Joost,
There have been many really good replies from the other responders, so I
won't repeat the good advice of others, but here is a tip. Connect an LED
from DC ships voltage to the hull in an obvious location. If the LED is on,
you have a ground fault, fix it. Secondly, make the engine common to the
hull and not common to DC return, it's less expensive than isolating the
engine and the result is the same. Thirdly, your problem might not be on
your boat, but on the dock. When connecting shore power to your boat, never
connect the safety ground at the outlet on the dock to your hull and always
connect your AC ships ground to the hull. However, to meet code the Dock
ground needs to be connected to the isolated frame of the isolation
transformer. If you don't have a transformer, you won't comply to code, but
the solution still works and is safe, because the sea is still common to the
reference at the distribution transformer.The problem is that other boats
electrical leaks can cause electrolytic corrosion to your hull because their
DC system is connected to their ships ground and it is common to their power
outlet on their dock. It is paramount that you isolate yourself from the
electrical problems of your neighbor(s). Lastly, if you have a paint scheme
fault, you must sandblast to bare metal and redo the entire bottom
correctly. Do not do the latter until you complete the former.
Steve

"joost" wrote in message
...
Hi,

Last year when I hauled out my steel boat I found many paint blisters
on the bottom. When I opened them, clear grey steel came out
underneath. I then suspected a ground leak causing electrolysis to be
the problem. There was indeed a leak on the RF ground of the shortwave
radio, which I fixed by adding capacitors in the ground path.
According to my measurements then, no more ground leaks existed. I
scraped off all the blisters and repainted.

Today I was overwhelmed by a feeling of sadness when I discovered that
there is a whole bunch of new blisters at and below the waterline. The
question that I would like to ask is:

Are there other causes for this type of blistering than electrolysis?
The size of the blisters is about 5-25 mm.

Thanks,
Joost.