WARNING: change when half worn.

[S/V Elusive]

While no chemist, I believe that as the sacraficial electrons of the zinc go on their merry way, it leaves an outer layer (filmy - whitish) of metal that does not have the extra electron to give up. And while there may be plenty of metal that does have the extra electron, it may have difficulty in tranistioning through the 'used' outer skin. So, while there may be the same surface area (because of the pitting), the flow won't be as robust and therefore not as protective.

I'd draw a target for those that want to shoot down that theory, but I'm no artist either.

[Alan Wheeler]

50% erosion has more surface area than a new, smooth, one,

Remember Gord said surface area and Mass. Normally a properly disapating Anode will remain reasonably smooth and should be dark grey. If touched, it should leave a black film on your hands. If it is shiney grey, it is possibly erroding way to fast. Pitting is a sign of impurities other than Zinc that have an ever so slightly higher nobility. Maybe Aluminium or or oxide trapped in the molten zinc when melted and poured. So even though it may seem that is has greater surface area, chance are, it ain't doing the job.
Thomas, firstly, if you have white material building up on the anode surface, you have another problem. This is a sure sign that the anode is either too protective, or oversize, or it has no electrical bond to the metal it is protecting. The whitish material is Zinc oxide and is non-conducting and insulative. White powdery stuff = Bad.

[S/V Elusive]

Hummm... I KNOW that I have good electrical contact to the metal being protected. I have one shaft zinc, and the mfg. recommended prop nut zinc. Don't think that I've got too much material ... Can you have TOO much protection? What is that? I know I can't be TOO rich .. but I can be TOO poor (was heading that way until the beginning of this year). I suppose I could be TOO good looking (but for some strange reason, I've never heard anyone say that).

[GordMay]

The surface of a sphere varies as the square of the radius, hence at 50% eroded diameter, a spherical anode would have 25% of the original surface area.
That would represent a lot of “dimple” area.
(A = 4 pi x r squared)

[Alan Wheeler]

Absolutley you can have too much protection. It's jsut as bad as not enough. It is easy to make assumptions here and jsut as easy to put you wrong. So the best course of action is to get a test and see if you are safe either way.

[seafox]

All too bloody hard. I'll just pull the beast out every 6 months and change it.
bloody
volvos, never had this hassle with the Yanmar.

[fstbttms]

Originally Posted by seafox

All too bloody hard. I'll just pull the beast out every 6 months and change it.
bloody
volvos, never had this hassle with the Yanmar.

A diver could do it in a few minutes for a fraction of the cost of a haulout.

[seafox]

I sold all my dive gear on trademe last year. We only pay $50 for a lift and hold at the Marina. There is a waterblaster where the boat is lifted so you get time to clean the prop, change the anode and wash down the hull for $50. I reckon a diver would charge me that for transport to and from the Marina here. (Our Volvo man charges us $40plus gst for transport whenever we use him)

[fstbttms]

Originally Posted by seafox

We only pay $50 for a lift and hold at the Marina.

If that were the case in California, I'd be out of business!

[Moby Dick]

Originally Posted by GordMay

I think Moby Dick's eyeballs are offering false testimony:
The surface of a sphere varies as the square of the diameter, hence at 50% eroded diameter, a spherical anode would have 25% of the original surface area.
That would represent a lot of “dimple” area.
(A = 4 pi x r squared)

Methinks Gords eyes are playing up as well: square of the radius it is.

The surface area of a rough anode would be HUGE compared to a smooth one as it is a fractal surface, appearing the same at any level of magnification.....

I think Alan may be correct though, the remainder may have more impurities, but it does all eventually erode away (or fall off..)

[seafox]

Mmmm fstbttms, maybe your way is better after all:

http://www.cruisersforum.com/forums/...acht-7481.html

Originally Posted by fstbttms

If that were the case in California, I'd be out of business!

[GordMay]

Moby Dick:
Thanks for the correction (Euclidian surface are varies as the square of the radius).

Although the dissolution of the anode zinc may result in a fractal surface topography; the anode effect (wherein the anode becomes virtually insulated from the electrolyte by an inhibiting gas film, and/or calcareous deposits, such as calcium carbonate or magnesium hydroxide) actually decreases the conductivity of the cathodic circuit. This may also result in anodic polarization (passivation), discussion of which is way beyond my expertise.

This phenomenon is suggested by the experimental method of measuring the rested corrosion, or open circuit potential. The commonly recommended potential is 200 mV (0.2 V) difference (silver-silver chloride reference cell)*.

*By referring to the Galvanic Series in Seawater tables*, and by knowing the metal alloy of which each fitting or structure is composed, you can determine whether any particular metal is at risk for corrosion damage.
For instance, if the composite reading is 200 mV or more negative to the metal that has the most negative natural potential of the connected metals, then all the connected metals are cathodically protected.
If the composite potential is positive to the natural potential of the alloy of any connected metal fitting or structure, that fitting or structure is most likely suffering corrosion damage.
If the composite reading is positive to the metal alloy that has the most positive natural potential of the connected metal fittings or structures, polarization by a stray electrical current from a DC electrical system fault is indicated, in which case all connected metals will most likely be suffering corrosion damage, very possibly severe corrosion damage.

* See Table 1, page 7:
Cathodic Protection ~ ABYC Section E-2
http://www.abycinc.org/committees/E-02.pdf

[Alan Wheeler]

There used to be many years ago, a system I thought was great. I haven't seen it for years. It has a user adjustable controll and a meter to show the level of protection. You can dial on more or less protection. this was a great idea I thought, as protection varies with Salinity and what other boats are around you. I have never seen a system sold by anyone.
Anyone have experiance with the system and does it work???

[GordMay]

Impressed current systems (Impressed Current Cathodic Protection - ICCP ) utilize an external power source, that provides a protective current to mitigate corrosion activity.
A typical ICCP system consists of (nonsacrificial) noble anodes connected to power supplies, reference cells to monitor hull potential state, and a controller to adjust the current output of the anodes.
They work.

[Steve Kidson]

I might be missing something here, but if it is likely to cost hundreds (or more) dollars for a quality replacement prop, why question paying $20.00 for a replacement zinc to protect it??

If your replacement prop is going to cost $1,000 you could buy a new $20 zinc every 6 months for 25 years, and still be in front.

I know that is very simple thinking, and does not include for haulout, but if you are able to careen, or can swim under, replacement is not so much a big deal.

Sometimes we try to complicate what should be simple and is obvious.

Just my 2c

Fair winds

Steve