Hi, I have put some pictures he
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Search for images named "sabina113"

I find when I take the yacht out of the water each year I have bubbles
in the paint mainly where things have been welded to the inside of the
hull. The bubbles are filled with salt water which is quite strong and
possibly acidy. When the bubbles are opened there is very clean steel
underneath. It doesn't seemed to have corroded the steet alot. The
zincs last a year ok.
I guess this is some sort of electrosis or stray current erosion but
was hopeing for a second opinion.
Also, if anyone looks at the pictures, have you seen a driveshaft
thrust bearing setup like in the picture. There is a small brass bung
on top where the thrust bearing is. Do you put oil in there?
Thanks
asdex

wrote:
Hi, I have put some pictures he
www.photobucket.com/
Search for images named "sabina113"
try this URL :-)
http://s45.photobucket.com/albums/f87/sabina113/

I find when I take the yacht out of the water each year I have bubbles
in the paint mainly where things have been welded to the inside of the
hull. The bubbles are filled with salt water which is quite strong and
possibly acidy. When the bubbles are opened there is very clean steel
underneath. It doesn't seemed to have corroded the steet alot. The
zincs last a year ok.
I guess this is some sort of electrosis or stray current erosion but
was hopeing for a second opinion.
Also, if anyone looks at the pictures, have you seen a driveshaft
thrust bearing setup like in the picture. There is a small brass bung
on top where the thrust bearing is. Do you put oil in there?
Thanks
asdex


I'd bet the bubbles actually contain an alkaline fluid. Its possible
that you are over-zinked, which is known to cause problems with the
paint like this.

I may not have the answers, but I do have some questions that ought to
be asked.

Are you in a marina with shorepower?
If so do you leave the boat hooked up and is there an isolation
transformer or galvanic isolator? (It may not be YOUR zincs that are
providing most of your protection!)

What primer was used? Then whats on top of that? Did you follow the
paint's instructions ACCURATELY, especially correct thinners,
over-coating interval, temperature and relative humidity limits?

Have you had the hull shot-blasted back to bright metal and fully
repainted at any time or have you been dealing with patches as you go?

Exactly what have you been doing to fix the patches and are the bubbles
coming back in the same places?

Have you consulted with your paint manufacturer's technical guys?

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & 32K emails -- NUL:
'Stingo' Albacore #1554 - 15' Early 60's, Uffa Fox designed,
All varnished hot moulded wooden racing dinghy.

Interesting.

I have a difficult time explaining your
bubbles in terms of galvanic currents,
since they occur mainly where stuff has
been welded on the inside of the hull.

My first reaction is to think in terms
of increased thermal mass in the welded
areas causing temperature differences.

Next thought is stresses on the hull at
those spots causing "stretching" of the
paint surface, maybe allowing water in.

Sorry I can't do better than that.

Good luck.

Chuck

Could it be that there is a slight difference in either the plates used in
the hull and the welding rod? Or even between the plates?
"chuck" wrote in message
ink.net...
Interesting.

I have a difficult time explaining your bubbles in terms of galvanic
currents, since they occur mainly where stuff has been welded on the
inside of the hull.

My first reaction is to think in terms of increased thermal mass in the
welded areas causing temperature differences.

Next thought is stresses on the hull at those spots causing "stretching"
of the paint surface, maybe allowing water in.

Sorry I can't do better than that.

Good luck.

Chuck

Hi,

I'm no expert, but I've been studying
electrochemical reactions including stray current
corrosion due to problems with the rudder post on
a boat that we recently acquired.

Since the steel is very clean under the bubble, I
would guess that you are correct about it being an
electrolysis type reaction. Otherwise it would be
a rust bubble.

The "Marine Electrical and Electronics Bible"
lists the following under Corrosion System
Troubleshooting:

Paint stripping off keel and hull:
Hull overprotected (too many anodes)
Severe electrical leakage to hull
Copper based antifoulant

The reason that the problems show up around the
welded areas is because that is where you are more
likely to have pinholes in your paint.

Don W.

wrote:
Hi, I have put some pictures he
www.photobucket.com/
Search for images named "sabina113"

I find when I take the yacht out of the water each year I have bubbles
in the paint mainly where things have been welded to the inside of the
hull. The bubbles are filled with salt water which is quite strong and
possibly acidy. When the bubbles are opened there is very clean steel
underneath. It doesn't seemed to have corroded the steet alot. The
zincs last a year ok.
I guess this is some sort of electrosis or stray current erosion but
was hopeing for a second opinion.
Also, if anyone looks at the pictures, have you seen a driveshaft
thrust bearing setup like in the picture. There is a small brass bung
on top where the thrust bearing is. Do you put oil in there?
Thanks
asdex

Thanks very much for all your information. We arn't on shore power but
I will start some checking today on any current leakage to the hull. We
have grinded the bubbles out and spot primmed with Altex primer and
then two coats of high build primer followed bu antifoul. That should
isolate the metal from the water. We have just two 2.5kg zincs on the
hull and one on the rudder which is isolated from the hull in nylon
bushes.Oh, also a zinc that hangs fron the deck by stainless steel wire
over the side. This was there when we bought the yacht in 1998 and have
kept it. It gets eaten away and is handy to pull up and see whats
happening.
Thanks,
asdex