The stabiliser fins are driven by a gyro that feels the ship roll, and the bow has a wave recorder that measures the sea, and on a calm night both read nothing. On one stretch of the crossing the gyro drives the fins hard against a roll of six degrees, the wave recorder reads flat, and the passengers feel the ship lean.
I am the second engineer on a short sea ferry, and the stabiliser system is on my watch.
How it works.
Two fins, one each side under the waterline, driven by hydraulics. A gyro in the hull senses roll rate and the controller moves the fins to oppose it, continuously. It logs the roll it senses and the fin angle it commands, several times a second.
The bow has a wave radar that measures the sea's height and period, and that goes on the same log, because a stabiliser that works hard on a flat sea is a stabiliser with a fault.
On every crossing, on a stretch about twenty minutes out from one of the two ports, the gyro senses a roll of about six degrees, port to starboard, with a slow period, and the fins drive to correct it.
The wave recorder on the bow reads the sea as it is, which on most nights is a metre or less, and on some nights is flat.
The passengers feel the ship lean, because a six degree correction on a flat sea is the ship being tilted by its own fins.
Six degrees is a beam sea of some size. It is not there. The bow says it is not there, my eyes on the bridge wing say it is not there, and the log says the gyro felt it and the fins fought it.
The stretch is always the same, and I have the positions from the log: about a mile and a half of water, over a charted depth of thirty metres, over nothing the chart marks.
What has been checked.
The gyro, replaced at the last dry docking. The controller, updated by the manufacturer. The fins, inspected, and their hydraulics pressure tested.
The wave radar, calibrated against a buoy the harbour authority runs.
And a second gyro, a portable one from the survey firm, on the deck plates amidships for four crossings, logging on its own.
It felt the roll. Six degrees, port to starboard, slow, on the stretch, on a sea the radar and a survey engineer standing beside it agreed was flat.
What that means.
The ship rolls. The sea does not make it. Something under the hull, on that stretch of water, takes the ship by the keel and rolls it six degrees to starboard and back, with the slow period of a big swell, and the fins fight it and lose a little, and the passengers feel it, and the bow sees a flat sea.
I have written that in a report in words the company could accept, which are that the system responds to a roll of undetermined origin on the stretch, and that the fins are operating correctly.
The other ships.
Two other vessels work the route and both have stabilisers. I asked their engineers, at the club, without leading them.
Both have it. Same stretch, same six degrees, same direction. One of them said his old chief had called it the nod, and had told him it had been there since the first ship with a gyro on the route, and that before that nobody could have known.
What is done about it.
The fins fight it and the ship leans and the passengers hold their coffee, and the log says roll of undetermined origin, and the marine superintendent has signed off the stabilisers as fully serviceable, which they are.
What I do.
I keep the watch.
The one thing I have changed is that I go up to the bridge for the stretch, rather than staying in the control room with the log.
The reason is a night in the winter with the sea flat and the fins driving, and the roll came, port to starboard, slow, and I was on the port bridge wing looking down at the water alongside, which was flat, and lit by the deck lights, and as the ship rolled towards me the water alongside did not come up to meet the hull the way water does when a ship rolls.
It stayed where it was. The ship leaned six degrees towards a sea that stayed flat and level and did not move at all, as though the ship were being tilted in air, and the fins fought it, and the water came back to the hull as we came upright, and the bow recorder read a flat sea, which it was.