Which anodes?

[Rivers & creeks]

We're going to spend a few months in the Baltic - an area of slightly salty water and regular brackish water but it'll be a month travelling to get there in proper salt water. What anodes would you recommend? It's really important as we have some very big bits of dissimilar metal at the back!

 

[Momac]

Aluminium ?
http://www.solentanodes.co.uk/collections/aluminium-bolt-on-anodes
.

 

[sarabande]

drying your 35 out in parts of the Baltic* may provide a problem if you want to change the anodes when you get there, unless you opt for a lift.

If you install magnesium anodes before you go, it's hard to imagine them eroding completely in the N Sea before you reach the fresh.brackish waters that magnesium is best for. Could you go oversize to be sure ?


Unless you rely on the semi-diurnal and wind-induced oscillations.

 

[Momac]

Magnesium anodes could easily fizz away to nothing after a a couple of weeks let alone a month in salt water.

I am based in fresh water where there isn't really any corrosion issue but if sticking to fresh water I would certainly use magnesium anodes. But we make occasional trips in brackish and salt water and have fitted aluminium anodes.
This is not without due consideration . The use of aluminium anodes received an agreeable nod of approval from the local volvo penta experts but also I know of others who have done so with reported success .

 

[VicS]

Magnesium for fresh water

Aluminium for brackish water

Zinc or aluminum for salt water

see http://www.mgduff.co.uk/leisure-craft/cathodic-protection.html

Therefore fit aluminium anodes but be sure they are good quality to the relevant US military standard or equivalent. No cheapies off ebay!

 

[Rivers & creeks]

Thanks. Tried to find aluminium collar anodes for a 1.75" shaft - not sure they exist! Does a zinc anode give any protection in brackish water?

 

[VicS]

Thanks. Tried to find aluminium collar anodes for a 1.75" shaft - not sure they exist! Does a zinc anode give any protection in brackish water?

MG Duff say

Not all anodes are suitable for every environment, for example the surface of a zinc or aluminium anode will if left in fresh water for some time become covered with an off white crust of oxide which effectively seals the anode and stops it working even when returned to salt water. Zinc Anodes suffer a similar problem even in brackish conditions whereas Aluminium will continue to operate effectively in river estuaries and other areas of brackish water indefinitely. The consequences of this passivation of the anode are that the next most anodic item within the anode bonding system will start to sacrifice itself which could of course be very serious.

 

[Rivers & creeks]

That's very helpful thanks. The steel rudder is easy, the tip is just below the surface so I can put an anode stud on that near the top replacing the one near the bottom, then put a zinc anode on before we go and change it for an aluminium or magnesium one when we get there - I'll check with the marina what most people there fit. The shaft collar anode will be a real pain, it's too deep for me and so far I can't find an aluminium one for 1.75" shafts. If I could rig a shaft bush thingy from the inside to the existing hull anode then it would make the collar anode redundant.

 

[VicS]

That's very helpful thanks. The steel rudder is easy, the tip is just below the surface so I can put an anode stud on that near the top replacing the one near the bottom, then put a zinc anode on before we go and change it for an aluminium or magnesium one when we get there - I'll check with the marina what most people there fit. The shaft collar anode will be a real pain, it's too deep for me and so far I can't find an aluminium one for 1.75" shafts. If I could rig a shaft bush thingy from the inside to the existing hull anode then it would make the collar anode redundant.

No need to mess about changing anodes unless you are going into fresh water and need the protection of magnesium anodes. You probably won't for short periods . Just fit aluminium at the outset.
The real trouble with taking zinc into brackish water and zinc or aluminium into fresh water is that if they become passivated they won't be effective when you return to salt water.

A shaft brush, or even a connection via the gearbox and bridged shaft coupling, to an aluminium hull anode is one satisfactory solution to providing protection of the stern gear... always assuming it actually needs it of course.