(This is an instalment in a series of blog posts about the ISAF Case book 2009-2012 with amendments for 2011. All cases are official interpretations by the ISAF committees on how the Racing Rules of Sailing should be used or interpreted. The cases are copied from the Casebook, only the comments are written by me.)
Rule 10, On Opposite Tacks
Rule 18.1(b), Mark-Room: When Rule 18 Applies
Definitions, Proper Course
When a starboard-tack boat chooses to sail past a windward mark, a port-tack boat must keep clear. There is no rule that requires a boat to sail a proper course.
Two close-hauled boats on opposite tacks meet at a windward mark to be left to starboard. S has adequate room to tack and round the mark with due allowance for wind and current but instead of tacking, S holds her course with the intention of forcing P to tack to keep clear. Can P disregard rule 10 if she considers S to be sailing beyond her proper course and to have sufficient room to round the mark?
No; rule 10 applies. Rule 18.1(b) states that the boats are not subject to rule 18 because they are on opposite tacks and the proper course for one of them (S), but not both, is to tack. Therefore, when S chooses to hold her course, P must keep clear. While in certain circumstances boats are prohibited from sailing above a proper course there is no rule that requires a boat to sail her proper course.
The synopsis of this case states that there is no rule that requires a boat to sail a proper course. Do not read that too literary. There’s no rule in this case that does that, but there are rules that require a proper course from the r-o-w boat. For example rule 18.4 (gybing at a leeward mark) and of course rule 17 – in part – because the right of way boat shall not sail higher than her proper course.
But back to Case 9.
This situation is frequently used in Match Racing. All mark roundings are starboard roundings. And to force the Port tack boat to tack, the Starboard boat sometimes continues past the mark. Starboard has to time her approach exactly, to pull this off. Because to defend, Port can duck Starboard and tack fast, becoming the inside boat.