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Sacrificial Anodes

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Antares_9

Antares_9

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Or you can exchange the prop nut for one that will accept an anode, usually a conical boss on the prop nut with a securing screw down the middle. Aluminium is as you suspect, the right thing for brackish, zinc will form an oxide layer when you are in fresh water thus rendering it next to useless. Really anodes are best chosen for your mooring rather than cruising ground as that's where most boats spend most of the time. Always check for stray current in a marina if you are on shore power and seriously consider a galvanic isolator if there is any suspicion. The rudder is an easy one, just drill through it and bolt a disc anode each side.

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When replacing the anodes on my boat, I googled the subject extensively (or maybe I should say "exhaustively" , as there's so many conflicting opinions on the web. :)

A major factor to also consider is the type of metal you're trying to protect. Most shaft drive boats only have steel and brass, so aluminium is judged ok by most people for brackish water, but if anyone is trying to protect an aluminium outboard or outdrive leg, then it's considered by many to be useless.

The consensus of advice to protect aluminium parts in fresh and occasionally brackish water is to use Magnesium anodes, they will fizz away rapidly in salt water, but give much more protection in fresh.

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Hockham Admiral

Hockham Admiral

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hi ,

you can,t put anodes on the prop, it would become unbalanced, rudder yes you can bolt them on ,you can also have them fitted to the hull ,but these must be bonded to the boats earth to be effective :wave

That's what I was asking but the article only deals with steel boats. Does an anode fitted to a grp hull have to be bonded?

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Antares_9

Antares_9

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You can bond a bar anode to both the rudder and prop shaft if you like John (using brushes on the shaft internally) but there is no substitute for direct connection or at least “line of sight†installation. As an example on my ship I have: Discs each side of the rudder through bolted, Prop nut anode, trim tab anodes directly bolted, three large bar anodes directly bolted to the keel to rudder protection skeg, one huge bar anode bolted to the GRP and bonded to the rudder and prop. The direct contact anodes are all doing their job, evidenced by the fact they need regular replacement (a little less regular since fitting a galvanic isolator). However the huge bar anode fitted on the transom and electrically bonded has shown no sign of erosion since the boat was new despite “brightening†with production paper at each lift, I have seen this arrangement on many boats with similar results which leads me to the conclusion that they offer little protection. The definitive resource for anode information can be found here http://www.mgduff.co.uk/leisure-craft/hull-anode-selection/ they live and breathe anodes and do nothing else.

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mbird

mbird

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A major factor to also consider is the type of metal you're trying to protect. Most shaft drive boats only have steel and brass, so aluminium is judged ok by most people for brackish water, but if anyone is trying to protect an aluminium outboard or outdrive leg, then it's considered by many to be useless.

The consensus of advice to protect aluminium parts in fresh and occasionally brackish water is to use Magnesium anodes, they will fizz away rapidly in salt water, but give much more protection in fresh.

I'd disagree with that from personal experience Strowy, as I think would a couple of other members on here with outdrives. I replaced the zinc ones on Serenity last year with aluminium, and upon inspection this year, they are certainly doing their job and the drive/props seem to be being protected okay. Admittedly, we have been berthed in brackish water so maybe the anodes are a little more active than if we were moored further upstream in purely fresh water.

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Hockham Admiral

Hockham Admiral

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Here you go John, complete replacement prop nut and anode kit, you just need to get the shaft size identified and bob's your father's brother.

http://www.mgduff.co.uk/mgduff-product-catalog/The+%91Martyr%92+range/Martyr+Prop+Nut+Anodes/

Many thanks, David. :clap:clap:clap

With this and the rudder done I guess I wont need to bond another to the prop shaft?

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Antares_9

Antares_9

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With this and the rudder done I guess I wont need to bond another to the prop shaft?

I see little point John, make sure you grind the area where the rudder ones are fitted back to good bright metal and you're sorted.

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Admittedly, we have been berthed in brackish water so maybe the anodes are a little more active than if we were moored further upstream in purely fresh water.

Quite right, that seems to be the nub of the matter, admittedly though, I'm only going by other people's experiences at the moment.

There's a lot of forum posts in the USA where outdrives are much more popular and they have vast fresh water boating areas (lucky bu**ers).

Here on the Broads we have a fairly rare situation of the 120 miles of navigation being composed of brackish and freshwater, just a few miles apart.

Boats moored at Wroxham or Horning sit in quite different saline levels to those moored at the WRC or Burgh Castle, for instance. I imagine Magnesium wouldn't last a season down there... :shock:

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