A bow thruster is a propeller in a tunnel through the bow, driven from a lever on the bridge, and its motor's current is logged with the lever position. At one berth, every arrival, after the lines are on and the lever is at zero, the motor draws full power to port for six seconds and the ship moves off the fender, and the mooring line load cells feel her go.
I am the master, and berthing is the part of the job where the instruments and I are closest.
How the thruster is logged.
The lever on the bridge commands the thruster, and the motor's current is logged with it, so that a push shows as a lever movement and a current draw together. The mooring lines run over load cells at the winches, so the tension on each line is logged too, and the whole picture of a berthing is on the recorder.
After the lines are on and the lever is at zero the thruster is idle, and the current is zero, and the line tensions are what the wind makes them.
At one berth, every arrival, about four minutes after the lines are on, the thruster draws full power to port for six seconds.
The lever is at zero. The log shows it at zero. The current shows a full push, to port, which at that berth is away from the quay, and the line load cells show the head line and the spring take the strain as the bow moves off the fender by, on the camera, about half a metre, and come back.
The bridge is manned when it happens. I am on it. The lever is under my hand and it does not move, and the thruster runs.
What has been checked.
The thruster. Motor, controller, the lever's electronics. Serviced, and the controller replaced. The push happens with the new controller.
Isolation. I have had the thruster's breaker opened after the lines are on, on three arrivals. With the breaker open there is no current to draw, and the log shows none. The line load cells show the bow move off the fender anyway, by the same half metre, for the same six seconds, and come back, and the camera shows the gap open at the bow.
So the thruster is not doing it. Something pushes the bow off that berth for six seconds, four minutes after the lines are on, and when the thruster is available it is the thruster that is used to do it.
The other berth.
We use two berths at that port and it is only the one. The other ferries on the route use it too, and their masters have the same log, and one of them told me his ship had been pushing herself off that berth since before he joined her.
The berth was rebuilt in the eighties on the site of an old one, and the harbour's own records, which I read one winter in their office, have a note from the sixties about a vessel that was crushed against that quay by another coming in too fast, with a loss of life, and the harbour master's recommendation that ships on that berth keep the thruster ready to push off.
What is done about it.
The push is logged as uncommanded thruster operation and the company has had the thruster surveyed twice, and it is fine, and the mooring arrangement has been adjusted so that the lines take the strain without parting, which they did once, in the early years, a head line, on a push that was longer than six seconds.
What I do.
I berth the ship.
The one thing I have changed is that four minutes after the lines are on at that berth, I stand at the lever with my hand on it, and when the push comes, I go with it. Lever to port, full, six seconds, and back to zero.
Since I started doing that the push is my push, on the log, commanded, and the ship moves off and back the half metre, and the line cells show it, and nobody has to write uncommanded in a log.
The reason I started is the arrival in the spring when I had my hand on the lever at zero and the push came, and the lever, under my hand, went to port. Not by itself. It was pushed, from the far side, by a pressure against my palm, the pressure of a hand on the other side of the lever pushing it over, and it held for six seconds, and I let it.