Overtaking boat is not a rule within the RRS, but it is quite a common misconception. The boat clear astern must keep clear by RRS 12, once there is an overlap then the windward boat keeps clear by RRS 11.

The only thing I can see that would limit the actions of the clear ahead boat would be RRS 16, changing course. RRS 15, acquiring right of way, could be used if the clear ahead boat changed course enough to establish an overlap.

Rule 16. "When a right of way boat changes course, she must give the other boat room to keep clear." 
Note that a windward boat turning down changes the angle of the overlap line. if the overlap occurs because of the actions of the (formally) clear ahead boat, she must immediately keep clear. She doesn't get the rule 15 initial extra room to keep clear. 
Finally, were I the clear-ahead boat, I would let the faster boat go to leeward. Seems like a better option than trying to fight them off on the windward side. 

To the best of my knowledge, there's no rule that precludes passing to leeward. Keep in mind that Rule 17 limits the leeward boat from sailing above her proper course once overlapped. The windward boat must keep clear (RSS 11) and if she feels the newly overlapped leeward boat is sailing above her PC, she should stay clear and protest.

Prior to 2009 there was a rule (17.2) that limited how low the windward boat could sail (without gybing) when the boat to leeward was within 2 boat lengths of her.  This included being 2 BL clear astern.  I think the thinking was that it was a "fairness" rule, i.e. the leeward boat can't come up and the windward boat can't come down.  Part of the problem with rule 17 at the time was that each was limited by their proper course which could be very different.  Windward boats would erroneously complain that they were not sailing below their proper course and the leeward boat was taking them up.

Colregs are, in some races , used at night. This is  usually stated in  the SI's .this changes the overtaking boats rights. 

Thanks everyone for your comments. 

We have a pretty tight racing group of one design boats and often one boat is just slightly faster than another on the off wind leg. On a couple of races, the boat clear ahead was being caught by the boat clear behind and simply wanted to remain ahead for a short distance to the gybe mark. The question was raised if the lead boat could alter course away from the mark to block and remain in front of the approaching boat so it could maintain the lead until the mark (while still preventing an overlap) or if it there was any proper course requirement.  

Room to keep clear should not apply as the boat behind is not overlapped, and does not become overlapped. I appreciate the overlap geometry will change as the relative angle of the boats changes.

Thanks again for all the comments

Neil,

'Room to keep clear' is only a thing a RoW boat has to give to a 'keep clear boat'.  It does apply if the boat clear ahead changes course. (R16.1).

If this were a reach, Ahead could bear away in front of the boat astern.  If she does, she must give Astern 'room to keep clear'.

In the diagram:
Example 1.  Yellow/Blue. Yellow is fine. - She is far enough ahead she creates no overlap.  When she changes course (both at her initial bear away and her straightening up), she gives Blue room to keep clear.  (RRS 12/16.1)

Example 2,  Red/L Blue. - Red is not fine.  She creates an overlap and breaks Rule 11.

Example 3.  Magenta/Yellow - Magenta is not fine. When she straightens up, she must give Yellow room to keep clear.  She does not and breaks RRS 16.1.

Some people have mentioned that 'overtaking boat' is not an RRS term, but a Colregs term.  They are right.

'Overtaking' describes a boat initially astern and her relative motion; one boat's advancement and passing of the other.

Well, RRS doesn't work on 'relative' motions.  RRS works on 'snapshots'.

In your case, when a boat is passing another to leeward (overtaking) there are 2 critical snapshots:

1.  In one snapshot Ahead is clear ahead, so Astern is clear astern and Rule 12 applies.  Astern must keep clear.
2.  Another snapshot at the exact moment Ahead's transom is aligned with Astern's bow (overlap is created).  If that's caused by Ahead's actions, then Rule 15 and Rule 11.  Ahead must initially give Astern room to keep clear.  Windward (old Ahead) keep clear. If not, then just Rule 11.

If you want to show you know the rules, NEVER shout "Overtaking boat!" to another.


Hope that helps.