If you have zero bank, and apply rudder you will begin
a
flat turn.

Wrong! UTTERLY WRONG!!!! A rudder yaws the airframe,
it does not 'turn' or 'steer' the aircraft.

Your whole problem seems to be in compairing a glider
with a boat. They may both have rudders, but they both
do totally different things.

Next time you fly in your glider, line yourself up
with a straight feature. Apply some rudder to yaw the
glider, but keep the wings level. All you will do is
continue in a straight line, sideways on. (commonly
known as a side slip)

However, the rudder does has a secondary control effect,
which introduces a roll moment due to differences in
lift between the wings. (which is why the first thing
you do is apply full opposite rudder to counteract
the rotation in a spin)

It is the ailerons that instigate a turn, the rudder
is used in a coordinated manner to 1) check the adverse
yaw (secondary effect of ailerons), and 2) to align
the airframe correctly into the airflow.

Please stop using language which inforces a belief
that the rudder is used to turn a glider in flight.
Your very action in doing so may well end up enforcing
that belief into a low airtime pilot reading these
posts and KILLING THEM!