In short: yes, 18.3 is broken. The rule says ‚cause‘, not ‚compel‘.

Makes sense, I agree.

That said, "cause" is a weird word. How about if I'm nowhere close to in their way, but a reflection of the sun off my boat gets in the other skipper's eye, causing them to mis-steer to above close-hauled...

Recently had a boat do exactly as described. Tacked in front of us in medium to heavy winds at the windward mark in close quarters with other boats in competitive keelboat one- design fleet.  We warned the other boat she didn’t have room. We were right. We had to go above close-hauled and the offending boat forced another to bear off. That boat protested. Result - 30% penalty. 

Nicholas: I see how the tacking boat's action broke 18.3 with respect to you (they caused you to sail above close-hauled), but I don't understand how their interaction with the other boat (the one that bore off) broke any rule. (The bearing-off boat didn't sail above close-hauled, after all.) If the protest committee found otherwise, can you explain why?

18.3 establishes two obligations: 1) do not cause the other boat to sail above close-hauled to avoid contact; and 2) if the other boat establishes an inside overlap, give mark room (presumably still without having to sail above close-hauled).

So if the starboard boat is overtaking and has to go above close-hauled to avoid contact and establish an overlap to windward, 18.3 is broken. If the overtaking boat comes down to establish an inside overlap and then can't make the mark, 18.3 is broken.

Question: 18.3 says that the tacking boat can't cause the other boat to sail above close-hauled to avoid contact. If the overtaking boat comes down to establish an inside overlap and then has to sail above close-hauled to make the mark but is able to make it around without touching the mark or the outside boat, has the boat that tacked in the zone broken 18.3? I kind of think not.

I believe our situation was fairly common. If we had been sailing dinghies the taking boat may well have tacked clear but in a 34 ft boat, tacking slows you up considerably so the loss of speed of the tacking boat means action is necessary to avoid a collision. Practically, with other boats to leeward, the only way to do that is to go inside the tacking boat. The tacking boat slowed and was then in the way of the next boat to leeward, that boat then has to avoid collision by bearing off. In essence the tacking boat doesn’t keep clear. It’s clearly a tactical  decision whether a port boat in the zone can tack and keep clear. No rule, though, allows you to barge through the fleet. Technically, the boat that bore away made a third party protest - we were two busy rounding ahead of all of them. The third party protest was made because too often boats in competitive fleets tack in the zone without enough room and then it is really difficult to show that the boat affected was really forced above close-hauled. Protests carried to the room are rare in our fleet unless there is damage but this was a case where a good rule needed to be enforced.

A boat that tacks ahead of another boat doesn't have to keep clear if they're clear ahead or leeward. They still have right of way. A boat astern, if going faster, must keep clear and if they run into the slower boat's transom they break 12 & 14. 

The tack taking place in the zone doesn't change right of way (12 is still on) but if the faster boat astern has to head up above close-hauled to avoid contact the tacking boat breaks 18.3.

If the third boat saw the tacking boat break 18.3 she's allowed to protest the breach even if she wasn't directly involved in the incident. 

I think it is about the objective of the rule (18.3) itself.  So, if the starboard boat has to luff above close hauled to avoid contact, the tacking boat has broken rule 18.3. And if she has to bear away to avoid contact, and then either do not make the mark or luffs up above close-hauled, tacking boat is again liable  for penalty.  The rule's objective, therefore, is asserted in that a port tack boat coming in the zone should not be a prudent option, and tacking definitely  not.  But, we sailors do that often, and will continue to do for gaining that few lengths at the mark.

If the port tack boat tacks above the starboard tack boat, and the starboard tack boat is forced to go below her to avoid contact, must the starboard tack boat jibe around below the mark, or may it proceed around the mark?  Obviously the port tack boat entered the three boat-length circle first.

John-- I'm not completely clear on what situation you're describing, but it sounds like once the starboard tack boat is overlapped to leeward, the other boat must keep clear. I don't think the (original) starboard tack boat has any obligations other than giving room to keep clear.

Can you say why you think the starboard tack boat might not be allowed to proceed around the mark?

Tim:

We need to be mindful of 15 too, which switched-on as soon as her 12 ROW does, so one can complete the tack clear ahead of a faster boat and break 15 by doing it too close.

I am sorry my question was not clear; the problem I am presenting is that the port tack boat crossed the starboard tack boat and then tacked well above it leaving it room to round the mark.  May the original Starboard tack boat establish the overlap to leeward and proceed around the mark?

Angelo Guarino said:
"We need to be mindful of 15 too, which switched-on as soon as her 12 ROW does,
so one can complete the tack clear ahead of a faster boat and break 15 by doing it too close."

Which is the limit of "too close"?
If the faster boat manoeuvring promptly in a seamanlike way ,
the boat clear ahead does't break rule 15 ?

THANKS !!!

Catalan, yes.

The term “clear ahead” is a static data-point.  As you know we measure that by freezing the picture at different points in time, draw extensions from the stern and determine. 

Breaking 15 is a both a static and a dynamic measurement.  It’s static because we measure it “When a boat acquires right of way”, which is a frozen moment in time.  Keeping clear is a dynamic analysis.  You are right to bring in “seamanlike way” because 15 provides a “boat room to keep clear”.  Room ties in “seamanlike way”.

So, yes, if the boat clear astern is able to keep clear in a seamanlike way, then the boat that tacked ahead did not break 15.  

All that said, in this scenario it’s more common to claim the tacking boat broke 13 instead of 15.  Typically the boat astern claims they had to change course prior to the tacking boat reaching her close-hauled course. 

Catalan, also when I think of maneuvering "in a seamanlike way", I always bring Case 35, 101 and 93 to mind.  The first 2 are about RRS 20, not 15, but I find them helpful when taken together in considering room to maneuver in a seamanlike way.

 Cases 35 and  101  also on the other end 93 .  

I think there is another case where a boat was forced to crash-gybe, but I can't find it right now.

Talking about  "in a seamanlike way", of course this was not in the zone, but is so cool !!

VIDEO LINK:
https://www.youtube.com/watch?v=TDpvoNup-uM

 

Catalan, that wasn't a room/seamanlike-way issue, rather WO broke RRS 13 (like I mentioned in my "more common scenario" for tacking too close). The IJ found (and as you can see) Comanche started changing course to avoid WO prior to WO completing her tack. - Ang