Having looked through the rules I'm a bit unsure of a situation that arose yesterday:

Two boats on starboard  are closehauled heading towards a windward mark. The mark is to be left to starboard so both boats need to tack to round the mark. The leward boat is on the layline while the winward boat is one boat length to windward and slighly over standing the mark. Both boats have been in this situation from about 8 boatlengths from the mark.

My question is does the leward boat have to give water to the windward boat?

If the leward boat forces the windward boat above the mark does she also have to follow?

If this situation only formed within the 3 boat length circle (i.e. the leward boat tacks in under the windward boat) does it change things?

"My question is does the leward boat have to give water to the windward boat?"

Yes. None of the exclusions of 18.1 applies.

"If this situation only formed within the 3 boat length circle (i.e. the leward boat tacks in under the windward boat) does it change things?"

L still has to give mark-room and also can't force W above close-hauled. See 18.3.

Few people have been as far-sighted or experienced in our sport as Paul Elvstrom, and I'd rate his opinion above that of any RO's of my aquaintance. Here's what he had to say about port vs starboard roundings.

He was talking about the tendency of boats approaching a port-rounded windward mark to form a long line of starboard tackers:

"...each succeeding boat has to tack further and further over to the starboard side in order to avoid backwind and thus loses that much more distance until the situation gets so bad that that a boat decides to come in on port tack and hopes to lose less by taking a chance. Only the starboard side of the course is used instead of the whole area. This causes congestion and restricts the tactics on the windward leg, making it less interesting.

"In large fleets if the weather mark is ordered to be rounded to starboard it is very much easier for boats to be able to round satisfactorily. In this case, if the port tacker overstands the mark by only about four boat-lengths she is almost certain to round in the clear. She will only have to sail six boat lengths further than a boat which rounds in the best position. In this case the whole of the racing course is used because boats can come into the mark on port or starboard tack always with a good chance of being able to round. In big fleets we do know that rounding marks to starboard seems to produce more protest situations but I still think it is better to round this way"

The RO's at the Helsinki and Melbourne Olympics, at least, seemed to agree with him as they both used starboard courses.

We've all been in fleets where the leader was virtually at the wing mark by the time the tail end Charlie got around the windward mark thanks to the queuing up on starboard tack phenomena, unduly spreading the fleet already at the first mark; very irritating (if you're not at the front!)

This has been quite a nice clarification I think in the new rules and nice pictures have followed in the "explained" stuff which is on the market!

When you enter the THREE boat zone (correct hull legnths now?) and both boats must tack then the inside boat has rights.

If the boat laying the mark was on port-tack outside the zone and the other continued on starboard then they have precedence with rule 10 UNTIL they breach rule 13 or 16/17. 

One thing to remind me of though, opposite tacks and one passes head to wind in the zone now, do they loose rights to water under the new rule 18? Will read it soon though, just for the sake of arguement on STB roundings which will be a little troublesome!

Until the first of them passes head to wind, then rule 18 goes off as long as the proper course of one is still to tack.

Let's not use 'precedence' it was got rid of in the rewrite.

Stbd does not have 'precedence':  she simply has right of way under rule 10 (limited by rule 16, not 15 or 17), and rule 18 does not apply until both boats are on the same tack, that is, until Stbd passes head to wind.

If Stbd tacks inside/below Port, she is subject firstly to rule 13, then regains right of way under rule 11, but has limitations not to cause a close hauled fetching boat to sail above close hauled under rule 18.3.

If Stbd tacks above Port, from the instant she passes head to wind, she must keep clear of Port, first under rule 13, then rule 11, and she must give Port mark-room under rule 18.3.