rudder, oar, name plate
榜 centers on a flat wooden board used for steering or display: a rudder or oar for directing a boat, and a name plate for posting information.
榜 combines 木 (wood) with 旁, which likely contributes the sound and the idea of a flat surface. The character originally referred to a wooden board, later extended to a rudder or oar and a name plate.
A wooden 木 board 旁 used for steering a boat or posting a name. Picture a wooden rudder that doubles as a name plate for the boat.
For ボウ, imagine a wooden bow of a ship where the rudder is attached: bow -> ボウ, and the wooden rudder steers the boat.