Provided that the sailplan and underwater hull balance their efforts properly, then steering, tacking and all of the other maneuvers work without extra power, assuming that there is enough wind to drive the boat, and not so much that the boat becomes uncontrollable. My comment about my first home build getting a motor was supposed to indicate that I discovered that I knew so little about designing sailboats that I dropped the idea of a sailboat and built it as a motor boat, not as a sailboat with an auxiliary motor.
Steering is by rudder and servo, only it usually needs a larger rudder than a powered boat, since there is no propwash, having a winch (and I expect to see some of your ingenuity here) to adjust the sails helps a lot. As TT mentioned, before radios, there were the self steering systems. These were good on waters with all-round access, but only tried to keep the yacht on a set heading to the wind after much adjustment. Even after you got it right, there was always the chance that a shift in the wind would give a new and unexpected course.