I know about ‘Electrolysis in Steel hulls, but how can it damage a wooden hull?

Both electrolytic and electrochemical corrosion of metal components can seriously damage a timber hull. This process is properly called ‘delignification’ as it is the lignin in the timber that is destroyed. Lignin is the chemical that fills the gaps between the cellulose fibres of the wood. It is in effect the glue that binds the bundles of fibres together and gives it strength. The stiffer the timber type, the higher percentage of lignin in the make up of the timber.

Lignin is insoluble in water, but is soluble in alkaline solutions. When a metal corrodes it forms an oxide of the metal. When corrosion occurs in water the oxide combines with hydrogen and forms a hydroxide solution with the water. Metal hydroxides are highly alkaline so the resultant solution starts to dissolve the lignin in the timber surrounding the corroding metal. This process starts by just darkening of the timber. Ultimately it severely weakens the timber, and results in the wood appearing soft and woolly with loose hairy fibres. Where this initial darkening is seen it should be treated before serious damage occurs.

When this process occurs internally such as in wet bilges the hydroxides dry out leaving a white crystalline residue. This can be treated by washing with an acid solution which neutralises the alkali. The best source for this is ordinary kitchen vinegar.  Externally all metal components where they pass through the hull, such as anode bolts, should be isolated from the wood by epoxy and foam rubber, fasteners must be dowelled over. Generally the process can be minimised by keeping the vessel in flowing water, i.e. on a swinging mooring, so that any hydroxide solution is swept away by the tide.