b393capt wrote:
Chuck,

{From Prior thread}
Here are two quick tests you can make:

1) Measure the voltage between the fish
and the engine stud, noting the
polarity. This may confirm that
something is adding to the galvanic
couple set up by the zinc and the prop.



-- With the engine connected to the black wire, and the fish on the
red wire, I got -127mV


2. Disconnect the battery positive
terminal wire and see if your symptoms
change. If you have a breaker at the
battery, or a switch, or a fuse, then
there is no need to remove the wire, as
long as absolutely nothing is connected
to the battery terminal. That includes
bilge pumps, etc. Be careful working
around the battery.


-- No change, between the case of batteries disconnected and running
minimal DC. Both had - 42 mA; However, when I increased the number
of
devices I had in use I saw a change to -21 mA. I realize these
readings are different then yesterday, I used better equipment and
measurement process (see other thread)

-- You made an earlier point, summarized here, that you don't expect
to see any potential if there are two zincs in close proximity both
connected to boat ground (the prop and the fish). That makes a lot of
sense to me, does that statement still make sense to you ?


-- Please look at the new thread and comment





I think 50mA is excessive under the
circumstances, and the 127 mV is not the
right potential difference between a
bronze (i.e., copper) prop and a zinc.

If I understand correctly, you have
connected the fish with a copper cable?

That's OK as long as the copper is not
submerged in sal****er. The zinc fishes
are normally supplied with a stainless
cable, both to support the weight, and
to discourage creation of a galvanic
cell consisting of the wire and the zinc
fish.

Try pulling the fish, hosing the
connection to the copper with fresh
water and submerging without the copper
touching the water. Yeah, I know. Can't
really be done. One thing that will work
in a hurry is a stainless welding rod of
three feet or so between the fish and
the copper cable. Ideally, I guess you
would silver solder the copper to the
stainless, but an alligator clip will do
the job long enough for a measurement.
I'd also attach a rope to the fish to
prevent losing it.

I still don't see how this would explain
your other symptoms, but it could
explain the 50mA.

Chuck

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Chuck,

The copper wire I was referring to is entirely unrelated to the
fish. What I was doing is comparing the reading I would get from
connecting the zinc fish to boat ground vs. connecting a bare (at the
tip) copper wire in the water to boat ground. (in both cases with my
meter inbetween measuring current). My expectation is that by looking
at a change in voltage and current, I could learn something about
what's going on. E.g. while I do measure a current with the fish,
maybe the fish is inducing it. Now that I have the readings ... don't
know how to interpret.

My keel is iron and ideally has a succesful barrier coat of paint
between it and the salt water, as I know the keel is connected to my
boats grounding system (lightening protection). I also have a rudder
that I don't believe is tied into ground, but I am not sure. So is two
or three points under my boat creating a battery ... is the only two
points between the prop and my fish ?

Dan

b393capt wrote:
Chuck,

The copper wire I was referring to is entirely unrelated to the
fish. What I was doing is comparing the reading I would get from
connecting the zinc fish to boat ground vs. connecting a bare (at the
tip) copper wire in the water to boat ground. (in both cases with my
meter inbetween measuring current). My expectation is that by looking
at a change in voltage and current, I could learn something about
what's going on. E.g. while I do measure a current with the fish,
maybe the fish is inducing it. Now that I have the readings ... don't
know how to interpret.

My keel is iron and ideally has a succesful barrier coat of paint
between it and the salt water, as I know the keel is connected to my
boats grounding system (lightening protection). I also have a rudder
that I don't believe is tied into ground, but I am not sure. So is two
or three points under my boat creating a battery ... is the only two
points between the prop and my fish ?

Dan



Hello Dan,

Sorry for my misunderstanding about the
copper wire. And sorry to lead you on a
"fishing" trip.

So with the meter between the fish and
the copper wire dropped into the water,
you measured about 0.8 volts? That's
what you should be getting with that
arrangement.

I hadn't even thought about the keel.
Somehow I thought it was lead. But if
the fish is protecting an iron keel,
that could explain the 40-50 mA as well
as the 0.127 volt measurement. It would
also explain the accelerated erosion of
your prop zinc.

