Advance
Copper Bottoms
Wood may be the ideal material for shipbuilding, but it tends to deteriorate over time. By fitting our ships with copper sheathing below the waterline we can reduce this deterioration, allowing them to better withstand long journeys.
Advance tree
The game files don’t say where this goes: it has no requires, so the game places it in the 
Reformation tree itself.
Modifiers
Navy Attrition−25%
Definitionin_game/common/advances/4_choices_mil.txt:261
263icon = ship_building_advance
264for = mil