One of the limitations in the rules on the right-of-way boat is contained Rule 17, ON THE SAME TACK; PROPER COURSE, which reads as follows:

If a boat clear astern becomes overlapped within two of her hull lengths to leeward of a boat on the same tack, she shall not sail above her proper course while they remain on the same tack and overlapped within that distance, unless in doing so she promptly sails astern of the other boat. This rule does not apply if the overlap begins while the windward boat is required by rule 13 to keep clear.

This rule is in Section B of Part 2, and is put in place so that a boat on the same tack as you can’t establish an overlap to leeward from clear astern and luff you above her proper course. Remember it is the proper course of the boat that has established the overlap to leeward that matters to this rule. Another important point about this rule; if you are a boat that has been overlapped to leeward and believe that the leeward boat is sailing above her proper course, you are still required to keep clear. Your only option is to keep clear and then protest. If you don’t keep clear you will be disqualified even if it is found that the leeward boat did indeed break Rule 17.

I’ll show a couple of cases that demonstrate how the rule works. In the first case a boat on the down wind leg gets an overlap from clear astern and luffs above her proper course, she breaks Rule 17. In the second case two boats are sailing different courses because perhaps they are in a PHRF race and they are different types of boats and sail different angles down wind. In this case even though the windward boat is sailing higher than her proper course it is the proper course of the leeward boat that matters, so Rule 17 is not broken. In the third example before the starting signal you are permitted to establish an overlap to leeward from clear astern and luff head to wind because by the definition of proper course, there is no proper course before the starting signal. However, after the starting signal she needs to head down immediately or she will break Rule 17.

This is the basics of Rule 17. If you want to get more in depth you can review US Sailing Appeals: 4, 13, 43 and 70. Also take a look at ISAF Cases: 7, 13, 14 and 46. If no one protests next month I’ll look at another rule.