On Mon, 28 Nov 2005 12:08:56 -0400, Terry Spragg
wrote:

Jere Lull wrote:

"Gogarty" wrote in message


Put a donut zinc between the strut and the prop.


Uhhh, We HAVE had a zinc on the shaft and the MaxProp's hub as well.
Both get half eaten each year. Before the MaxProp & its zinc, we lost
the same half of the shaft zinc.

In article ,
"Boots" wrote:


why don't you treat the problem and not the cause???


We have no AC connection to dock power. There is no electrical
connection to the shaft or strut; the shaft is isolated by a plastic
drive-saver; electrical resistance between boat ground and shaft is
essentially infinite, so it's not from the battery.

It happened while we were in the mooring field.

Thus, I believe we solved what you believe is the problem already.

The *only* thing it can be is the difference in potential between bronze
and stainless steel.

OR... the potential difference between both steel & bronze and zinc, and
that's what I've been thinking for the past few years.

Truth be told, I *have* thought about switching out the shaft for
bronze. If the zinc still disappears, it's the zinc's problem as there's
nothing left. My only problem with this solution is that bronze bends
quite a bit easier than SS, and I'm more concerned about proper shaft
straightness and alignment than the loss of relatively cheap zincs.


Then you are left shopping for a stainless prop. Zincs may be
cheaper. Go without and watch the prop melt until thr money saved
on zinc offsets the cost of a prop. Or not. A thin insulating
bushing and washer between the shaft, prop and nut may serve you well.


I guess I have mentioned this befo (Sir) Humphrey Davy sold the
British Navy a bill of goods to prevent erosion of their copper
bottoms. It was essentially electrolytic/cathodic protection.
The copper erosion stopped dead. But the marine growth started and
was overwhelming in months. So they dumped the cothodic protection.
The copper slowly dissolves and poisons the fauna.

So here's the scoop.

If you want to protect a lump of bronze from erosion, you could site
a lump of zinc close, or supply a lead or stainless anode close with a
one volt feed..

BUT
if you want to prevent marine growth on the bronze lump, you get rid
of the zinc or the protective anode current and let the copper slowly
dissolve. Yoiu get no marine growth.

OR if you want a really radical alternative to painting a bronze prop
on a stainless shaft, arrange for the shaft to be fed a small current
with say +1 volt wrt the prop. It may grow a garden, but it won't
dissolve, and won't need zincs.

Brian Whatcott Altus OK