Theoretically, the barrier coat ought to
protect the iron keel, as you said, but
it is not unusual for zinc anodes to be
welded to it anyway.

It may be worth having a corrosion
technician check things out in person.
Email analyses tend to by awfully
hypothetical as you've seen. You might
also contact the builder to see if they
have a recommended fix.

Meanwhile, the fish is probably a good
measure to take some of the load off of
your prop zinc.

If the rudder is not bonded to the
boat's ground system, it's not likely to
be affected by, or affecting, the fish.

Chuck

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"b393capt" wrote in message
ups.com...
| Chuck,
|
| The copper wire I was referring to is entirely unrelated to the
| fish. What I was doing is comparing the reading I would get from
| connecting the zinc fish to boat ground vs. connecting a bare (at the
| tip) copper wire in the water to boat ground. (in both cases with my
| meter inbetween measuring current). My expectation is that by looking
| at a change in voltage and current, I could learn something about
| what's going on. E.g. while I do measure a current with the fish,
| maybe the fish is inducing it. Now that I have the readings ... don't
| know how to interpret.
|
| My keel is iron and ideally has a succesful barrier coat of paint
| between it and the salt water, as I know the keel is connected to my
| boats grounding system (lightening protection). I also have a rudder
| that I don't believe is tied into ground, but I am not sure. So is two
| or three points under my boat creating a battery ... is the only two
| points between the prop and my fish ?
|
| Dan
|
|
|
|
Hi,
For lightning protection, your keel should be connected to the mast, not to
the boat's ground.
Disconnect the keel from ground and measure again. Also, there are 2 schools
of thought about bonding all bronze thru-hull fittings. Recommended reading:
Nigel Calder.
Do you have a saildrive or a shaft?
Cheers,
Wout

WBH wrote:

Hi,
For lightning protection, your keel should be connected to the mast, not to
the boat's ground.

These are not mutually exclusive
options, of course. Typically, the keel
is connected to the mast and to the
boat's "ground".

Isn't a fair amount of imagination
required in order to derive a benefit
from isolating the DC ground from the keel?

Disconnect the keel from ground and measure again.

If that can be easily done, it might
confirm the diagnosis.

Also, there are 2 schools
of thought about bonding all bronze thru-hull fittings.

There are some strange theories about
bonding metal thru-hull fittings as part
of a lightning protection strategy, but
from a corrosion perspective, the
science is clear.

A quality bronze thru-hull, unbonded,
and immersed in seawater may well
outlast a fiberglass hull! Bonding
(i.e., attaching) a bronze prop to a
stainless shaft is what creates the need
for a sacrificial anode in the first
place. If you hang the bronze prop over
the side on a rope, you will have to
wait decades before you notice any
corrosion. It won't propel the boat, of
course, but think of what you'll save on
zincs!


Recommended reading:
Nigel Calder.
Do you have a saildrive or a shaft?
Cheers,
Wout



Chuck

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"Chuck" wrote in message
...
| WBH wrote:
|
| Hi,
| For lightning protection, your keel should be connected to the mast, not
to
| the boat's ground.
|
| These are not mutually exclusive
| options, of course. Typically, the keel
| is connected to the mast and to the
| boat's "ground".
|
| Isn't a fair amount of imagination
| required in order to derive a benefit
| from isolating the DC ground from the keel?

