From: STU@mwvm.mitre.org
Subject: Re: Question about zinc...
Organization: The MITRE Corporation, McLean VA 22102
Date: Fri, 20 Nov 1992 13:05:27 GMT

For my 2 cents worth:
 
1.  The rate of erosion of the zinc is dependent of several factors.  Assuming
there are no stray electrical currents, zinc erodes roughly propostional to
the amount of less nobel metal present on the boat.  Consider a number line,
all metals fit on the line - stretching from a "-" point to a "+" point -
and roughly, metals generate a voltage - when placed in an electrolite such
as salt water - or battery acid - that is proportional to the distance they
are apart on the number line.  For example, if your boat were made of
carbon and zinc, you would have a potential of 1.54 volts (a common "D" cell)
between the two metals.
 
2.  Now, in theory, any two dissimilar metals serve as a battery - or actually
a cell.  Thus, if you have stainless steel and bronze present, the less nobel
would erode in favor of the more nobel - causing your thru hulls to fail
with corosion after a period of time.
 
3.  If you examine your wheel - or prop - and it was bottom painted, you
will probably notice severe corosion on the bronze wheel.  I had one fail over
the 4th of July on a borrowed friend's boat - because the bottom paint is
less nobel than the wheel - and they are in very close contact.  That is why
most people recommend not painting bronze thru hulls and wheels - and most
painters ignore the recommendation.
 
4.  Zinc is more sacrificial than either stainless or bronze - or the iron
of your engine block.  Thus, in theory, the zinc erodes and deposits a coating
of zinc on the parts you want to have last the life of the boat.  In theory
also ( a conflicting theory), all dissimilar metals interact independently -
thus you may not be surprised to see some wheel erosion on an otherwise
protected boat.
 
5.  How can you tell?  Buy or barrow a guppy or some other zinc piece to hang
over the side.  I honestly don't think it matters what the alloy is - since
the zinc is the sacrifical material and the test is done with new, clean
zinc.  Hang it on a wire, place a digital volt meter between the wire and
the ground point on the boat (the block, a ground plate, your LORAN ground)
and read the voltage.  Remember the zinc is the negative terminal of a cell -
so some voltage is always present.  In my experience, a voltage of 0.2 or
less translates to about 1 year of zinc life on a prop shaft.  I have seen
voltages of 8 or more at a marina with a poor ground connection.  Repeat
the test with the boat plugged into shore power and unplugged.  If there is
more than a 0.5 volt difference on plugging the boat in, either buy a
cathodic protection system, buy an isolaton transformer, or move marinas.
 
Don't necessarly blaim the marina for poor electrical service - it may
be other boats with aluminium outdrives that are forming a circuit with
your boat and there is nothing the marina can do.
 
One final thing, the red dodad in your prop shaft between your transmission
and the wheel is a "safe drive".  Its purpose is to prevent damage to the
transmission when you hit a rock, snag a crab trap or some other severe action.
 
Sometimes the drive survives the damage - and protects the transmission.
Sometimes the drive fails and protects the transmission.
Sometimes the drive fails and the transmission fails.
 
If you hit something, resist the urge to replace the bolts with normal bolts
for other than a short time in an emergency.  The bolts are "calibrated"
(look at their price!) to protect your transmission.
 
Happy zincing.  /Stu