How Many Zincs?

[markpj23]

Been roughly 2.5 years since last shaft zinc replacement. I have a single shaft zinc on a 1-3/4" shaft, it was about 50% wasted (man I love that isolation transformer!)

Just wondering - if 1 zinc is good then 2 must be better, right?

Is a single shaft zinc OK? My most recent experience here would indicate yes but wanted to ask.... Is there a formula for calculating required zinc surface area?

[mickmul]

Here's some research from my files (though concerning sterndrives etc) . .written by Mark Rigg ? I believe. . . hope it helps.

Determining which sacrificial anode will fully and safely protect a boat depends on a number of factors. As previously discussed, it is important to understand the physical properties of each anode material. In addition, boaters should also understand how each anode works with different hull compositions, sterndrives and types of water.
The best way to determine whether a boat is fully protected is by measuring its cathodic protection voltage or hull potential using a voltmeter. But, if one isn't available, here are some simple guidelines for selecting the right anode. The hull material of a boat determines, in part, which anode material to use. A fiberglass boat having an inboard engine with bronze and stainless metal parts needs less protection than an aluminum hull or a boat with an aluminum sterndrive. Zinc or aluminum alloy anodes will work well for these types of boats. The voltage generated by these anodes cannot overprotect, i.e., they cannot cause any damage no matter how much anode material is added. The maximum voltage generated is the voltage of the anode itself. Magnesium would also work with a fiberglass boat but only in freshwater. However, aluminum or wooden hulled boats can be overprotected by very active magnesium. Steel hulls can also be overprotected and the excessive protection voltage will rapidly lift the paint off the hull. Sterndrives and outboard motors, because of their very active aluminum assembly, are hard to protect. Initially, the anodes for these units were made of zinc. But corrosion problems in the early 1990s sparked the major engine manufacturers to start selling aluminum alternatives. The increase in protective voltage ensures that the sterndrive is protected. Today, in some cases, using zinc may invalidate an engine's warranty. Again, caution is needed when using magnesium anodes as they can overprotect. Water type is the final influential factor when choosing an anode. When used in freshwater, a zinc anode forms a coating of zinc hydroxide that insulates it and stops it from working. Magnesium used in saltwater can disappear very quickly, and if it is used on an aluminum sterndrive or outboard motor, it can be very dangerous. If piloting a boat with an aluminum hull or sterndrive down river and into the ocean, the overprotection of the magnesium anodes would cause hydrogen bubbles to form under the paint on the hull or drive resulting in it being literally blown off. Acid rain and pollution can also increase the conductivity of freshwater to the point where the same thing will happen. Aluminum alloy is the only anode material that is safe for use in all types of water and accepted by the major sterndrive manufacturers as the best material to use. It is lighter and protects better than zinc and is not so active that it becomes dangerous like magnesium. Aluminum alloy is also environmentally friendly, unlike zinc, which is considered a pollutant.

[GordMay]

A single Zinc should protect your shaft & prop, until nearly 1/2 eroded.

A “Google” search using the term:
zinc anode site:www.cruisersforum.com
will result in 17 returns on the subject, including
When to replace the Anodes ?
http://www.cruisersforum.com/forums/...odes-8773.html

wherein the following information, and much more, appears:

The calculation of the weight of anode alloy required to protect a structure is given by a simple calculation:

W = (A x C x L) ÷ Z
Where:
W = Weight in Kg
A = Area to be protected in M2
C = Polarisation Maintenance Current Density in Amperes / M2
L = System life in Hours
Z = Anode current capacity in Ampere Hours per Kg
The individual components of this equation, with the exception of design life, are known but only approximately*.

[markpj23]

I just knew Gordy had another formula in his pocket....

Thanks Gord & mickmul - good info!

