material, doesn’t afford adequate protection for Bravo III outdrives in any environment.

MerCruiser also recommends using its Mercathode system to suppress galvanic current between aluminum and stainless steel components. However, whereas once only a single Mercathode per engine was needed, now the engine maker recommends installing two.

Does the company have plans for new components or design modifications to prevent galvanic corrosion altogether? MerCruiser isn’t saying.

Are owners getting any help when they bring their corroded outdrives in for repair? Yes, to a limited extent. The logjam seems to be breaking, but mostly on units still under warranty.

Does BoatUS want to hear from Bravo III owners who either are experiencing problems or who have been helped by MerCruiser? Definitely! Contact the Consumer Protection Bureau at 703-461-2856 or by e-mail, ConsumerProtection@BoatUS.com.

(c) Copyright BoatUS Magazine, September 2002

**This article is a follow-up to Corrosion Burns I/O Owners and Owners’ Patience Erodes as I/Os Corrode**