Boats BL and BW, overlapped on the starboard tack, are overtaking boat A, also on the starboard tack but moving more slowly. Before the boats reached position 1, BW had overtaken BL from clear astern.

What are the applicable rules

Because BW had overtaken BL from clear astern before the boats reached position 1, rule 17 does not apply between BW and BL at any time during the incident.

While BW and BL are overtaking A, rule 12 requires both BL and BW to keep clear of A. Therefore, A is an obstruction to both BL and BW. However, A is not a continuing obstruction, as the last sentence of the definition Obstruction makes clear. BL may choose to pass A on either side (see rule 19.2(a)). BL chooses to pass A to leeward. During the interval of time that BW is between BL and A and both BW and BL are still clear astern of A, rule 19.2(b) requires BL, the outside boat, to give BW, the inside boat, room to pass between herself and A, the obstruction.

When BW becomes overlapped with A, the applicable rules change: BL becomes overlapped with A because BW is between A and BL (see the fourth sentence of the definition Clear Astern and Clear Ahead; Overlap); rule 12 ceases to apply; BL and BW obtain right of way under rule 11 over A, so A ceases to be an obstruction to BW and BL, and BL becomes an obstruction to BW and A; initially, rule 15 requires both BL and BW to give A room to keep clear; and, as soon as she is able to do so, A is required by rule 19.2(b) to give BW room to pass between A and BL.

When a boat is entitled to room under rule 19.2(b), is she required to hail for room? 

No. A boat entitled to room under rule 19.2(b) is not required to hail for room, although that is a prudent thing to do to avoid misunderstandings.

GBR 1977/6