My stray current, prop pitting experience.

When I first launched the Navigator back when I bought it, it was a brandy
new boat with new, shiney 30x30 props.
My first slip in Scituate was located right beside the marina main shore
power cables. After the first season we hauled the boat for hull
maintenance before taking it on the trip to Florida.

When it was hauled, we noticed that the edge of one of the props had some
very small, sharp edges pitted into the bronze. Somebody mentioned that it
looked like electrolysis and I should keep an eye on it. Although the boat
is equipped with an automatic electronic system that is supposed to
neutralize stray currents, along with the standard zincs, we suspected that
the close proximitry of the props to the submerged marina power cable was
likely the cause, particularly because only one prop (the one closest to the
cable) had any evidence of pitting.

When we got to Florida, I arranged for a monthly, in-water hull cleaning
service (required in the warm, Florida waters). I happened to be on the
boat one day when the diver was working on my boat and he surfaced to tell
me that, BTW, he was feeling a very mild "tingle" whenever he touched my
props or shafts. Not good. I investigated everything I could think of,
including disconnecting shore power and having the diver touch the prop.
Under this condition he couldn't detect any tingle.

I ended up calling Navigator, who in turn got me in touch with Volvo, the
engine manufacturer and installer. The engineer at Volvo told me that by
design, Volvo isolated the engines, shafts and props from the boat's bonding
system to prevent stray currents. H told me to look for any accessories
added to the boat that bonded the engine ...either through the battery or
directly.

It turns out that when the bow and stern thrusters were installed, their
dedicated batteries were connected to the main engine batteries for
recharging. The ground and negative leads of the thrusters were
electrically connected and the ground was tied to the boat bond, thereby
bonding the engines. The problem was resolved by adding a second, dual
battery charger dedicated to the thruster batteries.

One of those boat mysteries resolved and recorded in my head.

Eisboch

" JimH" jimhUNDERSCOREosudad@yahooDOTcom wrote in message
...

"Eisboch" wrote in message
news

" JimH" jimhUNDERSCOREosudad@yahooDOTcom wrote in message
...


I will be adding a magnesium tab on the outdrive and take up your
suggestion of the submerged magnesium fish connected to my grounding
system. A friend of mine who had an aluminum hull boat (Marinette) used
to dunk one of those anodes whenever he got into the dock.


Are you in salt water or fresh?

Seems to me that there are different anode materials for each.

Also ... this may help explain some of the mystery ... pertains to salt
water ..

http://www.boatus.com/boattech/casey/23.htm


Eisboch


I am in fresh water. I *assume* the anodes are magnesium, but they are
most likely original (since the boat lived it's previous life on a trailer
or in inland lakes) so who knows.

I am pulling the boat next week to do some work on it and will most likely
replace the anodes with magnesium anodes, plus adding a magnesium tab on
the cavitation plate.

Thanks again for all your help on this Richard.

BTW: I was obviously out of line with you in the recent past. You
continue to show yourself to be a class act and key contributor here.
Thanks for looking past that big mistake of mine and moving forward. You
are a good man and my observations were most obviously off base.

Can I safely say "I apologize to you" for making yet another mistake in
this NG?


No problem and thanks.

Eisboch

"Eisboch" wrote in message
news

" JimH" jimhUNDERSCOREosudad@yahooDOTcom wrote in message
...

"Eisboch" wrote in message
news

" JimH" jimhUNDERSCOREosudad@yahooDOTcom wrote in message
...


I will be adding a magnesium tab on the outdrive and take up your
suggestion of the submerged magnesium fish connected to my grounding
system. A friend of mine who had an aluminum hull boat (Marinette)
used to dunk one of those anodes whenever he got into the dock.


Are you in salt water or fresh?

Seems to me that there are different anode materials for each.

Also ... this may help explain some of the mystery ... pertains to salt
water ..

http://www.boatus.com/boattech/casey/23.htm


Eisboch


I am in fresh water. I *assume* the anodes are magnesium, but they are
most likely original (since the boat lived it's previous life on a
trailer or in inland lakes) so who knows.

I am pulling the boat next week to do some work on it and will most
likely replace the anodes with magnesium anodes, plus adding a magnesium
tab on the cavitation plate.

Thanks again for all your help on this Richard.

BTW: I was obviously out of line with you in the recent past. You
continue to show yourself to be a class act and key contributor here.
Thanks for looking past that big mistake of mine and moving forward.
You are a good man and my observations were most obviously off base.

Can I safely say "I apologize to you" for making yet another mistake in
this NG?


No problem and thanks.

Eisboch


Thanks Richard.

JimH wrote:
I took the day off and spent most of it on the boat with my wife and some
friends. We decided to do some swimming so we anchored at our favorite
swimming hole.

While in the water I noticed that the paint is starting to let go along the
edges of my outdrive. The outdrive has magnesium anodes in all the
correct places but there is no tab anode on the back end of the cavitation
plate.

This is the first time this boat has ever spent life in the water at a
marina........it was always a trailered boat in the past and the outdrive
was in pristine condition cosmetically when I purchased the boat last
summer.

Shore power is available at my dock but I do not use it other then when
working on the boat. The boat does not have a galvanic isolator.

I am planning on adding the tab anode on back end of the outdrive cavitation
plate. I am also considering adding a galvanic isolator to the boats DC
electrical system.

Is this a good decision or am acting prematurely before doing other trouble
shooting (and what would that trouble shooting include?)

If the galvanic isolator is a good add on ($150 on a Guest 50 amp on Ebay),
what amp size should I consider? 30? 50?

TIA for your help!

Is there corrosion where the paint is peeling? If not, it's probably
just the paint job.