Frogwatch wrote:
About three years ago, I moved my sailboat to a slip in a real marina
for the first time ever. I was surprised to see my shaft zinc go away
really fast. Then I noticed the edges of the bronze prop were really
rough and brittle. Uh-oh, So, I needed to get her bottom painted
anyway so had her hauled, not an easy thing to do around here (no
facilities). The bronze prop was a goner so I replaced it with a
PerfectPitch plastic prop that I like. However, the bronze shaft
strut also looked corroded but was good enough to last awhile. Now, I
have a diver check the zinc every 6 weeks.
So, I plan to haul her again soon and am thinking of replacing the
shaft strut.
Today, I dug out the old bronze prop whose blade edges were so
depleted of zinc that I could break off chunks of metal and examined
it. It was only near the blade edges that it was wasted of zinc
except a thin veneer of corrosion all over. The rest of it showed
good bronze when I used the dremel tool on it.
3 yrs ago when she was hauled, a small piece actually broke off near
the cutless bearing holder lip (not structural) causing me to think it
was wasted badly. This experiment makes me not so sure that it is
bad. I can only determine this by using the dremel all over the strut
when she is hauled. Anybody have any experience with this?
Now, I can have a strut made but that will prob take forever, they
have to use the old one as a model and cast it. I will not be sailing
again till next year, no good.

Hmmmm.......I've got a well equipped machine shop, so, maybe I can
make a strut. Maybe I can make a strut base from silicon bronze, make
a strut body from another piece bolted and brazed to base with a
cutless bearing holder brazed to that. Thoughts?


Sounds like that real marina has real bad leakage currents.
About fabricating a shaft strut: brazing silicon bronze just about
guarantees an electrolytic couple. Still it might be worth a try.

Thoughts turn to going medieval with cathodic protection if you can't
clean up the marina leakage. A couple of sheets of stainless close to
the stern, with a steady half volt POSITIVE applied to the plates
compared to the shaft/prop etc.

Or, getting fanciful he how about a chicken wire cage /skirt
draped all round the hull on standoffs, and secured to a good ground
or a little positive?

That would give the electrolytic corrosion something to think about! :-)

Brian W