Section C - At Marks and Obstructions
This part of the rule book is perhaps the least understood section of the rules. Here also is a section that has undergone a lot of changes. We have highlighted some text in red that deserves special attention.
I'm sorry we didn't include obstruction in the definitions we quoted last time. Since it is so relevant we provide it here.
Obstruction An object that a boat could not pass without changing course substantially, if she were sailing directly towards it and one of her hull lengths from it. An object that can be safely passed on only one side and an area so designated by the sailing instructions are also obstructions. However, a boat racing is not an obstruction to other boats unless they are required to keep clear of her, give her room or, if rule 21 applies, avoid her.
At the beginning of Section C is the preamble:
To the extent that a Section C rule conflicts with a rule in Section A or B, the Section C rule takes precedence.
Note - Section A is the Right of Way rules and Section B is the Limitations on the Right of Way rules.
18 - ROUNDING AND PASSING MARKS AND OBSTRUCTIONS
In rule 18, room is room for an inside boat to round or pass between an outside boat and a mark or obstruction, including room to tack or gybe when either is a normal part of the manoeuvre.
18.1 When this Rule Applies
Rule 18 applies when boats are about to round or pass a mark they are required to leave on the same side, or an obstruction on the same side, until they have passed it. However, it does not apply
(a) at a starting mark surrounded by navigable water or at it's anchor line from the time the boats are approaching them to start until they have passed them or
(b) while the boats are on opposite tacks, either on a beat to windward or when the proper course for one of them, but not both, to round or pass the mark or obstruction is to tack.
18.2 Giving room; Keeping Clear
(a) OVERLAPPED - BASIC RULE
When boats are overlapped the outside boat shall give the inside boat room to round or pass the mark or obstruction, and if the inside boat has right of way the outside boat shall also keep clear. Other parts of rule 18 contain exceptions to this rule.
Note - The inside boat must be given room to make a seamanlike rounding, while an inside boat with right of way must be given room to make a tactical rounding.
(b) OVERLAPPED AT THE ZONE
If boats were overlapped before either of them reached the two-length zone and the overlap is broken after one of them has reached it, the boat that was on the outside shall continue to give the other boat room. If the outside boat becomes clear astern or overlapped inside the other boat, she is not entitled to room and shall keep clear.
(c) NOT OVERLAPPED AT THE ZONE
If a boat is clear ahead at the time she reaches the two-length zone, the boat clear astern shall thereafter keep clear. If the boat clear astern becomes overlapped outside the other boat she shall also give the inside boat room. If the boat clear astern becomes overlapped inside the other boat she is not entitled to room. If the boat that was clear ahead passed head to wind, rule 18.2(c) no longer applies and remains inapplicable.
Note - Once the inside boat tacks, she loses protection of rule 18.2(c).
18.2(d) CHANGING COURSE TO ROUND OR PASS
When after the starting signal rule 18 applies between two boats and the right-of-way boat is changing course to round or pass a mark, rule 16 does not apply between her and the other boat.
(e) OVERLAP RIGHTS
If there is a reasonable doubt that a boat obtained or broke an overlap in time, it shall be presumed that she did not. If the outside boat is unable to give room when an overlap begins, rules 18.2(a) and 18.2(b) do not apply.
Note - The last part of 18.2(e) opens up some interesting possibilities.