Monday, 26 March 2012

(pillow)Case of the week (13/12) – 15

(This is an instalment in a series of blogposts about the ISAF Case book 2009-2012 with amendments for 2010. 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.)

(pillow)Case picture


Rule 12, On the Same Tack, Not Overlapped
Rule 13, While Tacking
Rule 18.1(b), Mark-Room: When Rule 18 Applies
Rule 18.2(b), Mark-Room: Giving Mark-Room
Rule 18.2(c), Mark-Room: Giving Mark-Room
Definitions, Mark-Room

In tacking to round a mark, a boat clear ahead must comply with rule 13; a boat clear astern is entitled to hold her course and thereby prevent the other from tacking.


Assumed Facts

A and B are approaching the windward mark which they are required to leave to port. They are close-hauled on parallel courses with A clear ahead. A expects B, when she can tack and fetch the mark, to tack to round it and head for the next mark. Instead, B holds her course as shown in the diagram and sails on well past the mark.


Has B the right to hold her course in this way and, thereby, prevent A from tacking?


Yes. While A remains on port tack, B is required to keep clear by rule 12 and, as A was clear ahead when she reached the zone, B is required by rule 18.2(b) to give A mark-room as well. Provided B keeps clear of A and gives A mark-room if A luffs (even if A luffs as high as head to wind), B is entitled to sail any course she chooses, including holding her course.

However, B is no longer required to give A mark-room after A leaves the zone (see rule 18.2(c)). If A were to pass head to wind, then at that moment all parts of rule 18 would cease to apply because the boats would be on opposite tacks (see rule 18.1(b)). In addition, A would no longer have right of way under rule 12, and B would become the right-of-way boat under rule 13.

RYA 1966/8


One  solution for Boat A to get out of this situation is to time his luff so that she will have enough speed to get head to wind next to the mark. Boat B will either have to luff and end up on the wrong side of the mark or bear down to behind A. If the latter happens boat A can safely tack while keeping clear.

120312 Case 15a

Boat B bears off behind A. She’s not entitled to mark-room although she get’s an inside overlap in position 3. RRS 18.2(c), first part.

120312 Case 15b

Boat B luff and finds herself on the wrong side of the mark. She can’t continue upwind to interfere with A, because A is already on the next leg and that means boat B would break rule 23.2.

She can also start pinching as soon as she’s entered the zone clear ahead to get closer to the mark that way. In both scenario’s boat B has to keep clear and has to give mark-room (until A passes head to wind_

120312 Case 15c

Boat B slows down between 2 and 3 to go behind A. And A can continue as lead boat, although she has lost some speed in doing so. The one thing boat A must not do is pass head to wind before boat B has committed to go behind. Rule 18.2(c), second part, also states that mark-room is lost once the boat having mark-room passes head to wind.


  1. Hi Jos, I have a question regarding the note on the second animation: "She can’t continue upwind to interfere with A, because A is already on the next leg and that means boat B would break rule 23.2."

    When exactly does A turn from being in the same to being in the next leg?

    1. She's on the next leg when she crosses the extension of the line from the previous mark trough this mark. If the previous mark is directly downwind than it would be as soon as Grey passes the line straight up from the mark on these diagrams.

    2. "passes" as in the way one passes the finish line, or as in leaving clear astern?

    3. The consensus is that it is like the finish line. Only the bow (or other equipment) is enough.

    4. Thanks Jos, I've had that question for a while now.

      In the second scenario, B can then continue as far as she wants without tacking, since A will not be able to be on the other leg unless she tacks and passes that imaginary line?

    5. That would be the logical implication if you use this 'leg boundary'

  2. We've been discussing a protest situation over at Sailing Anarchy ( where case 15 was also relevant. The only part I wasn't 100% sure on was whether A has to give B "opportunity to keep clear" (16.1). My understanding is while 18.2.b is still on, 18.5 switches 16.1 off and gives A the right to luff without making any allowance for B.

