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Corrosion Burns I/O Owners

Marine engines are complex and powerful, but in their hearts they're just big ol’ machines designed to transform energy contained in fuel into mechanical energy that can make boats go. To make this happen, a finely-balanced system of parts must work flawlessly.

A spate of problems reported to BoatUS recently suggest that certain Mercruiser I/Os are textbook examples of what can go wrong when one component of this balancing act is inherently unstable. Owners and mechanics from around the country say that recent model Bravo III sterndrives are being eaten up by corrosion and, in some cases, must be replaced because corrosion damage allows seawater to contaminate internal lubricated parts.

BoatUS has received 12 such complaints directly from members, but Internet chat rooms and comments from mechanics suggest the problem is widespread. Although the majority of reports we've gotten involve 1998 and 1999 Bravo III units, corrosion problems may date back to 1993, when the Bravo III was introduced. Mercruiser Alpha I, Bravo I and Bravo II outdrives don't seem to be affected nor do 2000 and newer Mercruiser outdrive models.

Consumer dissatisfaction with this problem is compounded by what many owners believe is Mercruiser's hard-line approach. Some who have complained to BoatUS said their sterndrives were first replaced under Mercruiser's three-year corrosion warranty, but when they failed again after the warranty expired, Mercruiser refused to provide further assistance. Replacement sterndrives and the labor to install them can cost upwards of $6,000.

BoatUS forwarded consumer complaint information, including engine model and serial numbers, to Mercruiser president Barry Eller in August but received no response.

Mercruiser's customer service personnel told us they are not aware of any corrosion problems specific to Bravo III outdrives even though late last year and again earlier this year, Mercruiser introduced replacement parts designed to halt corrosion damage caused by galvanic action between the dissimilar metals of the props, the prop shaft and a sleeve-like bearing carrier on the shaft. While the props and shaft are made of stainless steel, the bearing carrier is made of aluminum.

The Bravo III duo-prop system consists of two counter-rotating propellers on a single drive shaft. This feature sets Bravo IIIs apart from Alpha I, Bravo I and Bravo II sterndrives, all of which have single propellers. It also seems to be the source of the Bravo III problems.

BoatUS has learned that Merc's design originally did not allow enough room to install enough anodes - common examples are sacrificial zincs - to prevent corrosion from occurring at bearing carriers and seals near the propellers. The carrier is a sleeve that encloses bearings and seals intended to prevent water from entering the sterndrive housing at the drive shaft. Drive gears deteriorate quickly if lubricants are contaminated by water.

Even so, Mercruiser told one 1999 Bravo III owner in Port Richey, FL, "It has been determined that your [corrosion] failures were not associated with a product defect or any defect in materials or workmanship. The cause of the failure was determined to be lack of lubrication."

An owner in New York wrote, "My first drive lasted three seasons from May to October in fresh water before the lower case was corroded so severely that I had to replace the whole lower unit, a quick drop of $4300."

"Now after one season a dime size hole has developed in the skeg and Mercruiser wants to 'cut that part of the skeg off'. I find that totally unacceptable," he said, adding, "The bearing carrier is also corroded to the point it needs to be replaced, which they have agreed to do. On replacement drives you only get a 12 month warranty so this is my only chance for a resolution, after this I'm on my own."

Galvanic corrosion can occur when dissimilar metals, in this case aluminum and stainless steel, are immersed in sea water. Because the metals have different molecular properties, an electric current is set up between them. In the process, the less stable material, in this case aluminum, loses electrically charged ions to the more stable element, stainless steel. Blistering paint on metal components may be the first sign of galvanic corrosion, followed by a white powdery build-up on the aluminum surface and finally by deep pitting that leaves the aluminum weak and porous.

According to a marine corrosion expert and metallurgist now retired from Mercury Marine, there's no anode on the bearing carrier on pre-2000 Bravo III models "because there's no room." The drive shaft areas on Alpha and all Bravo sterndrives are identical but the twin propellers on Bravo IIIs take up more space. Dale Pluhar, now an independent marine consultant, explained that Mercruiser’s response was to counteract corrosion by coating the aluminum carrier with chromate material to prevent corrosion. Merc also issued a new prop shaft bearing that creates a better watertight seal and they added a second anode to the Bravo III's cavitation plate. These new parts first became available in December 2000 and in the spring of 2001.

Even though Mercruiser denies an overall problem with Bravo III outdrives, the engine maker nevertheless recommends that its customers use corrosion protection devices like Mercathodes or galvanic isolators, especially on boats equipped with stainless steel propellers. Mercathodes block the flow of destructive galvanic currents in boats' 12-volt electrical systems. Galvanic isolators block low voltage current through boats' shore power connections.

Experienced boat owners know that corrosion is a fact of life wherever metal parts come in contact with water. Preventive maintenance includes using anodes, replacing sacrificial zincs, maintaining adequate lubrication and, wherever possible, upgrading metal components to ones made of stainless steel, bronze or other stable materials. But, even the most conscientious boater will lose the battle with corrosion if the source is a design flaw.

Owners of these drives who have experienced corrosion problems should contact the BoatUS Consumer Protection Bureau (703-461-2856 or by e-mail at Be sure to have engine and outdrive model and serial numbers available when you call. BoatUS will continue working with the sterndrive maker to see if we can forge a workable arrangement for service on Bravo III outdrives.

(c) Copyright BoatUS Magazine, November 2001

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