Ólafur, I am sure our friends will want to know on which side is the mark to be passed. 

Is this a starboard or port rounding?

Ang

Its on starboard

Because Yellow is the inside boat.

Ang - looks like stbd based on the arrow between the mark and Y3.

Ólafur -
Rule 18 applies because

and none of the exceptions listed in rule 18.1 are true, i.e.

18.1(a) is not true because the boats are not on a beat to windward;
18.1(b) is not true because neither boat will have to tack to round the mark;
18.1(c) is not true because both boats are still approaching the mark;
18.1(d) is not true because the mark is not a continuing obstruction;
And room has not yet been given.

Since 18 applies, we look at 18.2 to see which boat is entitled to mark-room and which boat has to give mark-room.  In your diagram, the boats would have been overlapped when the first one entered the zone (see the definition of overlap).  The first sentence of 18.2(b) tells us that the outside boat must give the inside boat mark-room.  In this case Blue is the outside boat and Yellow is the inside boat, so Blue must give Yellow mark-room.

Yellow would be exonerated for breaking rule 10 as long as she is sailing within the mark-room she is entitled to because of rule 43.1(b) so Blue cannot claim that she does not have to give mark-room because Yellow is on port tack.

Simpler for Olafur:
The boats are OVERLAPPED with one another (by definition regardless of port/starboard when sailing downwind) and Yellow is the INSIDE boat rounding the mark. 

The boat is on opposite tack but overlap because angel of wind is more than 90° if I understand it right. Blue outside and Yellow inside you say. When the boats are on the same tack it is usually easy to see how is inside- and how is outside boat.  But what indicate that Yellow is inside boat and Blue outside boat? The wind come from opposite direction into the boat starboard side Blue and Port side Yellow. 

Can you tell me why Yellow is inside boat?

Olafur, sometimes it can be confusing. 

One trick is to image both boats going around the mark on the correct side from their starting positions. 

Next imagine a line being drawn by each boat’s wake such that the lines do not cross each other. 

When you do this, you will see one boat’s wake-line drawn inside the other. 

I would say that the simple way is to follow the definition of clear astern and clear ahead; overlap which says:
"They overlap when neither is clear astern" And because they are sailing both more than 90 degrees from the true wind and one of them is in the zone rule 18 applies.
Hope this helps.

Ólafur,
You asked why yellow is inside: here's one way to figure it out: both boats have to leave the mark on their starboard side. If you are on the blue boat and see a boat on your starboard side, and the mark has to also be on your starboard side, yellow must therefore be between you and the mark if you both happened to arrive overlapped at the mark at the same time. Next ask yourself was yellow also overlapped with blue on the inside when they reached zone? If yes, yellow has Mark room and therefore their inside position gives them rights over an outside boat. I.e. mark room. If yellow was not overlapped inside when blue reached the zone, yellow can claim to be "inside" but without the overlap when they reached the zone, yellow cannot claim the inside rights of "mark room".

Two boats going around the mark in the same direction at the same time:
Their tracks are concentric, with the mark as the center.
One track curve is inside the other track curve.

 “If yellow was not overlapped inside when blue reached the zone, yellow can claim to be "inside" but without the overlap when they reached the zone, yellow cannot claim the inside rights of "mark room".(Tim O Connell)

Can the speed influence this rule 18 and overlap? 

If Yellow is very fast and Blue slow or are they overlapped when Blue reached the zone and Yellow can claim inside right?

If the rule 18 is on, speed is not a factor. 18.2b) is quite important here because it describes 2 different scenarios within 1 rule,
(If boats are overlapped.../ If a boat is clear ahead...)

Olafur, as Arto stated speed doesn’t matter in determining 

That said, 18.2(c) does anticipate that their relative positions could change AFTER mark-room is determined ... (due to differences in speed and/or changes in a boat’s course)

In both scenario cases  Yellow is overlap with Blue because they are sailing both more than 90 degrees from the true wind and one of them, Blue is in the zone. rule 18 applies.

Then Yellow is inside boat and Blue are outside boat and Yellow can claim the inside rights of "mark room” in both cases

What about other rules,  15

Yellow is much faster than Blue, does rule 15 applied? 

Olafur, 15 has to do with a boat acquiring right of way (a boat did not have ROW one moment and does have ROW a moment later). 

Go through both drawings at each position and tell us which boat has ROW at positions 1,2,3 and which ROW rule applies.  

What you are looking for is a change from one position to the next. 

