Awayonmyboat

The centre of buoyancy of a box shaped vessel (which to all intents describes a narrow boat hull) will be exactly half way between the waterline and the underside of the bottom plate. The centre of gravity of the boat before modification would have been somewhat lower than this - or you would have capsized. When the overplating was done on the sides it would have added new material with a centre of gravity roughly the same height as the original centre of buoyancy, that is above the original centre of gravity. So when the new plating was added centre of buoyancy went down a little (due to sinking deeper in the water) and centre of gravity went up slightly (new steel CoG above original CoG). Net result the two are now closer and the boat indeed has become slightly less stable.