Section C
AT MARKS AND OBSTRUCTIONS
Section C rules do not apply at a starting mark surrounded by navigable water or at its anchor line from the time boats are approaching them to start until they have passed them.
18
MARK-ROOM
18.1
When Rule 18 Applies
Rule 18 applies between boats when they are required to leave a mark on the same side and at least one of them is in the zone.  However, it does not apply
  1. between boats on opposite tacks on a beat to windward, 
  2. between boats on opposite tacks when the proper course at the mark for one but not both of them is to tack.
  3. between a boat approaching a mark and one leaving it, or
  4. if the mark is a continuing obstruction, in which case rule 19 applies.
18.2
Giving Mark-Room
  1. When boats are overlapped the outside boat shall give the inside boat mark-room, unless rule 18.2(b) applies.
  2. If boats are overlapped when the first of them reaches the zone, the outside boat at that moment shall thereafter give the inside boat mark-room.  If a boat is clear ahead when she reaches the ;zone, the boat clear astern at that moment shall thereafter give her mark-room.
  3. When a boat is required to give mark-room by rule 18.2(b),
    1. she shall continue to do so even if later an overlap is broken or a new overlap begins;
    2. if she becomes overlapped inside the boat entitled to mark-room, she shall also give that boat room to sail her proper course while they remain overlapped.
  4. Rules 18.2(b) and (c) cease to apply when the boat entitled to mark-room has been given that mark-room. or if she passes head to wind or leaves the zone.
  5. If there is a reasonable doubt that a boat obtained or broke an overlap at the time, it shall be presumed that she did not.
  6. If a boat obtained an inside overlap from clear astern or by tacking to windward of the other boat and, from the time the overlap begins, the outside boat has been unable to give mark-room, she is not required to give it.
18.3
Tacking in the Zone
If a boat in the zone of a mark to be left to port passes head to wind from port to starboard tack and is then fetching the mark, she shall not cause a boat that has been on starboard tack since entering the zone to sail above close-hauled to avoid contact and she shall give mark-room if that boat becomes overlapped inside her. When this rule applies between boats, rule 18.2 does not apply between them.
18.4
Gybing
When an inside overlapped right-of-way boat must gybe at a mark to sail her proper course, until she gybes she shall sail not further from the mark than needed to sail that course.  Rule 18.4 does not apply at a gate mark.
19
ROOM TO PASS AN OBSTRUCTION
19.1
When Rule 19 Applies
Rule 19 applies between boats at an obstruction except
  1. when the obstruction is a mark the boats are required to leave on the same side, or
  2. when rule 18 applies between boats and the obstruction is another boat overlapped with each of them.
However, at a continuing obstruction, rule 19 always applies and rule 18 does not.
19.2
Giving Room at an Obstruction
  1. A right-of-way boat may choose to pass an obstruction on either side.
  2. When boats are overlapped, the outside boat shall give the inside boat room between her and the obstruction, unless she has been unable to do so from the time the overlap began.
  3. While boats are passing a continuing obstruction, if a boat that was clear astern and required to keep clear becomes overlapped between the other boat and the obstruction and, at the moment the overlap begins, there is not room for her to pass between them, she is not entitled to room under rule 19.2(b),. While the boats remain overlapped, she shall keep clear and rules 10 and 11 do not apply.
20
ROOM TO TACK AT AN OBSTRUCTION
20.1
Hailing
A boat may hail for room to tack and avoid a boat on the same tack. However, she shall not hail unless
  1. she is approaching an obstruction and will soon need to make a substantial course change to avoid it safely, and
  2. she is sailing close-hauled or above.
In addition, she shall not hail if the obstruction is a mark and a boat that is fetching it would be required to change course as a result of the hail.
20.2
Responding
  1. After a boat hails, she shall give the hailed boat time to respond.
  2. The hailed boat shall respond even if the hail breaks rule 20.1.
  3. The hailed boat shall respond either by tacking as soon as possible, or by immediately replying "You tack" and then giving the hailing boat room to tack and avoid her.
  4. When the hailed boat responds, the hailing boat shall tack as soon as possible.
  5. From the time a boat hails until she has tacked and avoided the hailed boat, rule 18.2 does not apply between them.
20.3
Passing On a Hail to an Additional Boat
When a boat has been hailed for room to tack and she intends to respond by tacking, she may hail another boat on the same tack for room to tack and avoid her. She may hail even if her hail does not meet the conditions of rule 20.1. Rule 20.2 applies between her and the boat she hails.
Section D
OTHER RULES
When rule 22 or 23 applies between two boats, Section A rules do not.
21
EXONERATION
When a boat is sailing within the room or mark-room to which she is entitled, she shall be exonerated if, in an incident with a boat required to give her that room or mark-room,
  1. she breaks a rule of Section A, rule 15 or rule 16, or
  2. she is compelled to break rule 31.
22
STARTING ERRORS; TAKING PENALTIES; MOVING ASTERN
22.1
A boat sailing towards the pre-start side of the starting line or one of its extensions after her starting signal to start or to comply with rule 30.1 shall keep clear of a boat not doing so until she is completely on the pre-start side.
22.2
A boat taking a penalty shall keep clear of one that is not.
22.3
A boat moving astern, or sideways to windward, through the water by backing a sail shall keep clear of one that is not.
23
CAPSIZED, ANCHORED OR AGROUND; RESCUING
If possible, a boat shall avoid a boat that is capsized or has not regained control after capsizing, is anchored or aground, or is trying to help a person or vessel in danger.  A boat is capsized when her masthead is in the water.

24
INTERFERING WITH ANOTHER BOAT
24.1
If reasonably possible, a boat not racing shall not interfere with a boat that is racing.
24.2
If reasonably possible, a boat shall not interfere with a boat that is taking a penalty, sailing on another leg or subject to rule 22.1. However, after the starting signal this rule does not apply when the boat is sailing her proper course.