Corrosion Burns I/O Owners
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
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
Mercruiser told one 1999 Bravo III owner in Port Richey,
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."
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.
to a marine corrosion expert and metallurgist now retired
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
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
Owners of these drives who have experienced corrosion problems
should contact the BoatUS Consumer Protection Bureau (703-461-2856
or by e-mail at ConsumerProtection@BoatUS.com). 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.
BoatUS Magazine, November 2001