Corsica River Marine Surveys

Annapolis, Maryland, USA
Serving Maryland, Delaware, Virginia, New Jersey, North Carolina & Central Florida

Travis L. Palmer, SAMS® AMS®, ABYC
Principal Marine Surveyor

Cell: 410-739-7097

Pre-Purchase Marine Surveys • Marine & Cargo Surveys • Insurance Damage Claims • Vessel Appraisal • Project Management and Oversight • Expert Witness • Damage Surveys • Insurance Survey

Common Corrosion Damage on Mercruisers Bravo 3 Outdrives

Corsica River Marine Surveys - Bravo 3 OutdriveThe Problem

Many of our customers have been hearing corrosion horror stories about the Bravo III The type of corrosion reported is Galvanic Corrosion. Galvanic corrosion (also called electrolysis) can be caused by many things. Loose wires hanging in the bilge, dock wiring with improper grounding, defective appliances and even steel rebar in the seawalls can cause galvanic corrosion. Even having your drive painted with the wrong type of paint can cause accelerated galvanic corrosion damage. Many times it is another boat moored close by that is causing the corrosion. Another item(s) that can cause galvanic activity are Dissimilar Metals. When a stainless prop is in close proximity to an aluminum housing in water, an electrical current is formed between the two items and the weaker metal will usually loose molecules into the water. This is Galvanic Corrosion. Galvanic corrosion can be a very destructive force and it can eventually destroy your outdrive. Something as simple as having a dead battery for a few days can result in additional galvanic corrosion problems. Usually we find it is a combination of the above items that cause drive damage.

What is the Mercathode?

The Mercathode is a special device that produces a protective field around the drive to Reduce galvanic corrosion damage. Most new Mercruiser engines come with the Mercathode installed from the factory. The Mercathode has two basic parts. The controller and the anode. The controller is usually mounted on the engine. It is either blue or black plastic. The controller is about 3 1/3″ wide and 2″ tall. It will have four terminals on the front to attach the power leads and the two anode leads. The anode is mounted outside the boat. The new style anode is mounted under the gimbal where the four hydraulic trim hoses attach. It is black plastic. The anode is connected to the controller by two wires, one orange and one brown. If you keep your boat in the water for any length of time you NEED to have a Mercathode installed. More about this later.

Bravo 3 OutdriveThe Facts

The Bravo III, when properly installed and maintained, can remain corrosion-free. It is up to you, the customer, to make sure your drive remains protected and undamaged. Customer education is important but unfortunately the selling boat dealer fails to properly educate the buyer for whatever reason. In some cases, it is the lack of product knowledge on behalf of the boat manufacturer, rigger or selling dealership. Mercury recommends that in areas where galvanic corrosion can be a problem that the mooring slip be tested and that a second Mercathode Controller be installed to offer additional protection against galvanic corrosion. Surprised? Maybe you should read that sentence again. Yes it’s true. You mean your selling Dealer didn’t mention that to you? Perhaps he should have! This “lack of communication” is the real problem. Mercruiser publications explain in detail how to properly rig your boat to protect it against galvanic corrosion damage. Here is a partial list of such items:

  1. Make sure your boats bonding system is complete and in good condition.
  2. Make sure your engine, gimbal and drives bonding system is complete and in good condition.
  3. Install a second Mercathode Controller unit in parallel to the existing unit.
  4. Maintain your zinc anodes and monitor them on a frequent basis.
  5. Use only Tin based antifouling paint on the drive and gimbal.
  6. Follow factory recommendations for bottom paint.


We will cover the above items one-by-one in more detail.

Bonding System:

The boats bonding system is basically a “Daisy Chain” of wires and cables. It links all major components together for a common ground and allows the zinc anodes and Mercathode to better protect the drive system from galvanic corrosion. The system is actually very detailed and is fully explained in Mercruisers’ Manual Number 11 Corrosion Guide Part Number 90-17431-4. To put it simply, the engine is connected to the inner gimbal, the inner gimbal is connected to the outer gimbal and steering lever, the steering lever is connected to the gimbal ring which is connected to the bellhousing. The bellhousing is bonded to the drive unit and the props are grounded to the propshaft. In addition, the trim cylinders are bonded to the gimbal ring. This Daisy Chain is accomplished through the use of wires, braided cables, bonding tabs and special washers. If any part of this chain is broken the drive will not be fully protected. Mercruiser does a great job of supplying the boat manufacturer with a pre-bonded unit.

It is the duty of the boat manufacturer to make sure that all of the bonding systems are properly hooked up during engine and drive installation. We feel it is also the responsibility of the delivering dealership to make sure the bonding system was properly installed and that the system be explained to the buyer. Servicing Dealers should be aware that elimination of any one of these components could result in accelerated galvanic corrosion damage. In short, one little cable left disconnected after servicing can cause severe damage.

