Paul,
Astern means behind, as you know. Using Astern instead of clear astern means any boat behind slightly as well as those completely astern. Also look at the history of the rule. I believe it used to end at "on the other's port side". There must have been situations not covered and so "or the one astern" was added to cover those situations.

This scenario often occurs as new situations are described, seen or imagined. The rules are modified to account for them.

Further, the rule does not say overlapped; it says "the one on the other's port side". All of this I think is to cover the situation to have no contact and not worry about the exact positions of the boats, e.g. overlapped, clear astern. So in ways it is a simple concept rule. Are you on my left or right.

Wayne

Rule 13 initially appeared in 1997 with the 'simplified' rules, and ended with: If two boats are subject to this rule at the same time, the one on the other’s port side shall keep clear.

A weakness in the rule first showed up in match racing. Consider the dial up after the initial entry where both boats end up head to wind after both boats had crossed head to wind during the maneuver. Yellow is on Blue's port side and must keep clear while the boats are still tacking. As the boats start drifting backwards, they end up on the same line with one boat ahead of the other - still tacking, still subject to rule 13, but with neither boat on the other's port side. A submission to add "or the one astern" was met at ISAF with "that only happens in match racing."

It did not take long to find fleet racing situations where two boats are tacking and neither is on the other's port side. Consider two Etchells overlapped on port tack with W about half a boat length to windward and slightly ahead. L luffs to tack and then W begins to tack to cover. After both boats cross head to wind, there is contact between the bow of L and the backstay on W. Both boats are tacking and subject to rule 13 and both boats are on the same tack, but neither is on the other's port side and neither boat is clear astern.

The Etchells situation was actually a protest during the '98 (or '99) Etchells Worlds in Marblehead and was used to argue for a change to rule 13. The current rule 13 first appeared in the 2005-2008 rule book.

Thank you for the background. That makes sense. I was having trouble seeing "visualizing" how that situation could occur needing that wording...

Then there is the possibility that two boats, 'Ahead' and 'Behind'
are tacking simultaneously, but a third boat overlaps them both
such that 'Behind' is astern, but not Clear Astern, of 'Ahead'
(by reverse osmosis) because now they are Overlapped