Monday, April 8, 2013

When does rule 18 turn off?

A question I hear often is "When does rule 18 turn off?"  At the simplest level, the answer is given by rule 18.1:

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
             (a) between boats on opposite tacks on a beat to windward,
             (b) between boats on opposite tacks when the proper course at the mark for one but
                   not both of them is to tack,
             (c) between a boat approaching a mark and one leaving it, or
             (d) if the mark is a continuing obstruction, in which case rule 19 applies.

If rule 18 does apply at some point during a mark rounding, part (d) clearly never applied.  Condition (b) is really just an extension of (a) for situations where there might be disagreement about whether boats are on a beat to windward, so we get the following summary: "Rule 18 turns off when either boat tacks at a mark to be rounded on its windward side, and when the leading boat leaves the mark while the other boat is still approaching it."

However, that's not the whole story.  The real question is, "When does the obligation to give mark-room end?"  And while that seems like the first question, it's actually quite different.  Basically, the obligation to give mark-room ends when the boat entitled to mark-room no longer needs it to pass or round the mark. 

Consider ISAF Q&A 2013-017.The animation at the left was used to draw the scenario in that Q&A; it shows two catamarans rounding a port-rounding offset mark, with the next leg directly downwind, in 10-12 knots of wind, no current.  At position 4 Yellow has passed the mark and both boats are on their downwind courses (which, they being catamarans, is not straight downwind).  Yellow jibes directly in front of Blue, breaking rule 10 (Port/Starboard).

Blue is required to give Yellow mark-room at this mark because Yellow is clear ahead at the zone.  According to the definition Mark-room, Blue has to give Yellow three things:
  1. Room to sail to the mark if her proper course is close to it;
  2. Room to round the mark as required to sail the course; and
  3. Room to leave the mark on its required side.
(I've changed the order of the requirements in the definition Mark-room so that they appear in time order, making them easier to discuss here.)

Consider the situation at position 4 of the animation.   Yellow's proper course is no longer close to the mark, so she no longer has the right to room to sail to it.  She is on her downwind starboard-tack course, so she has rounded the mark as required to sail the course.  And Blue has clearly given her room to leave the mark on its required side. Thus, Blue's obligation to give Yellow mark-room has been discharged and she is no longer obliged to do so.

That conclusion is particularly important because of rule 18.2(c)(2), which says:

(c) 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.

At position 4, Blue has become overlapped inside Yellow.  But because Blue has discharged her obligation to give Yellow mark-room, she is no longer required to do so and the initial phrase in rule 18.2(c) does not apply to her.  Therefore rule 18.2(c)(2) does not apply.  Yellow's breach of rule 10 is not exonerated under rule 21 and she should do her turns.

 Now consider a situation at a port-rounding leeward mark, with the next leg a beam reach (as with Mark 3 in a standard digital-N course in team racing). Blue is overlapped when she enters the zone, so she is required to give Yellow mark-room.  By luffing Yellow at positions 3-4, does she break rule 18?

I think the answer is no.  Refer again to the list of requirements from the definition Mark-room.  Blue has certainly given Yellow room to sail to the mark, and just after position 3 Yellow is on her course to the next mark, so she has been given room to round it.  At position 4 Yellow  has not yet passed the mark on its required side, but Blue is not preventing her from doing so.  Thus unless Blue luffs Yellow into the mark she has given mark-room as defined and has no further obligations.  

Note that this isn't as clear-cut as the previous example because not all the conditions of the definition Mark-room have been met when Blue luffs Yellow above her course to the next mark: Blue still has to allow Yellow to leave the mark on its required side.   So one could argue that until all the parts of the definition have been met, Blue has no right to interfere with Yellow.  But I don't think that conclusion is supported by the wording of rule 18 and the definition Mark-room.

If I'm right, this has an interesting consequence for team-racing: it essentially allows the outside boat to set a "post-mark trap" at Mark 3.  Suppose Blue has a team-mate close astern; then this play allows that team-mate to pass while Blue holds up Yellow.  You might say, Blue could have accomplished this by luffing later, along the leg, and that may be true -- but it may not, because if Blue allows Yellow to trim in and reach along the next leg, she may lose any real opportunity to luff Yellow above close-hauled, which she needs to do if she wants to hold Yellow up.  It's only the presence of the mark that forced Yellow to come down into the vulnerable position she's in at position 4. 


  1. John Grace writes:

    I’m interested in the last example in your post of 8 April 2013, where a boat entitled to mark room is luffed up above her proper course at the leeward mark. You wrote that unless Blue luffs Yellow into the mark she has given mark-room as defined and has no further obligations.