No, I don't think so. Isn't galvanic corrosion all about creating
"batteries" by putting dissimilar metals in seawater and by connecting them
galvanically to allow electron-flow? There will be no "battery action" if
the metals are not connected inside the boat.
For example, an iron keel and an alluminium saildrive can form a nice
battery if they are connected through the boat's ground. Of course there are
many factors that determine "how well" the battery works, such as the
distance between the 2 metals. Naturally, by galvanically connecting an even
less nobel metal such as zinc to the anode of the 'battery' (the saildrive),
this metal will be sacrificed first, but doesn't it make sense to disconnect
the dissimilar metals inside the boat in the first place, i.e. isolate the
keel from ground?
In a fibreglass boat like a Beneteau this should be no problem at all.
Most European boats do not have the thru-hull fittings bonded. For some
reason this is much more common practice in the USA. I've never understood
why. The theory is that the potential is equalized, but at the same time it
can form 'batteries' if the bonded metals are dissimilar.
On one of my previous boats I had mistakenly used the keel as a counterpoise
for my SSB installation, effectively connecting it to ground. As the Volvo
saildrive was also connected to ground, the zinc anode had been eaten away
in weeks and the galvanic corrosion process had started to eat away the
saildrive. This happened while moored and the batteries were switched off.
As soon as I disconnected the keel from ground and used a porous bronze
groundplate for the SSB instead, the zinc anode corroded at the normal rate
(1 every 2 years) again.

Wout

|
| Disconnect the keel from ground and measure again.
|
| If that can be easily done, it might
| confirm the diagnosis.
|
| Also, there are 2 schools
| of thought about bonding all bronze thru-hull fittings.
|
| There are some strange theories about
| bonding metal thru-hull fittings as part
| of a lightning protection strategy, but
| from a corrosion perspective, the
| science is clear.





|
| A quality bronze thru-hull, unbonded,
| and immersed in seawater may well
| outlast a fiberglass hull! Bonding
| (i.e., attaching) a bronze prop to a
| stainless shaft is what creates the need
| for a sacrificial anode in the first
| place. If you hang the bronze prop over
| the side on a rope, you will have to
| wait decades before you notice any
| corrosion. It won't propel the boat, of
| course, but think of what you'll save on
| zincs!
|
|
| Recommended reading:
| Nigel Calder.
| Do you have a saildrive or a shaft?
| Cheers,
| Wout
|
|
|
| Chuck
|
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News==----
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WBH wrote:
"Chuck" wrote in message
...
| WBH wrote:
|
| Hi,
| For lightning protection, your keel should be connected to the mast, not
to
| the boat's ground.
|
| These are not mutually exclusive
| options, of course. Typically, the keel
| is connected to the mast and to the
| boat's "ground".
|
| Isn't a fair amount of imagination
| required in order to derive a benefit
| from isolating the DC ground from the keel?

No, I don't think so. Isn't galvanic corrosion all about creating
"batteries" by putting dissimilar metals in seawater and by connecting them
galvanically to allow electron-flow? There will be no "battery action" if
the metals are not connected inside the boat.
For example, an iron keel and an alluminium saildrive can form a nice
battery if they are connected through the boat's ground. Of course there are
many factors that determine "how well" the battery works, such as the
distance between the 2 metals. Naturally, by galvanically connecting an even
less nobel metal such as zinc to the anode of the 'battery' (the saildrive),
this metal will be sacrificed first, but doesn't it make sense to disconnect
the dissimilar metals inside the boat in the first place, i.e. isolate the
keel from ground?
In a fibreglass boat like a Beneteau this should be no problem at all.
Most European boats do not have the thru-hull fittings bonded. For some
reason this is much more common practice in the USA. I've never understood
why. The theory is that the potential is equalized, but at the same time it
can form 'batteries' if the bonded metals are dissimilar.
On one of my previous boats I had mistakenly used the keel as a counterpoise
for my SSB installation, effectively connecting it to ground. As the Volvo
saildrive was also connected to ground, the zinc anode had been eaten away
in weeks and the galvanic corrosion process had started to eat away the
saildrive. This happened while moored and the batteries were switched off.
As soon as I disconnected the keel from ground and used a porous bronze
groundplate for the SSB instead, the zinc anode corroded at the normal rate
(1 every 2 years) again.

