Magnesium or Zinc anode

[Cookie Munster]

I have just purchased an Etap 26. It has Zinc anode attached to Penta 110 saildrive - anode is 80% intact. I will be sailing in freshwater - will zinc be ok for a season or two or do I need to replace with magnesium amode prior to launching into freshwater? thanks.

 

[Tejas]

Suggest that you replace with magnesium. We sail in freshwater and replace annually on haul out. Some years, there seems to be little erosion; other years considerable. The year-to-year difference might be a different amount of time on shore-power.

 

[VicS]

Magnesium for freshwater.

Aluminium for brackish or salt

Zinc for salt only.

If you leave a zinc anode on in fresh water it will become "passivated" ( coated in oxide I suspect) and not effective even when returned to salt water.

Magnesium is too reactive to use in salt water.

If using shorepower a galvanic isolator is essential if you leave the supply connected although sail drives are usually isolated from everything else. If so should not be affected. ( not sure if the 110 drive is isolated but think so)

 

[macd]

VicS is correct, as usual, except that zinc anodes can also become 'passivated' in brackish water with the effects he describes. I suspect this varies with the rate at which the anode erodes (it certainly varies with something), because at haul-out time in my home harbour some zincs looked as though they'd stayed active, some didn't.

The MGDuff website has heaps of info on what anodes to use, troubleshooting etc. http://www.mgduff.co.uk

 

[VicS]

zinc anodes can also become 'passivated' in brackish water

that is why I said they were for salt water only.

 

[Quandary]

The choice may be restricted to zinc or magnesium if it is for a saildrive, I have never been able to source aluminium, I wish I could.
Our boat spends most of the year in fresh water with expeditions to sea lasting between one day and about four weeks at a time, if we used magnesium they would not last more than a week or two in the sea, so we are forced to use zinc. The anodes on the propellor degrade normally but those on the leg get the passivated coating and hardly reduce at all, (the coating on the prop anode is probably eroded off by the rapid rotation). I have a zinc hanging anode wired back to the top of the leg which is deployed every time the boat is stationary, the passivated coating is cleaned off every second or third week with coarse emery paper, rubbing until it shines, this is intended to supplement the leg anode and so far has been very effective. The leg has never been antifouled, it is normally coated with lanolin wax (Propshield) and the surfaces look like new after four seasons. The wax coating on the leg starts to peel and has to be renewed ever second season, I tried it on the prop but it only lasts a week or two there, so I just polish the bronze and it comes out of the water clean.
Other local boats with shaft driven props are able to use aluminium shaft anodes and report that they are effective and durable in both fresh and salt water.

 

[Momac]

I have just purchased an Etap 26. It has Zinc anode attached to Penta 110 saildrive - anode is 80% intact. I will be sailing in freshwater - will zinc be ok for a season or two or do I need to replace with magnesium anode prior to launching into freshwater? thanks.

Magnesium - you must replace all of the anodes with the same material ( if you have more than one on the boat ).
Our magnesium leg anodes (motor boat) have lasted two years including a couple of days on tidal/brackish water and the sea.

 

[macd]

that is why I said they were for salt water only.

Are you always so touchy? Is "VicS is correct, as usual" too faint praise?

What you also wrote was "If you leave a zinc anode on in fresh water it will become "passivated"". I was merely pointing out that what you might more accurately have written, and probably meant, was that "If you leave a zinc anode on in fresh or brackish water it will become "passivated"".

 

[Cookie Munster]

Many thanks for all replies - magnesium it is! It's my first time being on this forum and it's great to know where to go for advice. Previous boat had outboard and this is my first boat with in-board diesel,saildrive, anodes etc.

 

[atlowers]

Interesting thread as my quandry is that when boat goes back in, I will be 3 or 4 weeks in solent(piled mooring no shorepower) before channel crossing and spring summer on French canals. What should I use? Was changing to magnesium but now worried by posts here suggesting they will not last more than couple of weeks in salt water.
Think the o.p is sorted so sure this doesn`t qualify as a hijack.

 

[macd]

Interesting thread as my quandry is that when boat goes back in, I will be 3 or 4 weeks in solent(piled mooring no shorepower) before channel crossing and spring summer on French canals. What should I use? Was changing to magnesium but now worried by posts here suggesting they will not last more than couple of weeks in salt water.

I'd be inclined to fit aluminium for the trip you plan. But, being lighter, it will make your air-draught greater by a micron or so
Galvanic activity is much less in fresh water since it's a relatively poor electrolyte compared to salt.
If you have a zinc that you'd prefer not to change, such as a shaft or prop anode, snorkel down and give it a scrub with a wire brush when you're back in full salt. It'll work fine again once the coating's removed.

 

[Quandary]

I suppose it depends on the weight and type of anodes you use. If you have a saildrive or a folding prop you are stuck with the standard anode weights and prop ones are particularly small. Saildrives have mix of vulnerable metals which adds to the risk. With shaft drive you may be able to put on two or three supplemented by a hanging or hull anode so that there is no risk of it not being protected. My previous boat was shaft drive with a two blade bronze folder without dedicated anodes fitted to it. I used zinc shaft anodes usually two and a hanging anode for back up. On one occasion the boat was in fresh water with only occasional trips to sea for 18 months without any visible ill effects.
If I were you, I would fit magnesium if I was only going to be in the sea for two weeks, I would fit zinc if in the sea for four; remember the rate of galvanic corrosion will be a lot slower once you are in fresh water. The risk would be much greater if you were going from fresh to salt. Check your anodes at the first opportunity, you will probably get away without any protection for a couple of months in fresh water without damage as long as you are not moored in a dodgy location, (shore power or metal boats)
In this case there is no advantage in mixing metals because the stuff you need later will be the first to go.

As above, use aluminium as long as it is not a saildrive, if you ever find aluminium anodes for VP legs please post the info.