TWL: Electrolysis
One of the freebie boating booklets suggested last Autumn that shore power
should be disconnected from boats being left in the water but not used
regularly to guard against electrolysis.
As I understand it, two different metals immersed in an electrolyte such as
salt water set up a current between them without any need for external
stimulation, and the lesser metal erodes. Since zinc is, to coin a phrase, a
"least metal", adding sacrificial zincs means the current flows to and
erodes them rather than some valuable and essential component of the vessel.
I assume, therefore, that the recommendation to "unplug" simply removes a
possible source of accelerated deterioration.
However, the latest threads on zinc placements got me wondering. How do you
determine where zincs should be placed, how big they should be, and when you
have enough of them?
My boat has "brush" arrangements on the propeller shafts. How effective are
they?
Finally, could someone tell me if Lehmans have pencil zincs and if so where
they are located?
Cheers, Garrett
My F-L 90s (same design as the 135) has one pencil zinc at the foreward
end, top side, of the heat exchanger. It is held in place by a six sided
nut which threads into the top of the exchanger; the zinc screws into
the bottom of the same nut. The length of the zinc is approximately 2
1/2 inches when new!
Garrett Lambert wrote:
Finally, could someone tell me if Lehmans have pencil zincs and if so where
they are located?
--
Bob McLeran rmcleran@ix.netcom.com
M/V "Sanderling" Docked at Point Patience Marina
Hailing port: Wianno MA Solomons, MD
Hampton 35 Trawler
However, the latest threads on zinc placements got me wondering. How do
you determine where zincs should be placed, how big they should be, and
when you have enough of them? My boat has "brush" arrangements on the
propeller shafts. How effective are they?<<<
There's what appears to be some good stuff on zincs, bonding and prop shaft
brushes in an article called "Corrosion, Zincs And Bonding" which is
available for download from this page:
http://www.kastenmarine.com/pdf_downloads.htm
As regards brushes on the shaft, the problem appears to be maintaining a
clean, grease-free connection which will not block the very low voltages
involved in electrolysis.
Regards, Robert Bryett
Sydney, Australia.
mailto:rbryett@ibm.net
My boat has "brush" arrangements on the propeller shafts. How effective
are
they?
Brushes can be VERY important if your shaft is insulated from the engine
ground - as ours is with a breakable carbon disk - or if your bonding system
is independant of the engine ground - as ours is as well. You are then
totally dependant on the brushes to connect your shaft and props to the
bonding system.
In general, brushes and their contact points on the shaft, should be cleaned
monthly. This is best done with emery cloth. I leave a couple of pieces
near my brushes to remind me to do the cleaning. I clean the brushes when
the boat is stopped but clean the shaft contact point by CAREFULLY reaching
down when the shaft is spinning while the boat is underway.
I have seen a number of boats where the tension on the brushes has not been
maintained. This seems to be a common failing and the result is a brush
that is hardly touching or not touching at all as was the case with a
friends boat that I inspected last week.
My Lehmans have zincs in the heat exchange units. By the way, it is a good
idea to run a ground wire back from the water pump to the end of the main
heat exchange to improve the zinc protection to the water pump. Of course
the older pumps should also be replaced before the teeth start to fail and
make a real mess.
Shaun aboard Rana III
"I'm lehman on a GB..." sung to the tune of ...
Finally, could someone tell me if Lehmans have pencil zincs and if so
where
they are located?
Cheers, Garrett