Prop shaft material

SG

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Feb 11, 2017
1,670
J/Boat J/160 Annapolis
Ah, one is electrolysis and the other galvanic. (galvanic requires bi-metallic preference). Thanks for reminding me, Charles.

http://www.pcmarinesurveys.com/AC DC electrolysis.htm

An excerpt from the cite (and site) above:

2. Stray Current corrosion : Corrosion that results from an electrical source causing a metal in contact with an electrolyte (water) to become anodic with respect to some other metal in the same electrolyte.

In simple terms a wire touches something it shouldn't, like a faulty bilge pump float or degraded wiring lying in the bilge sending current into the water, causing one metal to give up electrons and corrode. Again any vessel suffering from this type of corrosion is likely the master of it's own disaster but the culprit could also be a neighboring vessel. This type of corrosion can can eat metals at an alarming rate.I know of one 42' motoryacht that lost both shafts, both rudders and both propellers in a space of less than two weeks.

Complicating this picture somewhat is the fact that DC can be super-imposed on your AC wiring through the common ground on board or the ground in the shore power pedestal we all share on the dock. As all vessels in the marina are connected through shorepower grounds there is potential for widespread damage. Aside from concerns of corrosion there is also potential for electrocution if shorepower cords are allowed to lie in the water let alone the fools that leave their shorepower cord plugged in at the dock while they go out for an afternoon cruise.


Recent tests have shown that AC current from shorepower in the water can also cause corrosion to underwater parts although at a much slower rate than DC. This has been a long argued issue by people who know a lot more about this than me. Ground fault protection systems, galvanic isolators, isolation transformers and impressed current systems are some of the various methods attempting to combat corrosion.


Salt water is generally regarded as a more serious breeding ground for marine corrosion as the salt makes the water more conductive however, polluted fresh water can be even more conductive with the right contaminants.

With our aging fleet of pleasure craft it's likely that at some time, less than expert hands have played with your electrical system. If your vessel is suffering from any electrical faults or unusual corrosion consult with an American Boat and Yacht Council Certified marine electrical technician with specific corrosion control training or give me a call and I will try to set you up with an expert in this field.​
 
Jan 30, 2012
1,123
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
That term 'electrolysis' is not a favored term because it also refers to the dissociation of water molecules - but pcsurveys is a Canadian so we give him a little latitude -- heh heh.

Charles
 
Last edited:
Feb 14, 2014
7,400
Hunter 430 Waveland, MS
Recent tests have shown that AC current from shorepower in the water can also cause corrosion to underwater parts although at a much slower rate than DC
Well...
The damage is not caused by the volts, but the amps,
and AC can be semi-rectified to DC very easily in salt water or through a badly wired boat.

The simple reason for pure AC not being a big corrosion driver is it Alternates between Positive and Negative. Not so with DC currents.
_______
@Ken13559 if this is a new problem to your boat and you have not made any recent electrical changes, then I would suspect new arrivals at your marina as the source.

Jim...
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
But when the AC is "going forward" it does damage so 50%
 
Oct 29, 2005
2,355
Hunter Marine 326 303 Singapore
As a precaution for the moment, I've removed shore power connection when I leave the boat. I'll measure for voltage between shaft and seawater. How far down must the anode be lowered? Same level as prop shaft?
 
Feb 14, 2014
7,400
Hunter 430 Waveland, MS
But when the AC is "going forward" it does damage so 50%
:plus: for sure.

But galvanic corrosion is a battery or DC flow. AC is rapidly changing ± and the galvanic chemical reaction is slow to start, so it is somewhat less than DC which is slow and steady.
Tortoise and Hare effect.:)
Jim...
 
Feb 14, 2014
7,400
Hunter 430 Waveland, MS
Corrosion that results from an electrical source causing a metal in contact with an electrolyte (water) to become anodic with respect to some other metal in the same electrolyte.
That is not true.
Pure water is NOT an electrolyte!

If you dissolve salt in water, the salt will ionize and become an electrolytic [passes electricity].
This is another reason AC amps is less of a problem since the water ionization by AC is slow and is suddenly reversed. It is the salt, not the water that passes the amps.

That dang Hare[AC] thought he would win, but no one should ignore him.
Jim...

PS: A lightning strike can ionize pure water.:yikes:
 
Jan 30, 2012
1,123
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
I'll measure for voltage between shaft and seawater. How far down must the anode be lowered? Same level as prop shaft?
Not critical - over the side and a couple of feet from the surface works just fine.

Charles
 
Jan 30, 2012
1,123
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
Ken -

So you are around half the way to replacement time (a bit more actually.) So - how long since you changed zincs?

Followup questions:

Are other underwater metals bonded - that is wired one to the other? If so are any of these other metals - the shaft strut, rudder shaft - protected by their own zincs separate from the shaft zinc? Is the keel bonded (connected by wire to any other underwater metal) including possibly the motor block? If bonding exists and if there are no other zincs except the shaft zincs - then the shaft zincs are 'protecting' everything - hence much fast degradation of those shaft anodes will follow.

Charles
 
Last edited:
Oct 29, 2005
2,355
Hunter Marine 326 303 Singapore
Ken -
So you are around half the way to replacement time (a bit more actually.) So - how long since you changed zincs?
Followup questions:
Are other underwater metals bonded - that is wired one to the other? If so are any of these other metals - the shaft strut, rudder shaft - protected by their own zincs separate from the shaft zinc? Is the keel bonded (connected by wire to any other underwater metal) including possibly the motor block? If bonding exists and if there are no other zincs except the shaft zincs - then the shaft zincs are 'protecting' everything - hence much fast degradation of those shaft anodes will follow.
Charles
Anode lasted 6-9 months. About the only other wetted metal parts that's connected with ground wire is the Keel bolt.
 
Jan 30, 2012
1,123
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
The values your report are completely normal. A 6-9 mo rate of zinc depletion is not particularly rapid. But I am a little confused by your statement that you presently have '6- 9 months zinc life.' Was it 6 or 9 or somewhere in between? Also since you had 6-9 months zinc life the prior prop/shaft at same location then 6-9 months with the new prop and shaft is not out of line.

The normal voltage level means you are not exposed to stray current at the time you measured. 9 months of zinc life means you were probably not exposed to stray current since the last zinc change. 6 months suggests some possible stray current is at work some of the time. 3 months would be even more suggestive.

In any case continue to measure often and regularly. Keep track of the results. That way as conditions change - for example if a shaft zinc fell off or if current were passing via the shaft - you will know right away.

Charles
 
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Oct 29, 2005
2,355
Hunter Marine 326 303 Singapore
Question, anyone experience or heard of sailboat prop shaft breaking under motoring?
I'm still of two minds to change my pitted prop shaft or not.
 

MitchM

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Jan 20, 2005
1,011
Nauticat 321 pilothouse 32 Erie PA
check with your boat manufacturer before contracting out the job. for example, our boat has a stock p shaft which we can order from Vetus for 600$ with a one week delivery time.
 
Oct 29, 2005
2,355
Hunter Marine 326 303 Singapore
Received the prop shaft I ordered from SBO. Nice finishing! And come complete with prop nuts, slot key, cotter pin and coupler! Good deal!! :D
Delivery was fast. From placing order to shipping to my door within 10 days!
That's fantastic.
Thanks Dave G :)

Ken
 
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