PW and PL, two overlapped boats on port tack, were sailing up the first beat of a race in moderate air. Both were on closehauled courses. PL was about one hull length to leeward of PW and had her bow half a length in front. These two boats were converging with a third boat (S) who was on starboard tack. PL thought she was sailing into a header, and she wanted to tack to leeward of S. Therefore, she hailed PW for room to tack at the obstruction (S). PW, however, liked the right side of the course, and she wanted to bear off behind S and keep sailing on port tack. Since PW had an inside overlap on PL, she hailed PL for room to duck behind S (an obstruction). Unfortunately, it was not possible for PL to tack and for PW to duck at the same time. How do the rules resolve this situation?
Correct Answer: PW must respond to PL's hail for room to tack
According to the definition of obstruction, S is considered an obstruction to PW and PL because they were both required to keep clear of her. PL was entitled to hail for room to tack under rule 20 (Room to Tack at an Obstruction) because PL was closehauled, she had to make a substantial course change to avoid S, and she could not tack and avoid PW. Once PL does this, rule 20 applies.
According to the preamble to Section C, "When rule 20 applies, rules 18 and 19 do not." In other words, when PL properly hailed for room to tack at an obstruction, rule 19 turned off. That meant PW was not entitled to room to pass between PL and the obstruction (S). PW had to properly respond to PL's hail by either tacking as soon as possible or immediately replying "You tack" and then giving PL room to tack.