Q&A 2 Proper Course after the mark

I received another question about a 35 foot keelboat (L) in a midweek race.   Either just before or just after rounding the windward mark a larger boat (W) established an overlap from clear astern. Boat L bore away at the windward mark but not as far as their proper course to the gybe mark. Boat W was upset and yelled to L to sail her proper course. Eventually W dropped behind and sailed to the mark. 
The question from boat L. Were they right?
This is an example of people expecting more from the definition of proper course than is really there.
Proper Course   A course a boat would sail to finish as soon as possible in the absence of the other boats referred to in the rule using the term. A boat has no proper course before her starting signal.

If we look at the diagram blue establishes and overlap to windward of yellow. Yellow sails up to the mark and the bears away but not as far as the course to the mark. It is quite probable that yellow at position 6, 7 and 8 is sailing above her proper course. There is however no rule that requires her to sail her proper course. The closest two are rule 17 and rule 18.4.
            If a boat clear astern becomes overlapped within two of her hull lengths to leeward of a boat on the same tack, she shall not sail above her proper course while they remain on the same tack and overlapped within that distance, unless in doing so she promptly sails astern of the other boat. This rule does not apply if the overlap begins while the windward boat is required by rule 13 to keep clear.
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 no farther from the mark than needed to sail that course. Rule 18.4 does not apply at a gate mark.

Rule 17 applies to a leeward boat who establishes an overlap to leeward from clear astern. Yellow is to leeward but she did not establish the overlap from clear astern. Blue established it from clear astern.
Rule 18.4 applies to an inside overlapped right-of-way boat (all of which yellow is from position 3-7) who is required to gybe to sail her proper course. Yellow does not have to gybe to sail her proper course so this rule does not apply.
The last rule that might apply is the definition of mark-room. 
Mark-Room   Room for a boat to sail to the mark, and then room to sail her proper course while at the mark. However, mark-room does not include room to tack unless the boat is overlapped to windward and on the inside of the boat required to give mark-room.

Yellow was clear ahead at the zone. She is entitled to mark room from blue (and would have been if she was overlapped as well). She is therefore entitled to room to sail to the mark and room to sail her proper course at the mark. Blue has to give her that room. Yellow is still leeward boat and blue still has to keep clear just as if the mark was not there. There is no conflict between these two obligations.  
© Copyright 2010 Andrew Alberti