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. This process is known as blue boarding or historically blue flagging.
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. 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.
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. This must be transmitted every ten seconds.
Blue Flagging. Especially on rivers, look out for ships or barges with a blue flag or board display on the starboard side of the wheelhouse.
… meetings are conducted what is colloquially known as 'blue boarding'
Lots of blue flagging, as upbound vessels try to stay in the shadow of the current, and call for starboard-to-starboard passing.
we had already rigged an arrangement for blue flagging—a flag of Oxbridge blue run up on the starboard side being an indication that a vessel proceeding downstream on the Rhine, Meuse, Main, Neckar, Lahn, Ems, Weser, Elbe or Seine wishes to meet an approaching ship starboard to starboard instead of port to port. Or, more simply, that the skipper is keeping to the left instead of the right because he wants the current, or can't steer properly
Article 6.04 – Meeting: Normal rules … 4. Vessels proceeding upstream which leave the course of those proceeding downstream on their starboard side shall in good time and to starboard: … Display a light blue board in conjunction with a bright scintillating white light.
Article 3.03 – Boards, flags and pennants … rectangular. … (a) … neither the length nor the width is less than 1 m or, in the case of small craft, less than 0.60 m;
1.3.2. Navigation, short-term ahead … Navigation a short-term ahead is the decision phase in the navigation process. … — intention (blue sign),