Do you know what that blue board in the boat means?
We got plenty of first hand experience of this process known as “blue boarding” – or historically “blue flagging” – at river Rhine in Germany.
On navigable rivers boats normally pass each other like in right sided road traffic. But the display of the blue sign (board or a flag) and flashing white light signal intention to pass as in left sided traffic.
With blue flagging, the ships utilize the stronger/weaker current at the curves of the river. Or make sure they can get to a pier in a spot where there is a never-ending line of vessels going up- and downstream.
We zigzagged downstream River Rhine with 5 knot current and average speed of 8.4 knots for nearly 200 km. We switched sides of the river because of blue flagging so many times that we lost count. This is not a problem if all ships use the blue flag but things become interesting when one ship suddenly decides not to blue flag…
When cruising among blue flagging ships you need to keep close lookout because the huge cargo ships can raise/lower the blue flag at any given time.
Typically there is enough searoom for a small ship (like ours) to pass on either side of the blue flagging commercial ship but it is safer to play by the rules.