Excerpt from Understanding the Racing Rules of Sailing
by Dave Perry, Illustrations by Brad Dellenbaugh
- Now Available -

Rule 17.1 - On the same tack; proper course
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 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.

“What happens in the situation where L and W are both sailing their proper courses and the two boats are converging; who has to keep clear?”

W must keep clear of L under rule 11 (On the Same Tack, Overlapped). Rule 17.1 only requires that L not sail above her proper course. When L is on her proper course, W must keep clear. Note that the phrase in rule 17.1 “her proper course” clarifies that it is L who gets to sail her proper course. Therefore, when L is sailing on her proper course, W must keep clear under rule 11, even when W’s proper course may be a lower course than L’s. (See ISAF Cases 7 and 14.)
 

Whenever W becomes overlapped to windward of L, L can turn all the way up to head to wind for the duration of the overlap, provided she gives W room to keep clear in the process.

Remember that a proper course is essentially any course a boat chooses to sail in order to get to the next mark and ultimately to finish as quickly as possible. Therefore it is possible that there may be several proper courses at any given moment depending upon the circumstances involved. It is also obvious that two overlapping boats sailing for the same mark will converge. Note also that a boat’s proper course is not necessarily a straight-line course. It can change with changes in the breeze, current or waves, or with a change in the boat’s strategy (see the discussion of definition Proper Course). However, whenever L wants to change her course to a new proper course, she must give W room to keep clear under rule 16.1 (Changing Course). A hail that she intends to change course is strongly recommended.
 

L is slowly luffing up to a new proper course in order to get to a puff of wind sooner. Because she is not sailing above her proper course, she is not breaking rule 17.1; and because she is giving W room to keep clear, she is not breaking rule 16.1. W must keep clear under rule 11.

If L is “limited” and W thinks that L is sailing above L’s proper course, W must still keep clear and she can protest under rule 17.1. If the two boats hit, and it were decided by the protest committee that L was sailing above her proper course, both boats will likely be disqualified: W for failing to keep clear of L, and L for illegally sailing above her proper course.

“What happens when L wants to luff two or more boats and one of the middle boats is ‘limited’ to where she can sail?”

Because M becomes overlapped to leeward of W from clear astern, she is "limited" to sailing no higher than her proper course. When L luffs, M is required to keep clear of her under rule 11. Even in the absence of W, M would luff to keep clear of L. Therefore, M is not sailing above her proper course and W must keep clear of M under rule 11 and give her room under rule 18.2 (a).

Good question! Let’s take the situation where L and W are sailing down a reach about two lengths or so apart. A boat from astern (M) catches up and becomes overlapped between them. When M becomes overlapped on W, there was clearly room for her to pass between L and W such that she is entitled to room from W to pass L (rule 18.2(a), Rounding and Passing Marks and Obstructions; and rule 18.5, Passing a Continuing Obstruction). Now L begins to luff toward M and W. M responds by luffing. W must keep clear of M under rule 11 (On the Same Tack, Overlapped) and give her room under rule 18.2(a). And M is not breaking rule 17.1 because in fact M is not sailing above her proper course. Here’s the reason. Take the two boats involved, M and W. M became overlapped on W to leeward from clear astern. Rule 17.1 requires M, therefore, not to sail above her proper course. In determining her proper course, the definition Proper Course instructs us to remove the boats referred to in the rule using the term “proper course.” In this case, rule 17.1 uses the term and refers to the windward boat W, so we remove W. As M was sailing a course to keep clear of L, she would have been sailing the same course in the absence of W; therefore, M was sailing her proper course and not above it.

“What’s the purpose of the phrase in rule 17.1, ‘…unless in doing so she promptly sails astern of the other boat’?”

This is to close a very subtle, undesirable loophole in the rule. Here’s a potential scenario: on a beat to windward, a boat crosses you and tacks just ahead and about half-a-length to windward of you. Due to your greater speed you become overlapped to leeward from clear astern, but you realize that you won’t be able to sail past them enough to get your air clear. You want to tack out of there. Assuming that when sailing upwind your proper course is a close-hauled course, without an exception to the rule the question would be: “Could you sail above close-hauled and tack while you’re overlapped to leeward of W; or would you have to wait until you were no longer overlapped so you didn’t break rule 17.1?” Rule 17.1 clarifies that you can certainly luff and tack (i.e. sail above your proper course) provided you break the overlap with W at some point during your turn. If you luff and then realize that your bow won’t clear W’s transom and have to pull your bow back down, you have broken rule 17.1.