Wout

|
| Disconnect the keel from ground and measure again.
|
| If that can be easily done, it might
| confirm the diagnosis.
|
| Also, there are 2 schools
| of thought about bonding all bronze thru-hull fittings.
|
| There are some strange theories about
| bonding metal thru-hull fittings as part
| of a lightning protection strategy, but
| from a corrosion perspective, the
| science is clear.





|
| A quality bronze thru-hull, unbonded,
| and immersed in seawater may well
| outlast a fiberglass hull! Bonding
| (i.e., attaching) a bronze prop to a
| stainless shaft is what creates the need
| for a sacrificial anode in the first
| place. If you hang the bronze prop over
| the side on a rope, you will have to
| wait decades before you notice any
| corrosion. It won't propel the boat, of
| course, but think of what you'll save on
| zincs!
|
|
| Recommended reading:
| Nigel Calder.
| Do you have a saildrive or a shaft?
| Cheers,
| Wout
|
|
|
| Chuck
|
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Ah! Glad to see we're on the same
frequency regarding galvanic couples.
Isolation is a great deterrent to corrosion.

But the context of my comment was the
reference to lightning protection,
rather than to corrosion.

Chuck


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WBH wrote:
"Chuck" wrote in message
...
| WBH wrote:
|
| Hi,
| For lightning protection, your keel should be connected to the mast, not
to
| the boat's ground.
|
| These are not mutually exclusive
| options, of course. Typically, the keel
| is connected to the mast and to the
| boat's "ground".
|
| Isn't a fair amount of imagination
| required in order to derive a benefit
| from isolating the DC ground from the keel?

No, I don't think so. Isn't galvanic corrosion all about creating
"batteries" by putting dissimilar metals in seawater and by connecting them
galvanically to allow electron-flow? There will be no "battery action" if
the metals are not connected inside the boat.
For example, an iron keel and an alluminium saildrive can form a nice
battery if they are connected through the boat's ground. Of course there are
many factors that determine "how well" the battery works, such as the
distance between the 2 metals. Naturally, by galvanically connecting an even
less nobel metal such as zinc to the anode of the 'battery' (the saildrive),
this metal will be sacrificed first, but doesn't it make sense to disconnect
the dissimilar metals inside the boat in the first place, i.e. isolate the
keel from ground?
In a fibreglass boat like a Beneteau this should be no problem at all.
Most European boats do not have the thru-hull fittings bonded. For some
reason this is much more common practice in the USA. I've never understood
why. The theory is that the potential is equalized, but at the same time it
can form 'batteries' if the bonded metals are dissimilar.
On one of my previous boats I had mistakenly used the keel as a counterpoise
for my SSB installation, effectively connecting it to ground. As the Volvo
saildrive was also connected to ground, the zinc anode had been eaten away
in weeks and the galvanic corrosion process had started to eat away the
saildrive. This happened while moored and the batteries were switched off.
As soon as I disconnected the keel from ground and used a porous bronze
groundplate for the SSB instead, the zinc anode corroded at the normal rate
(1 every 2 years) again.

Wout

|
| Disconnect the keel from ground and measure again.
|
| If that can be easily done, it might
| confirm the diagnosis.
|
| Also, there are 2 schools
| of thought about bonding all bronze thru-hull fittings.
|
| There are some strange theories about
| bonding metal thru-hull fittings as part
| of a lightning protection strategy, but
| from a corrosion perspective, the
| science is clear.





|
| A quality bronze thru-hull, unbonded,
| and immersed in seawater may well
| outlast a fiberglass hull! Bonding
| (i.e., attaching) a bronze prop to a
| stainless shaft is what creates the need
| for a sacrificial anode in the first
| place. If you hang the bronze prop over
| the side on a rope, you will have to
| wait decades before you notice any
| corrosion. It won't propel the boat, of
| course, but think of what you'll save on
| zincs!
|
|
| Recommended reading:
| Nigel Calder.
| Do you have a saildrive or a shaft?
| Cheers,
| Wout
|
|
|
| Chuck
|
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An additional thought:

If your iron keel was exposed enough to
form a significant galvanic couple with
the saildrive, isolating it protects the
saildrive, as you indicated. But the
iron keel may now be pitting and
corroding if there are no sacrificial
anodes on the keel itself.

