For some sailboats, the "motor" is... OAR
Look on the right-hand side of the tools bar.
For boats that do have a motor, the icon looks like a boat Propeller.
That right there, is your motor control.
But don't use it unless you're running the RIB... because what's the point of a sailboat if you're going to motor about with it?
Have a great day and enjoy the Sim,
- Salty Dog