Excessive prop shaft anode corrosion?

[sebastiannr]

Hi there,

I'm sure this subject has been done to death, but I'm struggling to find info to answer my question, so apologies for flogging a dead pig.

The boat is a 10.8m GRP sail boat, currently in the Med. I recently inherited the boat, so I'm not overly familiar with her systems (a lot more complicated than on my previous boat). There is one anode on the hull, two on the prop shaft (I put two on because they were half the size of the one I removed, and were the only ones available) and one on the end of the prop.

After 2 weeks back in the water, there was already considerable corrosion to the prop shaft anodes - perhaps 10-20% (none noticeable on the hull anode). I would guess that they will have to be replaced well within a year if the current rate of corrosion continues. I'm reading through Calder's bible so that I can test the system, but I don't have a silver/silver half cell to put in the water to test for stray currents. As far as I can tell, the underwater metal parts are isolated, not bonded (there is no strap bridging the flexible shaft coupling).

When connected to shore power, the galvanic isolator status monitor shows no DC or AC leakage faults.

Does this excessive anode corrosion indicate that there is a serious stray current in the electrical system?


Any advice would be much appreciated,

Seb.

 

[Tranona]

Depends on what you are trying to protect. The shaft anode will deal with the prop in relation to the shaft. The hull anode can also protect the prop if it is connected to the shaft. The fact that there is little erosion of the hull anode suggests it is not doing anything and the shaft anode is doing it all. You can probably reduce the wear on the shaft anode by bonding the hull one to the shaft. The normal way is first to the gearbox and then with a strap across the coupling if it is the flexible type. Shaft anodes are very small so may erode quickly if that is all you have.

 

[Steve Clayton]

I would also be looking to query the source/quality of the anodes that you have purchased/fitted in the Med.

Recently replaced mine (Med/Spain fitted) and took the old worn/breaking up cone one on the end of the prop shaft to Mg Duff in Chichester - they couldn't identify the composition!; it seemed to have various layers of material. Also replaced the two anodes on the sidepower bowthruster; one being severely depleted and the other not far off looking like new.

 

[sebastiannr]

I would also be looking to query the source/quality of the anodes that you have purchased/fitted in the Med.

Recently replaced mine (Med/Spain fitted) and took the old worn/breaking up cone one on the end of the prop shaft to Mg Duff in Chichester - they couldn't identify the composition!; it seemed to have various layers of material. Also replaced the two anodes on the sidepower bowthruster; one being severely depleted and the other not far off looking like new.

OK thanks for that info, I'll bare it in mind.