    I’m not sure that this is correct. Your interpretation rests on a number of assumptions:
    (1) that “rounding the mark as necessary to sail the course” is limited to “turning a boat until you are facing the course to the next mark”.

    (2) that the steps involved in giving mark room then cease to apply – under your interpretation, the obligation to allow a boat room to “leave the mark on the required side” would be superfluous.

    (3) that the three obligations included in the definition of mark room operate totally independently of each other.

    If I was arguing for Yellow, I would emphasise that mark room is all about “room” ie space, the space required to manoeuver in a seamanlike way, and the space to do so promptly. Room is not only about allowing a boat to sail past a particular angle.

    Firstly, Yellow could argue that there is no reason to assume that Blue's obligation switches off as soon as Yellow is facing the next mark. Why can’t she simply say that she was not given “room” to “round the mark as necessary to sail the course” because she was forced to round the mark in a manner inconsistent with sailing the course in a seamanlike way?

    Secondly, Yellow can argue that the verb “rounding” is a transitive verb. It refers to boat moving around a central object rather than a boat itself turning round. When you round Cape Horn you are referring to passing Cape Horn from the time you approach it on one side to when you leave it on the other side. Not only the action of turning the rudder. When you sail round an island you are rounding it in the straight sections as well as the turns.

    Yellow can say that she was not given the room to round the mark as necessary to sail the course because she was not given the room she needed to sail the course while she still moving round the mark.

    Thirdly, Yellow can argue that Blue did not allow her “room” to leave the mark on the required side, because Blue was preventing her doing so, promptly in a seamanlike way. One cannot leave anywhere promptly when forced to sail above close hauled.

    Yellow’s argument also has weaknesses, but isn’t it just as valid as the argument in your example?

    If I am right, all that Blue needs to do is wait one further boat length – when Yellow would have left the mark - and then luff.

    1. I think your points are well made, and you may well be correct in your conclusion. Here are my responses to your numbered statements above:

      (1) I do think that, at a non-finishing mark, room to "round the mark as required to sail the course" only implies space to turn the boat in a seamanlike way the amount that's needed to sail to the next mark. In the given case, where the next leg is a reach with no other factors such as current, this means heading the boat toward the next mark, and I think Blue has given that room at position 3. There's nothing in rule 18 that says that, after giving that room, Blue has an obligation not to luff Yellow above her course to the mark. However, see my response to your point (3), below.

      (2) I don't think the term "leave the mark on its required side" is superfluous under my interpretation. Suppose the mark were a boatlength further to the right in the diagram, so Blue's luff took Yellow to the wrong side of the mark. Then Blue would have satisfied 2 of the 3 requirements of mark-room -- she would have given Yellow room to sail to the mark (and then some) and she would have given her room to round the mark as necessary to sail the course when the boats were at position 3. But she would not have given Yellow room to leave the mark on its required side.

      (3) Here's where I think I'm on the least firm ground. In the Q&A at the beginning of my post, Blue has satisfied all her obligations to give Yellow mark-room, whereas in the second example Blue has satisfied 2 of the 3 obligations -- she has not yet given Yellow room to leave the mark on its required side. I argued in the posting that as long as she gives Yellow room to leave the mark to port when she luffs, she breaks no rule; but I concede that this requires us to look at Blue's obligations piecemeal. If you look at them as a single whole requirement, at position 4 Blue is still bound by rule 18 to give Yellow mark-room, and you can argue that even though Blue was giving Yellow room to round the mark at position 3, she is no longer giving her that room at position 4.

      My biggest concern with your interpretation is, what if it's Yellow's initiative to luff around the mark as shown in the diagram? Do you claim that Blue has to sail on a beam reach for another boatlength and not follow Yellow up, for fear of blocking Yellow from sailing to the next mark? It seems to me that if that were to be the conclusion then we are giving Yellow the right to more room than the definition "mark-room" allows.

      I have no argument with your conclusion that if Blue waits another boat length, and only luffs after Yellow has been given the room necessary to leave the mark to port on a course to the next mark, Blue's obligation to give mark-room is satisfied and she can luff Yellow (subject to rule 16). But of course by that time Yellow will have had the opportunity to climb away from Blue (while Blue was dutifully sailing on a beam reach) and then sail over Blue if the latter luffs.

  2. If i am a third boat coming behide those tow boats can i round the mark using the gap that the blue leaves by giving room to yellow?


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