Defend the Dunk
The second sentence of Rule 17 states an exception to the rule’s first sentence—a leeward boat may sail above her proper course even if she establishes her overlap with the windward boat from clear astern and within two hull lengths of the windward boat provided that the overlap begins while the windward boat is required by Rule 13 to keep clear. That sentence was added to Rule 17 in 2001. I’m often asked why it was added and what its implications are.

Kim Downing

The sentence was added to discourage aggressive slam dunks by providing the “dunkee” with a tactic that can be used to prevent the “dunker” from tacking very close on top of the “dunkee.” The second diagram illustrates how this rule can be used by Paul (the “dunkee”) to thwart an attempt by Stan (the “dunker”) to make an aggressive slam dunk tack on top of Paul.
Stan and Paul are on opposite tacks on the last beat to the finish. They are well away from all marks. Paul has been enjoying clear air and the freedom to tack on the shifts. He has gained on Stan and, at Position 1, Stan, on starboard, is about to cross ahead of him. Paul sees Stan’s crew preparing to make a slam-dunk tack and wishes to thwart it so he can keep his air clear. The second sentence of Rule 17 can help.

Here’s how it works: Immediately after Stan’s stern is past Paul’s bow, Paul begins to luff from closehauled to head-to-wind. Initially, Paul must keep clear of Stan under Rule 10, but Rule 16.1 does not apply to Paul because he does not have right of way. While boats are on opposite tacks and sailing closer than 90 degrees to the true wind, they can never be overlapped (see the definition Clear Astern and Clear Ahead; Overlap). After Stan turns past head to wind, both boats are on port tack, and the terms clear astern, clear ahead and overlap begin to apply. Also, Stan looses right of way and becomes obligated by Rule 13 to keep clear of Paul.

At the moment Stan passes head to wind, Paul is either overlapped to leeward of Stan or clear astern of him. If Paul is clear astern, then as Stan continues to turn, Paul will become overlapped and, because of the second sentence of Rule 17, Rule 17 doesn’t apply to Paul. If Paul is overlapped when Stan passes head to wind, then Rule 17 does not apply to him because the overlap began as a result of Stan’s turn and not as a result of Paul becoming overlapped from clear astern. So, in either case, Paul’s luff above close-hauled (his proper course) does not break Rule 17.

At Positions 3 and 4, Paul is blocking Stan from bearing away onto a closehauled course on port tack. If Stan were to bear away after Position 4 he would break Rule 11. Paul can control the situation and choose when he wishes to bear away. He should be able to bear away back onto a closehauled port-tack course and continue to enjoy clear air. If the second sentence of Rule 17 were not in the rulebook, Paul’s luff might break Rule 17. Now you can see how that sentence permits a “dunkee” like Paul to use the rules to avoid being closely covered by a “dunker” like Stan.