Only Google it after you answer please
A bommy is a small metal plate attached to the side of old wooden ships. It would be attached outside the ship a few feet above the water line. A metal knocker was fixed to the bommy with a small float. If the ship started to take on water or loaded to much cargo, the ship would sit lower in the water and the knocker would hit the bommy, alerting the crew of potential problems.
Sounds good at least...