Anodes - Effectiveness and Location.

Karearea

Karearea

Active Member
Joined
24 Jan 2016
Messages
52
Location
Morrinsville and Hobsonville Marina NZ
Visit site
We are bringing a shaft driven boat to NZ from the UK after 11 years in fresh water.
It will going to a new life in the briny so what about the anodes?

The trim tab and shaft anodes are being changed to zinc but I believe there should be a hull anode and some on the rudders.
My understanding is that any magnesium in salt is a definite no-no and if there has been zinc or aluminium in fresh water then it may be "passivated' and not work when it hits salt water.

I also believe that salt water anodes should be all the same: either all aluminium alloy or all zinc alloy.
Having been in fresh water there may be one or more aluminium.
Would I be correct in saying that for full protection I need all new zinc (say) on prop shaft, rudders, trim tabs, hull and heat-exchanger?
The heat exchanger anodes would be also fresh water "passivated"?

Also would polishing/sanding the oxide coating off existing zinc anodes "de-passivate" (is that a word?) and re-activate them?
Is there an easy way to tell aluminium from zinc?

I have done a search and not found a definitive answer, but if this has been covered before, please point me in the right direction!

Cheers, G
 
assuming its a fibreglass hull with shaftdrives then change all the anodes to zinc or aluminium alloy. Don't mix materials. Rather than mess about trying to remove "passivation" oxides etc better to just bin the old ones with new.
There should be hull anodes, engine anodes, and trim tab anodes. There may be rudder anodes but often the rudders are electrically connected to the hull anodes internally so don't have separate anodes. You may not need shaft anodes if there are internal propshaft contact brushes.
If you aren't sure, get it all electrically checked for electrical continuity and quality of connection as poor anode set up will result in costly damage.
 
No reason why you should need additional anodes if the boat has survived 11 years with the current anodes. A hull anode is usually there to protect either the props or steering gear or both by being boded to them. For the props it is either through the gearbox or using brushes on the shafts. However on some boats just the shaft anodes are adequate. Not all steering gear needs and anode, depends on the metals use, but the fact that you boat does not have them suggests not needed.

Just change to zinc when you get to salt water and monitor depletion rates.
 
From personal experience - do not be tempted to leave her in salt water for too long if you currently have magnesium anodes fitted. I have had part-consumed
magnesium anodes disappear in weeks, not even months.

I think you were querying whether you should fit additional anodes. Salt water is a much more effective electrolyte than fresh (remembering schoolboy chemistry) so, other things being equal, electrolytic corrosion will take place more efficiently. I'm not sure there is adequate 'science' to back this up but many people claim that an anode must be able to 'see' whatever it is protecting. In my opinion that is not a bad rule of thumb and certainly cannot do any harm (apart from the downside of drilling more holes in your hull for mounting/bonding). On my last boat I had three anodes - two on the hull which could 'see' all the skin fittings and prop and a third in the bow-thruster tunnel. Interestingly, the bow thruster anode always appeared to waste proportionally quicker - possibly because it was quite small in comparison.
 
You must log in or register to reply here.

Share this page

Top