WestlawnMasthead27_Sept2013H.pub - page 30

The Masthead
Know It All Contest Solution
to the June 2013 Question
On what is causing bottom-paint discoloration and loss?
(This issue’s new Know it All question is on page 29)
The Know It All questions and correct answers are important design tips for students as well
as other marine professionals. We suggest that you file them away for future reference.
The Question Was:
Sept. 2013 Page 30
P
rancing Petunia
is a 1981, Chinese-built, fiber-
glass trawler. The fiberglass hull is sound and the
bronze hull fittings are of good quality and in good
shape. The zincs on the prop shafts are nearly gone,
with a great deal of marine growth on them. Both the
through-hull fittings and the rudder and strut are
painted with anti-fouling paint—the same paint as on
the hull.
Looking at the photos, you can see that there is a
clear ring discoloration and loss of paint around the
Photos: Lachlan Mackenzie
through—hull fittings and the strut. What phenomena is
causing this, and is it likely to be structurally harmful?
The Solution is:
Though we received several answers to this question,
none of the answers were quite on the mark.
The phenomena is
hallowing
from overprotection. Some
bottom paints have a high copper content, over 15%.
There was too much zinc and this overprotection is
causing the reaction in the high-copper paint.
If you examine the boat―most likely―you'll also find
that the bonding system is faulty as well. The reason for
this is that―if the bonding system were solid―then the
hallowing would be found to some degree on all the
metal underwater fittings. This problem is visible only
on the fittings shown. It would make sense to check if
the bonding system is faulty or if there's no bonding
system.
The solution is to clean to new metal and surface with
and epoxy barrier coat and then to reapply the anti-
fouling paint. It is also recommended to back down on
the anode protection level to 650- 700 mV.
Hallowing has no direct effect on the boat’s structure.
Thanks to Kevin Ritz , Ed Sherman and Lachlan
Mackenzie for insight into this phenomenon.
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