Drawing 1

1 Yellow have entered the Zone on PB and is ROW because of rule 18,2 b (inside boat)

1 Blue have entered the Zone on SB is not a ROW because of rule 18,2 b (outside boat)

2 Yellow ROW rule 18,2 b applied 

2 Blue is outside boat and must give Yellow room to round the mark (Rule 18.2 b applied) 

3 Yellow a ROW inside boat, Rule 18.2b 14 and 60,1a applied (Protest Blue)

3 Blue is outside boat and is bracing rule 18,2b (take a penalty 44.1)

 

Drawing 2

1 Yellow is PB boat, ROW rule 18 applied (inside boat)

1 Blue SB boat have entered the Zone on SB is not a ROW because of rule 18 (outside boat)

2 Yellow ROW rule 18 applied 

2 Blue is outside boat, Rule 18 applied

3 Yellow enter the Zone a ROW inside boat, Rule 18.2b 14 and 60,1a applied (Protest Blue)

3 Blue is outside boat and is bracing rule 18,2b applied ( penalty 44.1)

I can see the problem now.
There is a confusion between right-of-way and mark- room. In both pictures yellow boat is on port tack
and therefore it has to keep clear. Blue boat is on starboard tack and right-of-way boat. Nothing can change that fact.

But yellow "inside" boat has mark-room which means blue has to give that mark room. The key for this can be found from preamble (which I call hidden secrets)
which states: "SECTION A RIGHT OF WAY:
A boat has right of way over another boat when the other boat is required to keep clear of her. However, some rules in Sections B, C
and D limit the actions of a right-of-way boat."
And if you look carefully, you will notice that rule 18 is a rule of section C and is limiting the actions of right of way boat. (10-13)

Ólafur,
You're interpretation is making this too complicated. Read Arto's reply. Right of way is not the same as mark room. Read section B which identifies other limits over a right-of-way boat's actions . Consider mark room to be a temporary privilege granted to an 'INSIDE" boat ONLY if the inside boat was overlapped with an outside boat when EITHER of them reached the zone, and that privilege goes away once they have been given mark room. Simple. If the inside boat didn't have an overlap at the zone, but obtained one after they entered the zone, too bad. This rule is written this way to stop a boat charging into a mark too late for any room to be granted by boats already in the zone. Think about it. Imagine the chaos if the zone was one boat length instead of the present 3 boat lengths. As boats get even faster and reaction time reduced, there may be a need to make mark room even wider. In radio control racing classes that move and react very quickly, the zone is actually 4 boat lengths. 

Drawing 1   Action 2

1 Yellow enter the Zone on Port, rule 18,2 (a) applied (inside boat)

1 Blue enter the Zone, starboard boat is ROW rule 18,2 (a) (outside boat)

2 Yellow rule 18,2 a applied 

2 Blue is ROW outside boat and must give Yellow room to round the mark (Rule 18.2 a applied) 

3 Yellow a inside boat, Rule 18.2 (a) 14 and 60,1a applied (Protest Blue)

3 Blue is ROW outside boat and is break rule 18,2 (a) 

 

Drawing 2, Action 2

1 Yellow is PB boat, rule 10 applied

1 Blue, ROW SB boat rule 10 applied 

2 Yellow rule 10 applied 

2 Blue ROW rule 10 applied

3 Yellow enter the Zone, Rule 10 and 18.2 (b) applcied

3 Blue is ROW rule 10 applied 

 

Yellow is to late to claim room to round mark

I hope that is better

Olafur .. I've added a red extension line to both drawings. 

At position #1 in both drawings, is Yellow inside and overlapped with Blue when the first boat (either Blue or Yellow) enters the zone? 

If the answer is "YES", then there is no difference between the drawings as far as Blue's obligation to provide Yellow mark-room.

Dwg 1

Dwg 2

If Blue is fast enough to round clear ahead of Yellow or slow enough that Yellow can round clear ahead of her then Blue can simply maintain course & speed and give mark-room as she's required. Otherwise Blue must alter course to make room for Yellow to pass between her and the mark, even if the overlap is broken after the first boat enters the zone.

In scenario 2 Yellow is screaming into the mark and must gybe to round it. How much room does Blue owe her?

First of all, I will thank you for your patient and time you have given me.

The boats is then overlap in both cases. Yellow port and Blue SB ROW boat, rule 18,2 (b) applied to give yellow room to round mark

Tim re: “In scenario 2 Yellow is screaming into the mark and must gybe to round it. How much room does Blue owe her?”

Case 21 spells this question out.  Blue owes her room to make a seamanlike rounding given Yellow’s speed, maneuverability, conditions, etc. 

Important point is that Yellow is the keep clear boat here, therefore Yellow is not entitled to make a “tactical rounding” and neither does RRS 18.4 apply (Yellow is not ROW that will gybe at the mark). 

Therefore, the room taken by the inside boat as shown in Case 75 does not apply ... but I find it is helpful to examine 75 as a contrasting example as to what a ROW boat is entitled to and why ... and then by extension what a keep-clear boat is not entitled to. 

Ang