Boats with MerCruiser Bravo Three outdrives can avoid galvanic corrosion problems or damage with proper sterndrive installation, maintenance and protection
The MerCruiser Bravo 3 sterndrive is one of the most popular and innovative outdrive designs introduced to the marine market by Mercury.
Bravo 3 outdrives feature counter-rotating dual propellers that improve propulsion efficiency, speed, acceleration and handling on a boat. Two props are mounted in front of one another and turn in opposite directions. This innovative design eliminating wasted energy from the propeller wash and is capable of producing speeds of up to 65 mph for gas engines up to 525 HP.
Unfortunately the popularity of the MerCruiser Bravo 3 sterndrive comes with some controversy due to the high potential for corrosion problems. Mercury Marine and certified Mercury technicians have recommended several preventative measures for maintenance and protection from galvanic corrosion on the Bravo Three.
MerCruiser Bravo 3 Outdrives Prone to Corrosion Problems
The main cause of galvanic corrosion problems (also known as electrolysis) on the Bravo 3 is the dissimilar metals used in the outdrive design.
The Bravo 3 utilizes an aluminum casing and two stainless steel propellers. When the stainless steel props are in close proximity to the aluminum housing in water an electrical current is formed between the two metals. Galvanic corrosion occurs when the weaker metal begins to lose molecules in the water. The corrosion problem will eventually damage and destroy the Bravo 3 outdrive.
According to Mercury Marine, the dual props are stainless steel because there would be excessive cavitations damage with aluminum props as the two props counter-rotate and interact.
Galvanic corrosion (or electrolysis) originates from electrical currents in the water surrounding the boat. Electrical currents in water are most commonly caused by boat power lines in the water, loose wires hanging in a bilge, dock wiring with improper grounding or steel rebar in seawalls.
Installation of Mercathodes for Bravo 3 Corrosion Protection
MerCruiser recommends that boat owners with Bravo Three outdrives install a second Mercathode controller and anode in areas where galvanic corrosion can be a problem.
The Mercathode is a device that produces a protective field around the outdrive in an attempt to reduce galvanic corrosion damage. Most MerCruiser engines come direct from the factory with one Mercathode already installed as part of their anticorrosion protection. For boats that spend time sitting in the water (at dock or on a mooring), a Mercathode is critical in corrosion protection on a Bravo Three outdrive.
Sacrificial Anode Zincs and Anti-fouling Paint
Attaching sacrificial anodes to a boat is the easiest and least expensive way to protect outdrives from galvanic corrosion problems. Known more commonly as boat zincs, almost all sacrificial anodes no longer contain zinc and are made of various alloys such as aluminum and magnesium.
Sacrificial anodes are installed on or around an outdrive and are designed to absorb any galvanic corrosion and dissolve instead of the aluminum drive housing on a Bravo Three. Thus, they are sacrificed for the outdrive corrosion protection. When galvanic activity is high, anode zincs can dissolve quickly. Sacrificial anodes should be monitored closely and replaced as they become more than 25% dissolved.
Proper anti-fouling paint on a Bravo Three outdrive is equally as important. Most boat bottoms are painted with copper based anti-fouling paint, but copper anti-fouling paint should never be used on an outdrive unit. Paint with copper will eat holes right through aluminum drive housings. Be careful to leave a one inch border around the gimbal housing to avoid any copper paint from touching the aluminum drive. Three coats of a TBT (tri-butyl-tin) paint should be used to completely cover and protect the aluminum drive and gimbal of a Bravo Three.
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Maintenance Protection for MerCruiser Bravo 3 Sterndrive
MerCruiser supplies boat manufacturers with a pre-bonded unit that consists of a “daisy chain” of wires and cables that allow zinc anodes and Mercathodes to protect the drive system from corrosion.
MerCruiser technicians and boat owners should be careful to maintain the bonding system with any outdrive repairs.
With proper maintenance and protection, the MerCruiser Bravo Series outdrives, including the Bravo Three sterndrive, can remain free of corrosion problems and damage.
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