[hugosalt]

Isolation Transformer

Can some one confirm/enlighten...I'm under the impression that a isolation transformer is used when connected to shore power and would not be helpful on a mooring??? I am on a mooring most of the time (very little stray current around) and anchor out away from marinas and crowds the other times. In a slip with shore power
once a year if that! Have an allumiun saildrive that is getting old but in excellent condition, and I need to keep it that way. I change zincs every year with less than half used. Boat is fiberglass.
Thanks in advance!
Hugo

[GordMay]

Originally Posted by hugosalt

Isolation Transformer
Can some one confirm/enlighten...I'm under the impression that a isolation transformer is used when connected to shore power ...
Hugo

Absolutely correct.
The Isolation Transformer is an improvement upon the Galvanic Isolator, offering several advantages, but serving (more or less) the same purposes..

[David M]

Both the galvanic isolator and the isolation transformer stop stray DC currents. It is DC currents that are the primary cause of electrolysis. Without the transformer or the isolator, your boats metal can become the sacrificial anode for the entire marina.

[hugosalt]

Thanks Gord/David ... I do have a galvanic isolator so I have some protection.
I only wish I could remember all this great info!
Hugo

[fstbttms]

Originally Posted by markpj23

Just wondering - if 1 zinc is good then 2 must be better, right?

Is a single shaft zinc OK? My most recent experience here would indicate yes but wanted to ask.... Is there a formula for calculating required zinc surface area?

You don't need to get anal about how many zincs to install. If a single shaft zinc lasts over 2 years, I don't really understand why you feel the need to double-up, but you can, without problem. The physics involved say that if a single zinc lasts X amount of time, 2 zincs will last 2X amount of time.

[Extemporaneous]

Originally Posted by GordMay

Absolutely correct.
The Isolation Transformer is an improvement upon the Galvanic Isolator, offering several advantages, but serving (more or less) the same purposes..

Gord,
I've being doing quite a bit of reading and the the consensus, as you state seams to be that an isolating transformer has advantages over Galvanic Isolators. Problem is I can't find much information as to the exact reasons why.
Can you enlighten me?

Thanks,
Extemp.

[roberttigar]

You cannot over zinc a fibreglass boat, but you can harm wood or metal hulls with too much zinc. If you over zinc a fibreglass hull, you have now made the prop a more hosbitable place for those creatures that love to attach themselves to it. Otherwise, no problem.
You can check you zinc effectiveness using a multi-meter to check voltage produced. Bob

[sonnysoh]

Hi All,
I have a July, 2008 Hunter 33 and have replaced 5 anode since new, and I feel that the wear of the anodes is too fast, the boat is equip with isolation transformer. I use shore power from time to time for air cond and to charge batteries. Please advice is there any wrong and how to retify it if there is. Rgs. Sonny

[Albro359]

You need to determine what needs protecting

If your zincs are lasting 2.5 yrs ... maybe your prop's wasting..or yr bronze thruhulls

The only way to do it properly is with a DMM on mV range with an Ag electrode to measure the potentials at each potentially corroding site...ie
propellor and shaft , thru hulls, keel, skeg, rudder..... the highest readings need the most zinc

need to ensure you have a GOOD connection between propellor and shaft o'wise the zinc on the shaft is only protecting the shaft...which probably doesn't need protecting anyway

ALL bronze thru hulls need to be connected together and preferably connected to your keel bolts (esp. if you have a steel keel) and you need a connection to the rudder skeg and maybe the rudder also, and the steering shaft...

THEN you can have a shaft zinc and a skeg zinc and you're probabaly OK

OH ..AND you need to check the zincs in your engine heat exchanger regularly.

If you are in a marina, where other folks have shore power, you should be checking all zincs every 6 months...

AND make sure the connections are sound...use smallish gauge battery wire with CLEAN connections to connect everything up to ensure low resistance.

This stuff can kill you !!!

I lost a prop shaft strut many years ago because of lack of attention to this....

Cheers

Alan

[fstbttms]

Originally Posted by sonnysoh

Hi All,
I have a July, 2008 Hunter 33 and have replaced 5 anode since new...

How many zincs are you carrying on the shaft?

[Bash]

I carry two. That way if one comes off for whatever reason the prop is still protected.