I made a mistake when explaining what a rudder was last Saturday in my class in Guangyuan. I thought a rudder was like a steering wheel of a ship, which actually is a helm. I had been thinking that wrong way since my childhood, because there was a song named 《大海航行靠舵手》 (da4 hai3 hang2 xing2 kao4 duo4 shou3, Sailing the Seas Depends on the Helmsman).

Both rudder and helm are translated into one Chinese character 舵 (duo4). So I never bothered to ask myself a second time what a rudder really was. This even led me into the cockeyed version of the song's theme, for I used to say "Sailing the Seas Depends on the Rudderman."

Definitions for rudder, helm and tiller:

A rudder is a flat piece of wood or metal at the back of a boat or aircraft, which is moved from side to side in order to control the direction of travel.

A helm is the handle or wheel which controls the direction in which a ship or boat travels.

A tiller is a long handle fixed to and used to turn a rudder (= blade at the back of a boat used to control the boat's direction)

舵，舵轮和舵手的定义：

舵：船、飞机等控制方向的装置。例如：掌舵/升降舵/方向舵。

舵轮：（也做舵盘）轮船、汽车等的方向盘。

舵手：掌舵的人；比喻把握方向的领导者。