Have you ever heard (or participated in) an on the water exchange like:
"I'm going to need room at the mark."
"No room - we aren't overlapped."
"Oh yes we are - I need room!"?

If you don't know exactly what "overlap" means, you can't follow and enforce the rules on the racecourse.

In fact, "overlap" applies to several of the fundamental Racing Rules. It is referenced in rules

• 11 - Same Tack, Overlapped,
• 12 - Same Tack, Not Overlapped,
• 17 - Proper Course,
• 18 - Mark Room, and
• 19 - Room to Pass an Obstruction.

What is "Overlap"?

Like other critical terms, "overlap" has a specific definition in the rules. I strongly encourage you to read the "Definitions" section closely. Unlike the other terms listed there, however, "Overlap" is defined in a backwards way. The terms "Clear Ahead" and "Clear Astern" are defined. If one boat is neither clear ahead nor clear astern of another, then the two boats are overlapped. Here is the full text of the definition:

One boat is clear astern of another when her hull and equipment in normal position are behind a line abeam from the aftermost point of the other boat’s hull and equipment in normal position. The other boat is clear ahead. They overlap when neither is clear astern. However, they also overlap when a boat between them overlaps both. These terms always apply to boats on the same tack. They do not apply to boats on opposite tacks unless rule 18 applies or both boats are sailing more than ninety degrees from the true wind.

When the terms apply

The terms "Clear Ahead", "Clear Astern", and "Overlap" always apply to boats on the same tack. The figure below shows four pairs of boats, in four different colors, on the same tack. Between each pair, the terms apply. Either one boat is ahead and the other astern, or the two are overlapped.


The terms also apply to boats on opposite tacks if both boats are sailing more than ninety degrees from the true wind, or rule 18 applies. The next figure shows four pairs of boats, in four different colors, on opposite tacks. The terms "Clear Ahead", "Clear Astern", and "Overlap" apply to the green and blue boats, but not to the yellow and orange boats. The green and blue boats are all sailing more than 90 degrees from the true wind, but the yellow and orange boats are not.


The only time that these terms apply to boats sailing 90 degrees or less from the true wind is if Rule 18 is in effect. Imagine that two boats were broad-reaching on opposite tacks towards a leeward mark and as they neared the zone, the wind shifted 45 degrees. Now one is sailing dead downwind and the other is on a beam reach. Ordinarily, with one boat sailing 90 degrees (or less) to the true wind, the terms would not apply. Because rule 18 is in effect, however, they do. See the following diagram:


to be continued...