"Matt/Meribeth Pedersen" wrote in message
ink.net...

Especially if a wooden boat is overprotected, the more noble
fitting creates sodium hydroxide, which eats away the lignin in
the wood.

I don't understand what you mean by "overprotected".

I think I know what you are saying about the more noble fittings, which
would be the various through hull fittings.I have seen the wood surrounding
the fittings become deteriorated and spongy to the extent that the plank had
to be replaced.

Lots of woodenboat owners don't bond their through hulls together
and certainly don't put a lot of zincs on the boat.

Are you saying that they don't, but should? Or that they don't to avoid the
effects you are talking about?

Ten to 18 years ago I owned a 1956 Stephens, a wooden hull boat. The boat
was in poor condition when I got it and I spent a lot of time in the boat
yard over the 8 years that I owned it. I talked with lots of other wood
boat owners and did a fair amount of research. The general wisdom was that
you DID need to have a zinc anode and all the through hulls bonded to the
boat's electrical system.

If the through hull fitting was completely isolated electrically (such as a
seacock that was only connected via a rubber hose) then there would be no
current flow through it, assuming that the interior wood was dry so that
bilge water couldn't complete a circuit. In this case, that fitting would
be okay. But there are always fittings that you can't isolate, such as the
shaft log, rudder post and strut. These will create a current path,
especially when you connect up to shore power.

If you didn't have a zinc to provide an alternate path, then the current
flowing through the through hull fittings would create an "electrolysis
burn" on the wood. And NO, I am not saying the wood undergoes electrolysis,
I am saying that the effects of adjacent electrolysis damages the wood. Go
to a boatyard that has some wooden boats in it and look at the wood around
the fittings, it will be darker and in many cases severely deteriorated.

When the zincs are mounted you want to space the zinc away from the wood so
that you don't create an electrolysis burn around the zinc.

By the way, I understand that you can create the same situation when using a
Magnesium anode on an aluminum hull. I have heard stories of people who
directly bolted a magnesium anode to the transom of their aluminum boat,
then put the boat in salt water. The resulting "accelerated" galvanic
action with the anode "burned" the aluminum it was in contact with to the
point that an anode sized hole opened up in the transom and the boat sank.
Consider this "hear-say" as I can't confirm it, but I can believe it. It is
best to put bolts through the transom with nuts and washer on the outside,
then mount the anodes on the excess thread length spaced a 1/2" or so from
the hull.

Rod McInnis