T&T: Prop shaft tube - corrosion

[Mark Andrew]

Thanks, Lee,

The impressed current system (Siemen's CAPAC) I've got I think will handle
any galvanic current throughout the boat, so hoping I won't need to do the
brush device.  It has a meter on the control panel that reads out the
millivolts being impressed to counteract what the reference electrode is
sensing vis-`-vis the 4 anodes scattered around the hull.

The system comes with a silver alloy slip ring to mount on the shaft, and
silver-graphite brushes to bond the shaft to the system.  This keeps the
voltage difference between the shaft and the hull less than 40 millivolts

http://www.water.siemens.com/en/Product_Lines/Electrocatalytic_Products/Elec
trocatalytic_Products/Pages/capac_impressed_current_cathotic_protection.aspx

The engine was installed by the previous owner without engine
mounts...directly to the very beefy structural members the old Kahlenberg
had been attached to.  Everything is lined up so nicely that I have been
reluctant to lift the engine and reconfigure the steel supports to allow for
engine mounts.  In fact, it's the only thing that hasn't been pulled out of
the boat.  The entire interior, however, has been sandblasted and epoxy
coated with Ameron products (385 & Amercoat II).

I think a careful inspection of what I have inside the shaft tube is
warranted, now that I've read several responses from the listservs.

Thanks,
Mark

-----Original Message-----
From: Lee A Licata [mailto:lazilicata at gmail.com] On Behalf Of LA Licata
Sent: Sunday, February 10, 2008 2:10 PM
To: Mark Andrew; TrawlerList TrawlerList
Subject: Re: T&T: Prop shaft tube - corrosion

Mark,

I would get a monel, copper or (non ss) steel wire braid and ground
the shaft, like we do in the Navy. I would measure the milli-volts
from the shaft while turning underway and the grounding point for the
braid. I do not remember the exact # we wanted in the navy, but I
think it was 1 or 2.

If you want to get it really right, do not use a braid, but a serious
of carbon pads, like those found in DC motors.

But I think that is overkill.

A shaft zinc might also work, but if it does not, you will not know
for years.

But you are right, sea water is designed to flow down the shaft for
lubrication and to keep that area from getting into a low-oxygen
environment. I would also consider running this pump weekly (for an
hour or so) in port if not getting underway that week. Could help
keeping the critters at bay.

I might also consider a paddle wheel just to verify that flow is
actually occurring.

I also hope you will lift the engine, epoxy underneath, and put in
new engine mounts.

Good luck!

Lee


On Feb 10, 2008, at 2030, Mark Andrew wrote:

Greetings all,
...
I'm also assuming that the plumbing to the shaft tube provided fresh sea
water to the tube so that stagnant water would fester in the deep
recess of
the tube.  We plan to plumb from the seachest strainer thru a small pump
direct to the prop-shaft tube such that every time the engine is
turned on,
the pump will create a small flow into the shaft tube.  I figure this
was
the original setup.

My question:

How do we ensure minimal corrosion otherwise in the shaft tube?  I would
imagine like much of the boat, there may be a leaded wipe coating on the
steel, but if this isn't the case (and I've never had the shaft out
of the
tube), how do most steel boats like this protect an inaccessible,
essentially part-of-the-hull, area like this?  So people paint their
shaft
tube?  The tube on the boat is about 6 feet long.

Just wondering what standard practice is.  Worried that the whole
boat will
be epoxy coated and the weak point of the hull be become the prop shaft.

Thanks for any ideas or expertise,

Mark Andrew
"Black River"
Holland, MI
56' steel, circa 1955
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