Is S required to hold course after hailing starboard?
No, but if the right-of-way boat changes course, she must give the other boat room to keep clear. See rule 16.1. Also, if a starboard-tack boat bears away toward a port-tack boat after the starting signal, she must not force the port-tack boat to immediately change course in order to keep clear. See rule 16.2.
The hail "hold your course" really just means "I'm aware of you".
