WARNING: change when half worn.

[Alan Wheeler]

It's not the replacement of the Zincs that is the issue Steve, it is the question of, is the Zinc working correctly and thus saving the expensive prop or other parts of the boat from damage.

[NoTies]

It's also a question of the Volvo money generating machine making oddball parts that are very hard to justify aftermarket parts for, hideously expensive parts & service costs, and unavailabilty (except at a criminal cost) of parts & service manuals. Volvo are just another engine & drive system but their culture of holding you captive to their designated service agents makes owning one a liability.

[Steve Kidson]

Originally Posted by Alan Wheeler

It's not the replacement of the Zincs that is the issue Steve, it is the question of, is the Zinc working correctly and thus saving the expensive prop or other parts of the boat from damage.

I admit to having little experience in this, but won't the zincs be consumed at different rates in different situations, and that is the whole point?

In the same way that your antifoul paint will control growth at different rate in different areas; ie cold water vs tropical waters, surely the same would apply to the zinc consumption??

Is it possible that the boat in marina pen A for 3 months will suffer X amount of errosion, and if the same boat is then sailing in open waters for the next 9 months that a lesser amount of errosion is measured because of different environment??

I am trying to learn more about this prior to my boat going back into water.

Fair winds

Steve

[Alan Wheeler]

Your kinda right Steve. Yes you are correct that an anode will errode at different rates in different situations. Salinity, water temp, closeness of other vessels to you all have an affect on the errosion rate. However, the rate an anode errodes is actually irelevant. It is the level of protection an anode gives you that is the important part. You can easily have too much anode as well as not enough. Different metals close together in salt water act like a battery. An electrical current flows between them. As the current flows, carried along with it is the least noble metal. If the other more noble metal was close enough, it would slowly attract the other metal and eventually be plated in the other metal. By fitting another metal, in our case Zinc, the Zinc becomes the least noble metal. So the current flows from it to the most noble. The anode errodes away as it's electrons are carried away with the current. Due to water flow, seldom does the Zinc actually arrive at the other parts of the boat being protected. Fo good protection, the current needs to be within a parameter as Gord has shown above. If the anode is too big, it can actually cause too much current and this is sometimes seen as paint being blown off hulls. Sometimes the anode can actually corrode and take up minerals from the salt water. This then stops the anode from working and the result is as if the anode isn't fitted. To small an andode or badly worn will reduce the level of protection and the next metal on the nobility scale starts erroding. This is often the Zinc that is locked up in the Bronze props and the result is a damaged prop. I have seen alloy stern legs and otuboards that Anodes have corroded on. The owner still see's what he thinks is a good anode. But later to his horror, he finds ally that you can quite literaly poke your finger through.
So to concise all the above, yes you select an anode based on the variables around you and you replace the anode when the anode is getting about 50% down. You don't, although many do, leave it till the anode is just about gone, or you will loose your prop or worse.

[cooper]

Hi Gord
l have seen this system on a couple of 4 wheel drives (expensive ones) ; Impressed current systems (Impressed Current Cathodic Protection - ICCP ) but can it work on a boat when the boat is initself floating in an electolyte ? what would the current draw be , if it was possible.

[GordMay]

Yes, ICCP systems do work on boats. They aren’t cheap, and must be rigorously monitored & maintained. These systems are usually installed on larger yachts and metal boats.
The current applied to the anode is regulated by the anode control circuit, at a level protecting the structure, in accordance with the signals derived from the half cell & anode. For a small fibreglass boat (< 50'), I’d expect the current draw to be on the order of 15 watts or less (1 Amp).
I’m no expert, and suggest that anyone interested in impressed current systyems contact a manufacturer, such as Electo-Guard: Electro-Guard, corrosion and cathodic protection specialists for boats, yachts and small ships.
See also, ABYC Section E-2: www.abycinc.org/committees/E-02.pdf

[cooper]

So if lm correct, ..if the control circuit for what ever reason , allowed the electrical "balance" to fail, you would then have the opposite of what is desired. An automatic electrically driven metal mouse.