Mark Rounding Brain Teasers

October 25, 2001
David Norton

Since May, when my From the Experts article on new Rule 18 appeared in Sailing World, I have received several questions about mark roundings, most involving Rules 18.2(a), (b), and (c). Rule 18.2(a) is the basic rule that applies when two overlapped boats are about to round or pass a mark or obstruction. Rule 18.2(b) guarantees, or “locks in,” the rights of the inside boat if two boats are overlapped when the first one of them reaches the two-length zone. Rule 18.2(c) “locks out” a boat that is clear astern when a boat clear ahead reaches the zone. Those three rules are the most frequently needed parts of Rule 18. I will discuss three situations designed to increase your understanding of them.

Four-boat cluster: Ted, Alice, Bob, and Carol are running towards the leeward mark, which they must leave to port. The wind is fresh and the next leg is a beat. At the moment that Ted’s spinnaker enters the two-length zone, the four boats are in the positions shown in the first diagram.

First, let’s consider who overlaps whom. The defined term overlap applies between pairs of boats, so we have to examine each of the six possible pairs of boats. Obviously, Ted overlaps Alice, and Alice overlaps Carol. According to the definition of overlap, the term applies between boats on opposite tacks only if Rule 18 applies. Rule 18 begins to apply between two boats when both of them are “about to round” a mark (Rule 18.1). Given that the wind is fresh and these boats must douse their spinnakers before they round, they are all about to round, and probably have been, for at least a couple of lengths. Therefore, Alice and Bob are overlapped, as are Bob and Carol. (ISAF Cases 84 and 94 are very helpful for learning what the phrase “about to round” means. The ISAF Case Book is online at


Does Ted overlap either Bob or Carol? Ted overlaps Bob because a boat between them, Alice, overlaps them both. However, Ted does not overlap Carol because Alice is alongside them, and not between them. Instead, Carol is clear astern of Ted.

What rules apply from the time the boats reach the position shown in the diagram until they leave the mark astern? Rule 18.2(a) applies between all pairs that are overlapped. Bob overlaps the other three, which means that Bob is entitled to room to round the mark from each of them and, because he holds right of way as a starboard-tack boat, each of them must also keep clear of him until he jibes. Because he must jibe to sail his proper course around the mark, he is bound by Rule 18.4 not to sail any farther from the mark than his proper course would take him.

Because both Bob and Alice overlapped Ted when he entered the zone, Rule 18.2(b) governs Ted. Hence, both Alice and Bob would be entitled to room from him if the overlap between him and either of them was broken.


Carol was clear astern of Ted when he reached the zone. Therefore, Rule 18.2(c) requires her to keep clear of Ted even if an overlap is established later.

Does Rule 18.2(b) apply between Bob and Alice? The answer depends on whether Bob still has an inside overlap when Alice reaches the zone. If he does, then Rule 18.2(b) will entitle Bob to room from Alice if the overlap is broken. If Alice manages to pull clear ahead of him before she reaches the zone, however, then Rule 18.2(c) will apply, implying that Bob will not be entitled to room from Alice, and he will be required to keep clear of her as she approaches the mark and rounds it.

Alice holds all the trump cards over Carol. If those two are still overlapped when Alice reaches the zone, then Alice benefits from Rule 18.2(b) because she is on the inside. If Alice manages to pull clear ahead, then she benefits from Rule 18.2(c). A similar situation gives Bob the upper hand over Carol.


Three boats ahead, one behind: Let’s study another four-boat situation. In the second diagram, at Position 1, Tom, Dick, and Harry are running on port tack clear ahead of Larry. All four will leave the mark to port and round up to a closehauled course. Rule 18.2(a) entitles Tom to room from Dick, and both Tom and Dick are entitled to room from Harry. Because he must give so much room, Harry doesn’t enter the zone until Position 2 when he is turning toward the mark.

As the three leaders turn, each of them becomes overlapped with Larry. Tom and Dick enjoy the protection of both Rule 18.2(b) and Rule 18.2(c). Tom and Dick each reached the zone while clear ahead of Larry. Therefore Rule 18.2(c) applies to Larry and tells him that he must keep clear of both Tom and Dick and, even though he gets an inside overlap, he’s not entitled to room. Also, Tom and Dick reached the zone while overlapped with Harry. Therefore, Rule 18.2(b) applies to Harry and tells him that he must continue to give Tom and Dick room, and under no condition is he entitled to room inside either of them.

When Larry reached the zone, he was clear astern of Harry. The overlap between them began after Harry turned toward the mark. Rule 18.2(c) only applies if the boat clear ahead reaches the zone before the boat clear astern does. Because Larry reached the zone before Harry, Rule 18.2(c) does not apply between them. From the time Harry’s turn creates an overlap between him and Larry, Harry is obligated by Rule 18.2(a) to give Larry room to pass on the inside.


Rule 18.2(b) applies only if the boats were overlapped before either reached the zone. But Larry’s overlap on Harry didn’t begin until after Larry had reached the zone. Therefore, Rule 18.2(b) also doesn’t apply between Larry and Harry. If the overlap between them were broken while they were rounding, whichever boat pulled clear ahead would have right of way under Rule 12. (ISAF Cases 2 and 59 discuss similar circumstances.)

Tacking into a late overlap: Hillary and Bill were beating on opposite tacks to a windward mark that they must leave to starboard. Bill was on starboard tack on a closehauled course to pass just to leeward of the mark, and Hillary came in on port tack and completed a tack to leeward of him. What rules apply when the boats are in the positions shown in the third diagram right after Hillary’s tack?

When the boats reached the zone they were on opposite tacks, so Rule 18 did not yet apply (see Rule 18.1(b)). When Hillary passed head to wind, they were both on starboard tack and overlapped. Rule 18 began to apply at that time. Rule 18.2(a) required Hillary to give Bill room. Also, Rule 13 required her to keep clear, but when she completed her tack and was on a close-hauled course, that rule ceased to apply and Rule 11 began to apply, giving Hillary right of way as the leeward boat.

Rule 18.3 never applied because Bill was not fetching the mark. A boat is fetching a mark when she is on a course that will enable her to pass to windward of it without tacking. Rules 18.2(b) and 18.2(c) also didn’t apply because, at Position 1 when the boats reached the zone, they were neither overlapped nor in a clear ahead/clear astern relationship.

From Position 2 until Bill turns past head to wind during his tack to round the mark, Hillary holds right of way under Rule 11, but she is required by Rule 18.2(a) to give Bill room to round the mark inside her. If the overlap between them were broken during that time, Rule 12 would give right of way to whichever boat pulled clear ahead.

E-mail for Dick Rose may be sent to [email protected]


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