Blue sign

A blue sign or blue board is used by inland waterways vessels within the Trans-European Inland Waterway network when performing a special manoeuvre or passing on the starboard side. On navigable waterways vessels normally pass each other on the port-side, so the display of the blue sign and flashing white light signal intention to pass each other on the starboard-side.^[1] This process is known as blue boarding or historically blue flagging.^[2][3][4][5]

Blue sign activated, during starboard passing

Rotating-style blue sign laying flat when not active. There is a 5 cm white border and the light is in the centre

The Code Européen des Voies de la Navigation Intérieure (CEVNI) regulations require upstream vessels operating on the opposite side to display a light-blue sign and scintillating (flashing) white light.^[6] Article 3.03 states that the board must be rectangular and 1-metre × 1-metre for large vessels, or 0.6-metres × 0.6-metres for small vessels.^[7]

The presence and status of the blue sign is transmitted by the ship's Inland-Automatic Identification System (Inland-AIS) transponder to other vessels. The status of the sign is transmitted using two bits of the "regional application flags"/"special manoeuvre field" in the AIS position reports.^[8] This must be transmitted every ten seconds.^[9]