Prop anode

[QBhoy]

Bin the fixation with 12months --where did that exactley come from ? Show me the evidence ps ?

Replace anodes when they are deemed worn 1/2 or what ever worn , current opinion advises .

This may mean in certain situations like props , the shaft , the bolts , the p bracket ,the cutlass bearings , the coupling inside , with all those dissimilar metals there's a lot of potential electrolytic activity leading to rapid prop anode wear ,not making a 12-/12 time line .

Change them when ready ,regularly inspect etc .

I can Snorkel them and have a mini diving kit .The Med issued really relates to a wet suit or thickness of wetsuit .
Cold water UK does not automatically mean they have /need to be done @ the annual lift ! Conveniently that may be .

I can't see anyone suggesting anything about 12 months replacement

 

[rafiki_]

Another variable, the stbd anode erodes much quicker than the port one. Any more thoughts? Stbd trim tab anode 50% eroded, port one 5% at most. Hull anodes have lasted 2 years. Similar erosion rate on port and starboard.

 

[Portofino]

I can't see anyone suggesting anything about 12 months replacement

Assumed because they seem not to last between lift annual out s --- there's a problem and the Op is being presented with various solutions to stretch life to when its Convieniantly easy to replace .

I,am questioning that's there's is not an issue at all in the first place .

Just change them with the boat in the water when they are ready .

 

[QBhoy]

fair play Portofino...

The un symmetrical erosion of the anodes is a wonder.
Can only think that one is working harder than the other for continuity or galvanic reasons....surely ?
Like I mentioned, you shouldn't need prop anodes if you have a conjoined earthing-anode set up.

 

[superheat6k]

Another variable, the stbd anode erodes much quicker than the port one. Any more thoughts? Stbd trim tab anode 50% eroded, port one 5% at most. Hull anodes have lasted 2 years. Similar erosion rate on port and starboard.

If the hull anodes aren't eroding I would check their bonding. An isolated anode won't dissolve much on its own.

More one side than the other suggests either a difference in the bonding, or possibly something nearby contributing to the problem, such as a pile or pontoon. What side to do you berth ? It could be your stbd anodes are helping protect some other metalwork.

Check also the following ...

Bilge pump float switches

Polarity of shore power lead

Have you any bonded fittings on our stbd side ?

Any power cables from other boats trailing in the water - check within say 50m of your boat.

Measure the volts across your GI - if it is above 0.0 but below 1.4 v then galvanic current is trying to flow, but the GI is working.

No false neutral to earth links - a neutral to earth link on the boat will re-establish the path to the shore earth, bypassing and making the GI useless. If you have a generator or inverter check the neutral line is switched off downstream of earthing link off when these are not providing power (these should have two pole isolations).

Using an LED as a shore earth / neutral failure indicator can provide the same path to bypass the GI.

With everything turned off except permanent services disconnect the battery pos and measure voltage between the pos post and the connector - should be 0.0 volts.

I have some good manuals on underwater corrosion and bonding. I will PM you some links tomorrow.

 

[QBhoy]

Good point superheat. Further elaboration on my point.
Especially the jetty or pontoon theory.
I remember a while ago (at an unnamed fancy 5 star marina near me in bonnie Scotland) there was an abnormally high number of boats suffering severe corrosion to the soft metals. In particular the mercruiser trim pipes and fittings.
The local mercury service agent had a good year, that year.
It turned out that some of the pontoons weren't earthed properly, causing a galvanic reaction to any boats nearby.

 

[Tranona]

The prop anode will wear first because it is directly connected to the prop, but as stated earlier its mass is small compared with what it is protecting and a season's life is not unreasonable. The quickest way to increase bulk near the prop is using a shaft anode attached about 20mm in front of the P bracket. Expect the wear will be shared between the two.

Next stage is to use a hull anode located near the prop and bonded to it through the gearbox housing with a bridge across coupling if it is the flexible type. A hanging anode is another option, particularly if you want to avoid having lumps of zinc attached to the bottom of your boat. logically most of the wear occurs when the boat is moored so makes sense to just have them in the water during that time. You can buy them from MG duff or make your own and they need to be bonded to the gearbox in the same way as a hull anode.

What are your current hull anodes bonded to - that is what are they supposed to be protecting? follow the cable run inside the boat from the anode to the connection at the other end.

 

[Tranona]

The gearbox idea would be brilliant if the gearbox were immersed -

Completely irrelevant. The gearbox is only being used as an electrical path from the external hull anode to the prop. Perfectly normal arrangement used on thousands of boats.

 

[superheat6k]

Completely irrelevant. The gearbox is only being used as an electrical path from the external hull anode to the prop. Perfectly normal arrangement used on thousands of boats.

Erm, I was being flippant. If the boat has a significant galvanic issue then having the bonding route pass via the gearbox bearings is not very good for the bearings because the sparking can cause surface pitting of the bearings, which is the reason for using shaft brushes.