All,

I took some more careful measurements. One of the things I noticed is
that the current I read needs 30 seconds or so to settle down
sometimes (e.g. it reads a higher current at first and then goes
lower), also the analog and digital meters have their strengths so I
used a combination of the two to get better readings last night.

Boat = Beneteau 393, 39 foot long 14 foot wide, at a slip in salt
water, cable and zinc fish below are off starboard side abreast
cockpit, approx. abrest the propeller. Slip is on the starboard side.
Power boat on my port side, sailboat across the walk on my starboard
side.

Zinc Fish = brand new fish with 25' cable, typical west marine type
product, at a depth of 3 feet

Copper Cable = new 25' 10 gauge copper cable, stranded, with 1/4 inch
exposed to water at a depth of 3 feet.

Boat ground = a 19 foot, 14 gauge cable that runs from the starter
battery upto a stainless steel bolt mounted in cockpit locker for zinc
fish to attach too, brand new, all connections are free of corrosion.

All measurements below are with the black cable on the VOM meters
attached to boat ground. The Analog meter, a small portable, has a
range of 10uA to 250mA. The Digital meter has a range of 1mA to 10A.
Unless otherwise written shore power cord is disconnected.

[1. With minimal power on (Link 20, some lights)]
Copper cable to boat ground = 0.4 mA (e.g. 400uA), 4.5 Ohms, 0.733
volts
Fish to boat ground = -42mA, 0.6 Ohms, -0.127 volts
Cooper cable to fish (black) = 3.5 Ohms, +0.84 volts
Fish to copper cable (black) = 37 Ohms

Interesting !! Is the fish causing this ? Is it harmless ?
Should I consider not connecting the fish to boat ground ?

[2. With All batteries disconnected, everything off accept bilge pump
and Link-20]
Fish to boat ground = -42mA
House Load to boat ground = 30 KOhms (anything better than 1kOhms is
considered acceptable in most books, as evidence of very low
leakage).

[3. With virtually all DC power on (27.5 amps)]
Fish to boat ground = -27mA

[4. With minimal power, shore power connected, and battery charger
running]
Fish to boat gorund = -50mA to -70mA, varies as battery charger cycles
thru charging batteries (it cycles between charging the starter
battery and house battery)

What action should I take ?

b393capt wrote in
ups.com:

I took some more careful measurements. One of the things I noticed is
that the current I read needs 30 seconds or so to settle down
sometimes (e.g. it reads a higher current at first and then goes
lower), also the analog and digital meters have their strengths so I
used a combination of the two to get better readings last night.


To put the ammeter in series, you had to disconnect the fish,
temporarily, while it was still in the water. This allowed its ion
stream to wash away so when you reconnected it there was a great rush of
new seawater on the surface of the fish, resulting in an inrush of
current you measured. After a few seconds to minutes, the water
surrounding the fish was saturating with zinc ions that had been corroded
off the fish, insulating it some from further galvanic action unless the
seawater was rushing by. So, you saw the current drop as time increased,
which is quite normal. Super accurate readings are useless....here.

Larry
--
Grade School Physics Factoid:
A building cannot freefall into its own footprint without
skilled demolition.

b393capt wrote in
ups.com:

What action should I take ?


NONE! 50ma is nothing! You're seeing the current between the zinc anode
and all the underwater metal it is PROTECTING! You have a zinc anode and
a some-other-metal (brass, stainless, etc.) prop/shaft/skeg/etc. The
direct connection of these creates a SHORTED BATTERY in a salt water
electrolyte. The small emf this produces creates the current that you
are measuring, which is attempting to PLATE the boat parts we want to
save and EAT the fish, which is exactly what we want because that's what
makes it work. The readings you're seeing are wonderful!

(PS - you cannot make resistance readings with any meter in the presence
of an external voltage (the EMF the fish/prop battery is producing).
NEVER connect your ohmmeter to ANY source of voltage which may destroy
it...ok? Thanks.

Larry, Electronic Metrology since 1965
--
Grade School Physics Factoid:
A building cannot freefall into its own footprint without
skilled demolition.

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

On May 15, 12:13 pm, Larry wrote:
b393capt wrote roups.com:

What action should I take ?

NONE! 50ma is nothing! You're seeing the current between the zinc anode
and all the underwater metal it is PROTECTING! You have a zinc anode and
a some-other-metal (brass, stainless, etc.) prop/shaft/skeg/etc. The
direct connection of these creates a SHORTED BATTERY in a salt water
electrolyte. The small emf this produces creates the current that you
are measuring, which is attempting to PLATE the boat parts we want to
save and EAT the fish, which is exactly what we want because that's what
makes it work. The readings you're seeing are wonderful!

(PS - you cannot make resistance readings with any meter in the presence
of an external voltage (the EMF the fish/prop battery is producing).
NEVER connect your ohmmeter to ANY source of voltage which may destroy
it...ok? Thanks.

Larry, Electronic Metrology since 1965
--
Grade School Physics Factoid:
A building cannot freefall into its own footprint without
skilled demolition.

Larry,

Will this actually plate my prop, shaft, etc ? Is that desirable ?

Nothing seams out of sorts with the measurements above ? Doesn't it
seem strange to get these reading when I already have a zinc
protecting my prop ?

Thanks,
Dan

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 in
oups.com:

Will this actually plate my prop, shaft, etc ? Is that desirable ?

Nothing seams out of sorts with the measurements above ? Doesn't it
seem strange to get these reading when I already have a zinc
protecting my prop ?


I just read about an iron keel. Is that yours, too? The more metal
underwater, the higher the current's going to be.

It won't plate your prop so you'll notice because the currents and
distance between the zinc and the prop will carry off the ions. Any ions
passing your prop/shaft that get sucked into them by the galvanic current
will plate (and I suppose protect) them, which isn't bad at all.

The more zincs you put in the water, the more total galvanic current will
result from the exposed surface area of more and more zinc anodes. The
current you're measuring is just from the fish. There's even more
current from all the other zincs, especially the shaft zinc wrapped right
around the shaft with much less series resistance.

Want to see lots more current? Connect a big battery plate, lead,
through an ammeter to the ground post in the boat and drop it over the
side. Current will go up when it hits the water.

What you're effectively doing with all these extra zincs is making a
bigger shorted battery with larger plate surface areas. This ISN'T going
to "save" the shaft zinc, by the way. Its natural galvanic current isn't
determined by the additional fish zinc's galvanic current, at all.
You're just making the battery bigger.

Now, if we could only get the damned zinc current from the fish to LIGHT
THE CABIN LEDs!....(c;

Larry
--
Grade School Physics Factoid:
A building cannot freefall into its own footprint without
skilled demolition.

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