Mercathode Protection

As mentioned above, the Mercathode system is a very important part of the Mercruiser anticorrosion protection system. Without the Mercathode your drive unit will most likely suffer galvanic corrosion damage. Mercruiser has advised boat manufacturers that in possible extreme conditions of galvanic activity that a second Mercathode device be installed. The Mercathode Controller is the device in the engine compartment that sends a special signal voltage to the Mercathode Anode device on the bottom of the gimbal. Installing a second Controller virtually doubles the protective field around the drive. The second unit is simply wired in parallel to the first unit. Make up four 8″ jumper wires (14 to 12 gauge) with good ring terminals on each end.

It is preferable to use the correct color codes which are black, red, brown and orange. Just connect red to red, brown to brown, orange to orange and black to black between the two controllers. It is important to realize that the Mercathode MUST be properly wired directly to the boats battery terminals.It must NOT turn off when you turn off the boats battery switches. Improperly wired Mercathodes may fail to protect the drive. It is also important to note that the Mercathode will kill your batteries unless you have a constant battery charger hooked up or installed. If you are keeping your boat in the water it should be rigged with a good fully regulated battery charging system. If you have no battery charger the Mercathode will drain the battery and then you will have no galvanic protection or automatic bilge pump operation!

If the Mercathode drains the batteries your boat could sink! The Mercathode Anode device on the bottom of the gimbal must remain clean of barnacles and underwater growth. If the Anode is damaged or covered with growth it may not operate and your drive will suffer galvanic corrosion damage.

Sacrificial Anodes (Zincs)

Sacrificial anodes are an extremely important anti-galvanic corrosion component. We know them as “zincs” although most sacrificial anodes no longer contain zinc but are now made of various alloys such as aluminum alloys and magnesium alloys. Sacrificial anodes are designed to absorb the galvanic corrosion current. The sacrificial anodes will dissolve instead of the aluminum drive housings. The more galvanic activity around your boat the faster the anodes will dissolve. Monitor your sacrificial anodes on a daily basis! You can get a good “feel” for the level of galvanic activity around your vessel by how long the sacrificial anodes last. A properly operating Mercathode system will lengthen the sacrificial anodes life. Sacrificial anodes should be replaced after they have dissolved 25%. Remember, it’s YOUR FAULT should your drive be damaged from lack of sacrificial anode protection. Have your local Mercruiser Dealer show you how to maintain your sacrificial anodes. He should also be able to show you the basics of your bonding system and Mercathode operation. If he can’t, find another Dealer who can.

Anti-Fouling Paint

The type of bottom paint and how it is applied is very important. We sometimes find that selling boat dealers have a sales person who pressures the service department to get a boat ready for delivery as soon as possible for the least amount of labor. This can leave your vessel improperly prepared for the marine environment and can actually lead to accelerated destruction. Most boat bottoms are painted with a copper based antifouling paint. Copper antifouling paint should NEVER be used on an outdrive unit. The copper in the paint will eat the aluminum drive housings and can put “fist sized” holes right through them. Before a boat bottom is painted it should be protected with an epoxy barrier coat. The barrier coat prevents water from absorbing through the gelcoat surface. If water gets absorbed through the gelcoat surface a chemical reaction can occur called “blistering”. This blistering of the hull can ruin your boat. Make sure you ask your sales person to properly apply a good three or four coats of barrier coat before applying the bottom antifoluing paint. The gimbal housing must have a 1″ boarder around it which has NO PAINT. This boarder is extremely important in reducing galvanic corrosion damage. If copper type antifouling paint is permitted to come into contact with the gimbal housing a galvanic “stray currant” will be produced and damage will occur. The drive unit must only be painted with a TBT type of antifouling paint. TBT (Tri-butyl-tin) paint is a tin based paint that will not react with the aluminum drive housings. Make sure you get in all the nooks and crannies of the drive and gimbal for complete protection from marine growth. A good three coats is desired. Follow exactly the application method on the can.


The Bravo III is a wonderful drive but if not properly protected can be damaged.

In the end, it is up to the CUSTOMER to educate himself about galvanic corrosion and how to protect his boat. Don’t rely on your dealer to do it for you.

Your engines owners manual should also be reviewed for important information.

Have a surveyor fully explain and inspect your Mercathode and Sacrificial anode protection system. He should also come to your mooring slip and test the level of galvanic activity around your vessel. He has a special meter for this procedure.

Test DVA Meter with special test probe.
Check your boat on a daily basis and adjust your maintenance accordingly.
Keeping your boat in the water is convenient but potentially destructive.
Haul it whenever possible and dry storage is usually worth paying for.