Chuck

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"Chuck" wrote in message
...
| WBH wrote:
| "Chuck" wrote in message
| ...
| | WBH wrote:
| |
| | Hi,
| | For lightning protection, your keel should be connected to the mast,
not
| to
| | the boat's ground.
| |
| | These are not mutually exclusive
| | options, of course. Typically, the keel
| | is connected to the mast and to the
| | boat's "ground".
| |
| | Isn't a fair amount of imagination
| | required in order to derive a benefit
| | from isolating the DC ground from the keel?
|
| No, I don't think so. Isn't galvanic corrosion all about creating
| "batteries" by putting dissimilar metals in seawater and by connecting
them
| galvanically to allow electron-flow? There will be no "battery action"
if
| the metals are not connected inside the boat.
| For example, an iron keel and an alluminium saildrive can form a nice
| battery if they are connected through the boat's ground. Of course there
are
| many factors that determine "how well" the battery works, such as the
| distance between the 2 metals. Naturally, by galvanically connecting an
even
| less nobel metal such as zinc to the anode of the 'battery' (the
saildrive),
| this metal will be sacrificed first, but doesn't it make sense to
disconnect
| the dissimilar metals inside the boat in the first place, i.e. isolate
the
| keel from ground?
| In a fibreglass boat like a Beneteau this should be no problem at all.
| Most European boats do not have the thru-hull fittings bonded. For some
| reason this is much more common practice in the USA. I've never
understood
| why. The theory is that the potential is equalized, but at the same time
it
| can form 'batteries' if the bonded metals are dissimilar.
| On one of my previous boats I had mistakenly used the keel as a
counterpoise
| for my SSB installation, effectively connecting it to ground. As the
Volvo
| saildrive was also connected to ground, the zinc anode had been eaten
away
| in weeks and the galvanic corrosion process had started to eat away the
| saildrive. This happened while moored and the batteries were switched
off.
| As soon as I disconnected the keel from ground and used a porous bronze
| groundplate for the SSB instead, the zinc anode corroded at the normal
rate
| (1 every 2 years) again.
|
| Wout
|
| |
| | Disconnect the keel from ground and measure again.
| |
| | If that can be easily done, it might
| | confirm the diagnosis.
| |
| | Also, there are 2 schools
| | of thought about bonding all bronze thru-hull fittings.
| |
| | There are some strange theories about
| | bonding metal thru-hull fittings as part
| | of a lightning protection strategy, but
| | from a corrosion perspective, the
| | science is clear.
|
|
|
|
|
| |
| | A quality bronze thru-hull, unbonded,
| | and immersed in seawater may well
| | outlast a fiberglass hull! Bonding
| | (i.e., attaching) a bronze prop to a
| | stainless shaft is what creates the need
| | for a sacrificial anode in the first
| | place. If you hang the bronze prop over
| | the side on a rope, you will have to
| | wait decades before you notice any
| | corrosion. It won't propel the boat, of
| | course, but think of what you'll save on
| | zincs!
| |
| |
| | Recommended reading:
| | Nigel Calder.
| | Do you have a saildrive or a shaft?
| | Cheers,
| | Wout
| |
| |
| |
| | Chuck
| |
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120,000+
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|
|
| An additional thought:
|
| If your iron keel was exposed enough to
| form a significant galvanic couple with
| the saildrive, isolating it protects the
| saildrive, as you indicated. But the
| iron keel may now be pitting and
| corroding if there are no sacrificial
| anodes on the keel itself.
|
| Chuck
|

No Chuck, because if it is not connected to anything inside the
(fibreglass)boat, there is no 'battery circuit' with the keel as anode and a
more noble metal as cathode.
My present boat has a cast iron keel without any zinc anodes. It is not
connected to ground, just to the mast to provide a lightning path.

For the steel to get pitted, there would need to be a cathode of a more
noble metal. Surely if a zinc anode is bolted to a cast iron keel, it may
(slowly) be eaten away, but not necessarily because it protects the keel. I
have often seen zinc anodes on steel keels that had been on it for years.

The corrosion problem I had due to the SSB counterpoise was actually with a
lead keel, but that is irrelevant, because both cast iron and lead are more
noble (cathodic) than aluminium.

Wout