Galvanic Action in Wood Vessels

So what this galvanic stuff all about?

When you have dis-similar metals in wet (salt water) soaked wood - you set up a galvanic cell or a battery. When, due from various reasons, the current potential reaches (based on my observations) about 350 - 400mv, then the galvanic current creates alkaline substances in the wood which break-down the glue (ie., lignins - an organic polymer) which keeps the wood togther.

The pictures are from a recent survey of a Sabercraft where galvanic current (from a transom mounted plate zinc) has damaged the plywood surrounding the plate's thru-hull fasteners.