The ship's whistle sounds automatically in fog, once every two minutes, from a controller the officer of the watch switches on, and a sound level meter on the bridge wing logs it for the noise survey. On one stretch, in clear air, with the controller off, the whistle sounds, and the meter records a second whistle, lower, from ahead, where the radar shows open sea.
I am the master of a short sea ferry, and the whistle is mine, in the sense that everything on the ship is.
How the fog signal works.
The whistle is compressed air through a horn on the funnel, and it can be sounded by hand from the bridge or by an automatic controller that gives one prolonged blast every two minutes. The controller is switched on in restricted visibility and off when it clears, and the switch position is logged.
For the last two years there has been a sound level meter on the port bridge wing, logging continuously, because the port wants a noise survey of ships on the approach.
On one stretch, with the controller off and the visibility clear to the horizon, the whistle sounds.
One prolonged blast. Two minutes later, another. For the length of the stretch, which is about eight minutes, four blasts, and then nothing until the next crossing.
The controller's log shows the switch off. The whistle's own log, which records each blast and what commanded it, shows the blasts commanded by the automatic controller, which is off.
The meter on the wing records our blasts, at the level our whistle makes.
Between them, about a minute after each of ours, it records a second blast. Lower in pitch, longer, quieter, from ahead and to port by the meter's directional pickup, at a level that puts its source about two miles off.
The radar shows open sea two miles ahead and to port. So does the lookout. So do I.
What has been checked.
The controller, which has been replaced, and which sounds the whistle on the stretch with its switch off and its power isolated at the breaker, on the one crossing I did that.
The whistle. Air pressure, valve, horn. It is a whistle.
The meter, which is the port's and which I have no access to beyond the log they send me monthly. The port's acoustician has marked the second blasts in the log as unidentified vessel and asked me, in an email, whether we regularly meet another ship on the stretch.
We do not. The stretch is ours. The other two ferries are in port when we are on it.
The pitch.
Ships' whistles are pitched by size: a big ship has a low whistle so that you know from the sound that it is big. The second whistle is lower than ours. Considerably.
The old ship on this route, which was broken up in the nineties, was bigger than ours, and her whistle is on a recording in the maritime museum, made on her last crossing, and I have listened to it and then listened to the meter's log of the second blast, and I am not a musician, and they are the same note.
What is done about it.
The whistle sounding in clear air is a regulatory question, because a fog signal in clear visibility is a signal without meaning, and I have reported it as an equipment malfunction, which the log supports and which is the only description available to me.
The answering blast is not in my report, because it is not my equipment, and because I do not know how to write it.
What I do.
I have the ship.
The one thing I have changed is that on the stretch I stand on the port wing with the meter rather than in the wheelhouse.
The reason is a night in clear air with a full moon, when I was on the wing at the second blast and looked ahead and to port, along the bearing the meter gave, at open sea lit by the moon, and saw, at about two miles, a ship's lights.
Not a radar contact. Lights: a masthead light and a lower one behind it, the way a big ship carries them, steady, on a steady bearing, closing. The lookout beside me saw them. The radar showed nothing. The lights held for the length of the stretch and then were not there, and the meter's log for that crossing has a fifth blast in it, from ahead, close, that I did not hear and that the lookout did.