Electrolysistic Problems with alluminium prop

[EPRIMA14]

Good morning,
I have a SD-20 Yanmar salidrive and aluminum propeller 3-blade. A few days ago I was diving to see the state of the anode and I saw that the anode is very damaged and the alluminium propeller begins to lose the paint and It is starting to deteriorate.
I changed the anode 4 months ago.
Has anyone had this problem?
Thanks to all

 

[Bill Roosa]

You have a loose connection somewhere. Recommend disconnecting from shore power soonest, installing new zincs, check out the voltage available in the bilges and inspecting weekly till you can confirm what the problem is.
The shore power cable has a ground terminal which connects you with all the other boats attached to that shore power circuit (your dock). They act as an anode you your boat's prop and zinc is cathode
You may also have the bilge pump shorting into the bilge water which can cause the same problem.
The same goes for ANYTHING you leave energized while away from the boat. Turn off main power when away from the boat to eliminate these sources of electrical corrosion.
There is not much you can do about the shore power ground except disconnect. You can determine if the bilge pump is shorting out by measuring the voltage of the bilge water to ground. Find a long jumper cable and attache to battery ground and measure between that wire and a multimeter's probe stuck into the bilge water near the bilge pump. You should get 0.00 volts on the lowest voltage scale (tenths of volts range) If you get ANYTHING above 0.2 volts you have one of the energized wires leaking into the bilge. Disconnect the power from the bilge pump at the electrical panel and recheck.

 

[EPRIMA14]

Thank you very much for your response. This weekend I've been on the boat and these are my comments:
This problem had not before when the boat was in another area of the port. I had not the shore power connected, I changed the anode every 2 years and the propeller was in perfect condition.
When I changed to another port area, the problems started. Now I have the shore power connected because I put on earth wire a galvanic isolator (zinc saver) to avoid (or to minimize) the problem. If I remove the zinc the problem increases.
I have also connected the battery charger
This weekend I checked the bilges and they have no water, the bilge pump works correctly.
I have a plastic box that collects water from the shower with a sump pump, but I see no problem, the pump works properly. I measured the mass of the battery to the battery terminals but I see nothing wrong.
Any comments?
Thanks