Rule 15 and definition of room

   Hypothetical Situation

2 dinghies, P and S, are sailing close hauled on opposite tacks. P is give way boat and S is right of way boat.  P is unable to cross S so tacks directly infront / slightly to leeward of S.  In order to avoid collision, S begins her own tack on to Port but a slight collision occurs with no damage.

Both boats agree that P had completed her tack on to Starboard ahead of any impact, and as such there is no Rule 13 “While Tacking” infringement. However, there is disagreement over rule 15 “Acquiring right of way”. Both boats agree that P has become right of way boat before impact through either rule 11 or 12, but P had to initially give S room to keep clear. P argues that S could cleary see that P was tacking and that she would complete the tack before any rule 13 violation, and if S was truly manoeuvring  promptly in a Seaman like way there would have been no collision.

S argues that it doesn’t have to do anything until P becomes right of way boat which is when P completes her tack. Only from that time does S have to manoeuvre promptly in a seaman like way, which she does and there is still impact.

Who is correct? I know there is a Case that confirms that a boat is not expected to know that another boat will break a rule, but I’m not sure if that really applies here?  The parties are essentially pitching the opening words of Rule 15 “When a boat acquires right of way” against the definition Room with its “manoeuvre promptly in a seaman like way”