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Salty Sea Dog

Anode on Rudder

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Hi All

I have an aluminium rudder (stock) - which is showing some very small signs of corrosion. The yacht doesn't have a common ground between skin fitting (or rudder) - as it has been manufactured in the 'new way' of isolating everything.

I want to stick a hanging anode over the transom when I am in a marina - and attached to top of rudder stock (only) - to provide some protection to just the rudder stock. The rudder is isolated from everyone on the yacht at the moment.

Anyone got any advice on how to use a multi meter, between the anode and the rudder, to test if it is giving enough protection or if it is over protecting (as I have read you can over protect aluminium).

Would I set the multi meter to V and see what the difference is showing - compared to the 2 metals as listed in the galvanic scale ?

Any suggestion welcomed.



4 Replies

  • What material is the hull?

    What material is the rudder itself?

  • Hi, I am an experienced commercial diver and have a couple of questions.
    1. Is your boat sitting in Fresh, Salt or Brackish water?
    2. What type of boat to you have?
    3. Boat Propulsion - Inboard / Outboard, straight shaft, V-drive, etc..
    4. when was the Rudder Zinc installed, how many months in the water?
    5. Does your shaft or I/O drive/s have zincs?
    6. Is your boat harbored in a marina or at anchor?
    7. Hull material - steel, aluminum, fiberglass, cement, other?
    8. How often do you use your boat. Weekly, monthly, water condo?

    This should give some ideas on how to help your situation.

    Zincs and anodes are designed to "corrode". They are Nobel metals designed to protect your boat. There are three reasons and many factors that can cause the corrosion process to occur. The reasons are:
    a. Internal to the boat. Meaning your grounding or wiring is bad. This will show itself after I clean the prop and the prop starts boiling underwater, similar to putting a cold tablet in a class of water.
    b. External forces
    b.1 - shore power or neighbor issues - If the boat hasn't moved or turned it's shaft for a month or more. A zinc will show more pits or correction on one side. the side facing the issue.
    b.2 - movement of salt water around the zinc. Normally a more unified correction with some excess corrosion depending on the hull configuration, tides and currents. Daily and weekly use will also cause excessive corrosion if used in saltwater.

    I would be glad to narrow down the issue once I get the above questions answered.


  • Hi Kees

    Hull... GRP.
    Rudder Blade... GRP.



  • Hi SV Blue Lagoon

    1 - salt.
    2 - GRP sailing yacht.
    3 - Volvo sail drive.
    4 - no zinc has ever been installed on the rudder
    5 - no zincs on rudder shaft - but sail drive has plenty of protection
    6 - marina
    7 - GRP
    8 - out of the water in winter - rest of year sailed every week (ish)

    Any ideas on how to check the anode is offering protection by sticking a multi meter in line between the hanging anode and the rudder stock ?

    Many thanks