Extra Zinc Anode

[bartvdv]

Hello,

Small question here:

In adjunction to my zinc anode on the prop shaft, i would like to install an extra zinc anode on a cable. This to make the prop zinc last longer.

One end of the cable: the extra zinc anode hanging in in the water.
Other end the cable: hangs on the SS lifeline around the boat (with a SS carabiner).

I would then electrically connect a stanchion nearby to the propshaft (under deck).

Prior to sailing, i can easily remove this extra zinc anode and stow it away.

My propshaft is SS with a zinc and bronze prop.
Green-yellow cable of AC-shorepower is not connected to negative DC.

Is there anything i have forgotten why i should not electrically connect propshaft to the life lines?

[mitiempo]

To be effective the hanging zinc should be within 12" to 18" of what you are trying to protect.

As far as the shorepower ground it should be connected to the DC ground for safety. A galvanic isolator will solve possible corrosion problems - $99 from Yandina.

[Fstbttms]

Originally Posted by mitiempo

To be effective the hanging zinc should be within 12" to 18" of what you are trying to protect.

On what do you base this claim?

[sailingdog]

Fstbttms—

Given the fairly low voltage differentials involved, having the zinc any significant distance away will render it ineffective. Especially if that distance is through something like stainless steel rigging, which has about 33x the resistance of copper wire IIRC. Also, I'd point out that the connections that Bart is planning to use are low-contact-area, high-resistance connections. From his OP:

One end of the cable: the extra zinc anode hanging in in the water.
Other end the cable: hangs on the SS lifeline around the boat (with a SS carabiner).

I would then electrically connect a stanchion nearby to the propshaft (under deck).

A stainless steel carabiner clipped to a stainless steel lifeline is going to be a really lousy connection... he'd probably do better by clamping battery jumper cables made from tinned wire to the shrouds...

[mitiempo]

Yes proximity is important. One neighbor was replacing his shaft zincs quite often. He used a zinc fish over the side but maybe 10 or 12 feet from his stern and while the shaft zinc dissolved quickly his fish did not. Moving it closer solved the problem. If closer is better closest is best. Also like dog posted a clip to a stainless lifeline is not very effective. The fish should be on a good gauge of wire like #6 and the boat end should ideally have a good crimped ring fitting and be attached firmly to a good ground.
The green shorepower earth should be connected to the ship's ground for safety. If there is a fault the current has to go somewhere and this gives it a safe path. You say there is no connection on your boat? That would mean no battery charger where AC and DC reside in close proximity and AC can leak across to the DC side in some cases and if there is a fault. Remember that many chargers have a case ground that should go to DC as well as the DC output. A fault in that charger can make the DC system hot.
A galvanic isolator is also a must I believe. It will eliminate the currents that cause zincs to dissolve quickly but still pass current when there is a high voltage fault. It will not however protect your boat from stray current coming from other power sources like dock issues.