Excessive rot on Yamaha 300 lower unit

We own a 2021 R27OB. We have noticed a lot of rot on our lower unit. We boat in freshwater and currently have 200 hours runtime.

By “rot” I’m assuming you mean corrosion to the lower unit on your outboard.
Fresh water boats will get corrosion on the drives if the metal gets exposed to electrical current in the water. The current can be from the boat itself if it's system is not properly grounded or if it's mercathode system is not working properly. The current could also come from other boats or any electical systems at a marina where the excess current goes into the water. If the boat is kept down stream of an electrical source the corrosion would be worse than if it was upstream as the water current will actually carry the electrical current with it. Basically, any drive that has the outer layer of protective paint damaged to the point the underlying metal is exposed, can and probably will show some signs of corrosion. The corrosion is the metal composition chemically changing due to the contact with the electrical current.
If it is not bad, you can sand it down and prime it prior to doing a repaint. Powder coating is also an option after the corrosion is cleaned up. This does not guarantee the corrosion will not come back, but it will definitely slow the process.

Lplooard":69srs4rt said:

We own a 2021 R27OB. We have noticed a lot of rot on our lower unit. We boat in freshwater and currently have 200 hours runtime.

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Forgive my ignorance but doesn't the unit tilt up out of the water when not in use?

It is always out of the water when not in use as we trailer it. That’s why it’s confusing. We have another boat that’s 12 years old with twin Yamaha 150s on it. We bought it in Florida and have spent a winter in the Bahamas on it. Not corrosion on those. Very concerning as this engine is only 1 year old.

If the motor is 1 year old, then it's certainly covered under warranty. I'd start by contacting Yamaha, I've had engines berthed in salt water marinas over 10 years without corrosion issues.

That’s fine to say but I believe Yamaha will blame it on the boat. My gut says there a ground issue on the boat. Not sure where to even start looking. We have one of the first lithium battery boats. Not sure if it’s related to that.

If you do not already have a galvanic isolator, I strongly recommend that you install one, in case your lower unit damage is a result of through-water galvanically-driven electrical current. This happened on my 2016 Cutwater 24 docked in salt water. Installing a galvanic isolator solved the problem.

Take the boat to a Ranger dealer that knows how to troubleshoot electrical issues. If that is not possible contact a reputable ABYC certified electrician to troubleshoot the issue. A boat that is normally stored on a trailer that is one year old and has that type of electrolysis damage has something wired incorrectly. The isolator will protect you from Dc voltages that pass on the shore power ground. From your description of use your boat is not sitting in a marina all the time that could be the culprit. I would start ant the lower unit grounding and work to the boat. Something is wrong!

I am not sure if this is relevant or not, but we have noticed we have 1 aluminum annode on the engine mount which showed signs of use. We have a zinc annode on the lower unit. Not sure why they are different materials. The zinc annode looks brand new. Not doing anything.

Lplooard":371ysloy said:

I am not sure if this is relevant or not, but we have noticed we have 1 aluminum annode on the engine mount which showed signs of use. We have a zinc annode on the lower unit. Not sure why they are different materials. The zinc annode looks brand new. Not doing anything.

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Zinc anodes provide no protection in fresh water. They are for salt water only.

If you boat in fresh water only, magnesium anodes are what you want to use. They provide no protection in salt water. They are fresh water only.

If you boat in both fresh and salt water as I do, use aluminum.

These folks are the best place for anodes:

http://www.boatzincs.com

It is not a good idea to have mixed anodes. This is counter productive. It is also not a good idea in my opinion to use zinc anodes to protect aluminum alloy lower units and transom brackets.Aluminum anodes work slightly better than zinc because the alloy is working at a lower voltage. This is important when protecting aluminum hulls or outboards and sterndrives. The extra driving potential provides much better protection. That’s the reason all the major outboard and outdrive manufacturers have switched to aluminum anodes as standard. The other advantage is the aluminum works in all types of water fresh, brackish and salt. the aluminum will not oxide when introduced to air. A zinc anode will be of no use if it is submerged in water then taken out of water. the oxide coating that forms does not allow it to work. Why would anyone put Zinc anodes on a trailerable boat??? Then try to use them with an outboard! My opinion is they should not. I used the Aluminum anodes on my inboard and found the protection and longevity to be good. I cruised from fresh water to salt water then loaded the boat on the trailer, relaunched in salt water and used the boat in south Florida for 3 months. The anodes pictured below were used for 12 months in mixed waters.
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I think you may have more going on with your boat but a start would be to put fresh Aluminum anodes on to protect the LU. I would have the electrical system inspected by a ABYC certified Electrician that specializes in Bonding.