    1. Yes, if you have mark-room rule 18.5 switches off 16.1, but only AT the mark and only if you sail your proper course. If boat A suddenly changes course in any other direction, rule 16.1 is still in effect.
      But when sailing her proper course rounding the mark, she can change course as fast as she pleases and boat B has to give mark-room to do this and keep clear. (If A is subject to 16, she can only be the ROW-boat. If she's keep clear boat she's has no 16.1 limitations)

    2. Why just AT the mark? The definition of "mark-room" includes "Room for a boat to sail to the mark". If I needed to tack to reach the mark, surely I would be attempting to "sail to the mark" by going up to head to wind? I can't go any further due to the limitations of my rights but I'm trying to "sail to the mark".

    3. Rule 18.5 has been divided in two parts:
      You get exonerated for all ROW-rules in 18.5(a) while sailing TO the mark, but not for 15 or 16.
      Only AT the mark those last two rules are added in 18.5(b).
      Any course change while still sailing TO the mark, must still be done within rule 16 limitations.

    4. Ah, got it. Is there a good definition of "at the mark"? Something like "overlapped with the mark" or "less than one boat length from the mark"?

    5. There was a RRMR Call:

      Definitions Mark-Room
      Question 1
      When is a boat entitled to mark-room sailing ‘to the mark’?
      Answer 1
      A boat entitled to mark-room is sailing ‘to the mark’ when she is sailing a course towards the mark, that will leave the mark on the required side, until she reaches a position where she becomes ‘at the mark’.

      Question 2
      When does a boat sailing ‘to the mark’ become ‘at the mark’?
      Answer 2
      A boat that is sailing ‘to the mark’ will be ‘at the mark’ when one or more of the following conditions apply:
      (a) She is no longer able to alter course, in a seamanlike way, towards the mark and pass it on the wrong side.
      (b) Any part of her hull overlaps the mark and she is closer than half of her hull length to the mark.
      (c) She reaches a position where she would usually alter course to round or pass the mark on the required side in order to start sailing the next leg of the course.

      Published 3 February 2010
      This call is valid until 1 January 2011

  3. Question:
    We are sailing F18 catamarans at my club....they are fast but slow a race we got this issue:
    1. A & B are both on layline heading towards the mark (speed: 10 knots)
    2. No over lap established....2m apart.
    3. A tacks directly around the mark...
    4. B ...very angry...feels that she could easily T-bone A while she was in the tack....

    Ex olympic sailor on boat A...boat B leaves room....not happy.....who was right?

  4. Boat A gets mark room because she entered clear ahead. But looses that mark room as soon as she´s passed head to wind. She´s then subject to rule 13 while tacking. (Her mark room does not include room tack, because boat B was not overlapped on the outside of her). If boat B had to bear off because of boat A, she broke rule 13 and is NOT exonerated under mark room.

  5. Thanks for a quick reply! when is a tack completed really? When you are on the 'new course' right?

    If A could get to the new course before the 'collision' then A is safe right?

    Ok thanks for bringing light into our game

  6. Yes, once A is on a close hauled course she has completed her tack and is no longer keep clear boat under 13. But she does have a rule 15 limitation. Initially she must give boat B room to keep clear. If boat B can keep clear by luffing boat A has fulfilled her obligations. If boat B cannot keep clear (by only responding after A completed) than A breaks rule 15 and B is exonerated for breaking rule 12.

  7. Ok..I think I'm slowly getting this...thanks!

    Catamarans just stop when they tack....and if boat B had waited to start keeping clear until boat A had completed the tack.....then the collision had been hard to avoid.....I was on B and when A started to tack we started to bear away ..and we just passed behind A...nearly hitting him.

    Would you recommend A not to tack until B tack in a similar situation in the future?


    1. Yes, In order for A to fulfill her obligations she must make sure she keeps clear while tacking and initially provide room for B to keep clear after she finishes her tack.
      If there is not enough space between the boats to do that, she must wait until B tacks and then tack herself.
      Or, like I show in the diagrams, not pass head to wind until B has born off and becomes overlapped to leeward of her.

  8. Once again....thanks for your patience explaining the rules above.
    We just sailed the F18 Worlds and thanks to you we were really confident around the top mark!


Related Posts Plugin for WordPress, Blogger...