I have had blowout [aka haloes] occur around various thru-hulls from time to time. I repainted my bottom in May of 2013 and my diver has reported some blowout around the stabilizer cutter bars. This is bit premature, usually happens before the three year mark.
I have had multiple discussions with divers and ship-yard managers and have determined the following.
In a bonded system, especially in a hot marina environment, there can be a galvanic reaction between the grounded system and the bottom paint. It requires a very high copper content paint. I use Pettit Trident SR [same as Sally W. 37VT42 -ed.] which is very high copper. Over the years the exact formula changes and I believe the low VOC brings more loose copper to the surface allowing for greater reaction surface and looser bonds. In essence the copper in the paint is sacrificed. This brings up the old debate about not bonding all underwater parts but placing sacrificial zincs outside. On wood hulls due to wood corrosion, systems are generally not bonded.
This time I have reaction between the stainless cutters and the paint. The cutters are not bonded and the dissimilar metals of stainless and copper seem to be reacting. This has never occurred in the last 15 years but I suspect a change in paint formula and stray currents from a possibly hot boat have contributed to the blowout. I have attached two photos, one of thru-hulls and the other the cutters.
Due to this problem I am considering switching from a high copper hard paint to a lower copper sacrificial paint.
Ken Smith, kenes@earthlink.net
Polar Mist, 49VT07
Polar Mist, 49VT07
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