Simple Simon wrote:

Wrong, it's a valid signal.

And I since gather that it's an inland signal...


What is the vessel ahead of you doing?

Approaching a drawbridge.


What kind of vessel is it?

One which is too high and/or wide to fit through the closed bridge, in spite
of "any nonstructural vessel appurtenance which is not essential to
navigation or which is easily lowered".


What should you do?

Listen for an acknowledgment signal from the bridge and/or monitor the
appropriate VHF channels, if known, with a view to establishing whether the
bridge is open-on-demand (and thus about to start opening), or opens only at
specific times. Until activity at the bridge is clarified, I would be
tempted to stop, or perhaps slow down to minimum speed if I know I'm a safe
distance from the bridge and the other vessel. Once communication between
the bridge and the other vessel has been concluded, sound the signal
appropriate to my vessel and its status. If I also wish to use the bridge,
signal the bridge with one prolonged blast and one short blast, and/or hail
on the VHF.


--
Wally
www.makearatherlonglinkthattakesyounowhere.com
Things are always clearer in the cold, post-upload light.