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Q: I keep my boat in a river which has a lock at the entrance, so it is pure fresh water. I go out to sea about twice a year for 4 to 6 weeks. What is the best solution for hull & shaft anodes? Boat is Moody 34, GRP.

For cathodic protection the correct anode for salt water use is zinc, brackish water is aluminium and fresh water is magnesium. Aluminium is the only anode material that can be used in more than one water type (brackish and salt water). It usually best to pick the anode material that will suit the water type the boat is moored in, as this is often where leisure vessels spend most of their time. For you, this would mean magnesium. However when you travel off into salt water the magnesium will become very reactive and start to waste quickly.

We recommend if you are using magnesium not to take a boat into salt water for more than seven days at a time or fourteen days in any one year. If you are planning to spend longer than this (as it appears you are) the anodes should be changed for the correct type. This can prove quite a problem! And impractical.

The solution is to use hanging anodes. You will require two hanging anodes, one in zinc and one in magnesium. Whilst the boat is moored in fresh water, then magnesium hanging anode can be hung over the stern at a depth to line up close to the prop. The cable then must be attached inside the boat to give electrical contact to the items you wish to protect, prop, prop shaft. This can be achieved by attaching the wire to existing anode bonding wires or attaching to the engine block (this assumes that if you have a flexible coupling it is bridged to keep continuity). Hanging anodes have the advantage that you can lift them up at any time and check to see that they are wasting away and therefore are working. You will need - MD57LKIT plus a ZD57L. This new hanging anode from MGDUFF features an interchangeable anode so you only need to buy one kit.


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Tags: hanging anode, interchangable, fresh water, salt water, Moody, GRP

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